Adaptive Combined Space-Time Receiver Structures for DS-CDMA Systems

Abdelgader M. Legnain, B.Sc., M.Sc.

A Thesis submitted to the Faculty of Graduate Studies and Resemh in partial fdfilhetzt of the requirements for the degree of Doctor of Philosophy.

ûttawaCPrleton Institute for E I d d and Cornpliter Engineering Facalty of Engheerhg

Department ofsystems and Cornputer Engineering Carieton University

ûttawa, Ontario, Canada October, 2000 c8 copyright

2000, Abddgader M. L e p a h

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Abstract

Direct-Sequence Spread-Spectnmi (DSISS) systems offa severai amactive features such

as interference r'ection, mdtipath divei.sity, multiple access capability, and accurate

ranging. Recently, a greater interest has beai dexoted to the use of DS/SS systems in many

mobile radio applications incliiciing digital cellular systems and indoor wireless networks

due to the expected increase in capacity and an increased danand for new semices. This

interest motivates gmwing research on SS systems as a result of the critical d e that it

plays in the operation of SS systems.

W1th the dwelopment of wireiess and personal communication systans, there is a

considerable interest in using multiuser daection techniques and adaptive array

processing tedtnology to improve the system capacity m both present and fbture

genaation wireless systems. This thesis presents novel design ideas for DS-CDMA

systems which use a combination of adaptive muitiuser detection and adaptive array

pro-g*

Three novel adaptive cumbïned spacetime receiver structures are proposeci in this thesis.

These ncaVa structures jointiy combine the spatiai end temporal domain to improve the

o v e d system pafommce. These stmtmes are calexk 1) the linear adaptive combined

spawtime receiva, 2) the deesion feedback adeptive combined qace-time receiveq and

3) the centdimi decision feedback adapîive combined space-time reœiveL Analyticd

and simulaîion &ts are prescnted and perfiormances an cornparecl with the time oniy

adaptive mdtiuser structures. The anaiytical and shdatÎon r d t s show that a signifiant

impvement in c a p e and paformance can be achimd by atiliPng the adaptive

combineci space-time receivers.

The paform~pce of the adaptive space-time receiva is also studied Ïn fkequency selective

Êding charmek Analytid and simulation remlts show tbat the space-time stmture has

the ability to perfonn the task of the RAKE fecenfex It is found that the pmposed space-

time &ers -bines the fimction of multiusef detection, adaptin array pcessing, ISI

equaher and RAKE reception in a single structure.

The nomdized least mean squares (NLMS) algorithm is used to adapt the fiiter

coefficient. It is found that even though the adaptive spacetmie recdva has more

coefficients to adapt, its training paid is much less than that of the thne only multiuser

receiver and its training paiod l e s sensitive to the numba and power of intafems. Also

a shidy of the effect of thne varying user populations and packet transmission shows the

ability of the adaptive space-time stmctirres to hancile this kind of environment and makes

than a vay attractive soluîion for high capacity packet wireless systems.

I wish to express my sin- gratitude to my thesis supervisor, Professor DaGd D. Falconer, for his Wise advice, cumpeteat guidance, gentle encouragement and genuine fiendlines throughout my thesis research, His fine examples of professionsir and personal conduct have lefi and indelible impression not only on me but on evay body who has the opportmityto meet him.

1 aiso want to express my sin- gratitude to Professor Asnu Shiek for his help during my firstyearinthePh.D.program.

1 taanks Professor Roshdy Hafa for his help, valuable discussions, and cooperation dming my graduate studies at the department of Systems aad Cornputer Enpinesring at Carleton University.

I also wodd iike to express my thanks to my fiend, Dr. Mohamed G. Eltarhmi, for his patience while listening to my ideas and interestmg discussions that foIlows. Also 1 wodd iïke to thanks him forhis help durïngmy stay in Canada

1 thank ail the snrdent in the WCBS lab, previously called PCS Iab, for puthg up a nice workiag emimnment. Moreover, 1 am qprechtm of the fhculty and staff of Carleton University and the department ofsystems and Compuîer E n g i n d g who provideci much support through the period ofmy stady.

I am indebted to my parents, Munay Lepain and Salma Mobark, and my brothers and sisters, for th& steadnist love, support and encouragement, I dedicate this work to my fkther and mother.

Fmally and most importantly, 1 am also indebted to my Me, Hanan, and my cfiiIciren, Fatma, Asma, Murray, and Zakeriya, for th& love support and encouragement.

Table of Contents

List of Figares x

Chapter 1 Introduction 1

1.1 Background 1

1.2 Thesis Contri%utions 4

1.3 Thesis Structure 5

Chapter 2 Spread Spectram MuItipIe Access 7

2.1 ReIated Research on Spread Spectnmi Systems 7

2.2 The Proposed Structures 23

Chapter 3 Linear Adaptive Combined Space-Time Detector for CDMA Sys- tems 25

3.1 Signal and Channel Mo&l 25

3.2 Anaiysis of the Tap weights and the MMSE 28

3.2.1 InSynchnous CDMA Systenis 28

3.22 In Asynchuous CDMA Systems 34

3 3 Perfiormmce Eduafion of the Linear Adaptive Space-Tme Structure 39

3.4 The &ect ofthe d e r of mtenna eIements on the MMSE 40

3.5 Capacity CornpariSon 42

3.6 Near-Far ResiStaace 44

3.7 The eff'ect o f AOA on the MMSE 46

3.8 Conclusion 48

Chapter 4 Decision Feedback Adapüve Combined Space-Time Detector for CDMASystems 49

4.1 Introduction 49

4.2 Signai and Channe1 Model 49

43 TheProposed Structure 52

Chapter 5 Centralized Decision Feedback Adapüve Combined Space-Time Detector for CDMA Systems 64

5.1 Ijntroduction 64

5.2 Signal and Channel Model 68

53 The Proposeci Structure 71

5.4 NPmenCat R d t s 76

5.5 Conclusion 81

Chapter 6 Performance of the Adaptive Linear Space-Tlme Structure m Fre- quency Seleetive Fadmg ChuuieIs 82

6.1 Introduction 82

6.2 Channe1 mode1 including AOA 86

6.4 Analysis of the Lm= Structure in Ffe~uency Selective Fadmg Channe1 90

6 5 Channel Modd and System Parameters 95

6.5.1 Channel Mode1 Parameters 95

6.52 System Parameters 98

6.6 The e f f i of the number of fornard taps 98

6.7 The e f f ~ of the numkr of rrsolvable paîhs 100

6.8 The effect of the ntnnber of antexma elements 102

6.9 The effect of nmber of users. 104

6.10 Nem-Farresistance 104

6.11 The Outage probabiIity of the M E IO7

6-12 Conclusion 107

Chapter 7 Practîd Issues Relnted to the S p a ~ e - ~ m e Structures in Cellular CDMA Systems 116

7.1 Introduction 1 f 6

7.2 Transient behavior 1 17

75.1 Transient behavior anaiysis of the MSE 1 17

7.23 Amdyticalresults 118

723 The adaptive NLMS al go^^ 123

7.2.4 Remarks on the transient behavior 124

73 EEect of time varyhg user population and packet tmmhion 124

7.3.1 Simulation R d t s 125

73.2 Remarks on sudden birth and death 127

7.4 Effect of spteading codes on the @ormance 132

7.5 Out-off-cd interfiefence 136

7.6 Effect of Shadowing on the optase probability 149

7.7 BitenorrateresnIts 151

7.8 Conclusion 164

Chapter 8 Conclusion and fkture research 166

8.1 Conclusion 166

8 2 Future work 170

Refennces 172

List of Figures

Figœ(2.1): The block diagram of the çomentional recefYef.

Fig-(2.2): The block diagram of the optimum muitiuser &ver.

Fig42.3): The block diagram of the decomlating receivava.

Fig-(2.4): The block diagram of the MMSE receiver.

Fig-(2.5): The block diagram of the p d e I interference ~811ceIIation r e c h

Fig-Q.7): The block diagram of the adaptive hear rnultiuser receiver.

FigG8): The block diagram of the decision feedback multiuser feceiwx

Fig(2.9): The co11ventiond adaptive antenna array.

Fig43.1): CDMA CeiI Site.

Fig-(3.2): DS-CDMA trammitter and anternia elements stnicture.

Fig43.4): The &ect of 8nten.a elements on the MMSE.

Figœ(3.5): C$~)acity Cornparis~n.

Fig-(3.6): The Cfféct ofthe Near-Far probIem on the MMSE.

Figw(3.7): The effect of interferences wiîh same AOA as the desired user in synchronous CDMA system*

Fig.(4.2): The e f f i of the No. of antenna e1eaients and the No. of users on the hdMSE.

Fig.(4.3): Cap- cuwe fm the decision feed back combineci space-time stmture

Fig.(4.4): Capacity compWson betweea the pposed mctares whae AOA of the various ISI components are assumed to be widely disûi'buted,

Fig.(4.5): Capacity cornparison betwem the pposed structures where AOA of the various ISI components are assumed to be the same.

Fig.(5.1): Decorrelating Decision Feedback Mdtiuser Detector [12].

Fig.(5.2): Adqtive CenttatXzed Decision Feedback M u i h e r Detector, [87l.

Fig.(53): Cenîdhd Decision Feedback Adaptive Combineci Space'ïime Receiver.

Fig.(5.4): The effect of the No. of antenna elements and the No. of users on the MMSE.

Fig.(5.5): Capacity curve for the proposai stmctme.

FÏg.(5.6): Capacity camparison between the proposed structures.

Fig-(6.1): Multipath Propagation in ceUuiar systems.

Fig-(62): Mpltipath Propagation Model.

Fig.(63): Channel mode1 used in the simulation.

Fig-(6.4): The e f f i of number of f o m d tabs on the MMSE

Fig.(6.5): The efféct of number of resolvable paths on the MMSE.

Fig.(6.6): The efféct of number of elements on the MMSE.

FÏg.(6.7): Capacity compsrison.

Fig.(6.8): The e&ct of near-far problem.

Fig.(6.9): Outage pbability on the MMSE P(MMSD-6 dB).

Fig.(6.10): OiItage probabiiity on the MMSE P(MMSE>-IO dB).

Fig(6.11): ûutage probability on the MMSE P(MMSE>-14 dB).

Fig.(6.12): m e probability on the MMSE P(MMSE> -1 8 dB).

Fig.(6.13): Cumdative distnLbution hction ofthe MMSE with 2 usas.

Fig.(6.14): CumuMve distnion hction of the MMSE with 4 mers.

Fig.(6..15): Cumulative distriMon hction of the MMSE with 14 users.

Fig47.1): Transierit behavior cwes of the space-thne and thne ody receivers.

Figœ(7.2): Effect of the stepsize on the training m e .

Figœ(7.6): Efféct of packet tmsmMon.

Fig47.7): Ramp powa aocss technique vs. fblI power access for the Iinear space-time receiver.

Figo(7.8): Effect of the spreahg codes on the pafo~mance of the hear MMSE receiver (time ody receiver).

Fig-(7.9): E f f i of the spreading codes on the performance of the Iinear spacetime receiver.

Fig-(7.10): Effcct of spnading code selection on the outage probability of the lin- spacetime receiver.

Fig-(7.11): E f f i ofout-ofail interfierence with same spreading code as the deSned usa on the hear MMSE &er (time only &).

Figœ(7.12): Effect of out-of~ceu interfixeme *th same spnadmg code as the desired user on the iinear MMSE receiver ( the only rrceiver).

Fig-(7.13): E f f i of out-of-ceU intdefence with same spnading code as the desired usa on the hear spacetime receim.

Fig-(7.14): E f f i ofout-ofell mtderetlœ with seme sprradmg cade as the d e e d usa on the linear spacetime recenfer.

Fig-(7.15): Effect of out-ofkeli interference with seme spnading code as the desireci user on the linear MMSE receiver (time oniy reCmra).

Fig47.16): Wect ofout-ofell intdeteace wita slmie spreading code as the desired usa on the hear MMSE nxeher (time only receïm).

Fig(7.17): E&ct ofouî-ofkeU hiderence with same spreading code as the desircd user on the

Fig47.18): Efféa of out-ofaii interfice with same spreaduig code as the desired usa on the I inearspacet ime~ .

Fig-(7.19): Effect of out-of-ceIl intderez~ce with same spreading code as the desired user and di5 fermt powcr.

Fig(7.20): E f f i of out-of-celi interfimalce with saxne spreading code as the desired user and dif- faaa power.

Fig-(7.21): Effect of Wowing and distance rclated attenuation on the MSE.

Fig-(7.22): E f f i afshnAowing and distance related attendon on the hem space-tirne receiver.

Figœ(723): Outage probability of the me in shftnowing enviroxxnent

Figœ(7.24): Outage probability of theMSE in shadowing environment

Figœ(725): Capacity camparison, out-of fd interference is modeled as gaussian random vari- able Norma1(0247Ns, 0.078Ns).

Fig4726): Outage pmbability of the MSE where outsff-tell interference is modeled as gaussian random miable NormaI(O247Ns, 0.078Ns).

Figœ(7.27): Bit aror rate performance ofthe hear space-time receiver.

Fig47.28): ûutage pmbability of X=10-2.

Fig-(7.3 1): Outage pmbability of X=10-5.

List of Abbreviations

AAA . . . . . . . Adaptnre Antenna Array

AAP Aàaptive Array Proeesshg

AOA Angle Of Arrinl

AWGN ...................... Additive White Gaussian Noise

BER ......ee....HH.W....H-.. Bi Emr Rate

BPSK . - - Binarg Phase Sbln Keying

CDMA - e - . . . a . . . . Code Division Multiple Access

CMF .--....e--..O, Ch@ Matched Fier

dB -.rm....mH...................... deci i l

DFE a.o--*-*-o.aawe.a-o Decision Feedback Equalizer

DSCDMA - - Direct Sequence - Code Division Maitrp1.e Access

DSlSS Y , . , . Direct Seqaence I Spread SpectrPm

PDMA - - , . Frequeney Division MuiüpIe Aceess

FHBS -........ Fnquency Hopphig I S p r d Spictriun

FIR . F i i Impdse Response

FPA --..-.-.....,....-.... FdI Power Access

ISI . Intenymbol Interference

LMS e . LeastMeanSqaatre

MAI Miiltiple Acœs Interference

MMSE .-------MinimpmMcrnSqiureEmr

MSE . . Mean Sqiinre Error

MUD ..........-.~..- Mdti-User Detection

NFP .,....,,,,,,,,-----Y Near Far Pmblem

NLhiS . . NolZIIlljjZed Least Mean Sqgare

PG . . Proassmg Gain

PIC ...---YIY.......---... P d d Interference Cancektion

PN ...........,.----Y-Y--...- Pseud~~Random Noise

RPA . . Ramp Power Access

SIC ,,-,..............,,- Successive Interfereace Canceiiation

SIR ,..............., Signal to Interfierence Ratio

SNR , . Signil to Noise Ratio

ss ".,,*........",."' Spread speetnim

TDMA - , Tiine Division Mtiltiple Access

Chapter 1

Introduction

The danaed on wireless communication due to freedom in

location and time has increesed recently CochanneI intdefexlce due to this rapid i n m e

in danand on wire1ess communication becomes a major prob1em. As the dernand on

wireless communication continues to grow, and due to the E t e d available spectrum, the

need for the dedopment of an efficient transmission and muitipIe access techniques to

dciently utilia the spectrum, cancel cochsuuie1 interference, and i n m e the system

capacity recentiy became a mearch cfiallenge. Code division muItipIe access (CDMA),

tEne division multipIe a m (TDMA), and Eecpency division multip1e access (FDMA)

are lmown multiple access techniques that allow many users to share the same wire1ess

channeIo

1.1 Background

In CDMA multiple users are ailowed to share the same bandwidth ova a common channe[

by using a pre-essigned code for each usa. In TDMA each usa is assi@ a tene slot to

conmiecate tbrough. In FDMA each user is @en a certain fiequency channe1. TDMA

and FDMA tend to be bmdwidth limited techniques, while CDMA tends to be interference

hnïted. Glhousen a ai prove in [23] that CDMA offas large capacity hcrease over the

TDMA and FDMA by exploithg the rmiitipath remlution, voice activity, antexma

sectonzation and robustness to mterference.

Frequency hoppïng spread spectmm (M-SS) and Direct secpence spread spectnna @S-

SS) sre the two main techniques used in CDMA. In direct sequence d e division multipIe

access (DS-CDMA) the transmitted data is multiplied by a passigned code to spmd the

tmdtkû en- over a wider bmdwidth. This pre-assignecl code has higha rate than the

fnmmmitted data The same assigneci code wiîl be used at the recehr to recover the

traosmitted data. In fkquency hopping code W o n mdtipIe access (nr-CDMA) the

transmitted carrk fiequency is h g e d accordhg to a specinc code sequence. DS-CDMA

is the most commonly used techique in CDMA systems.

Wmless communication c)ianneis d e r h m many impairments which place fimdamental

limitations on the pediomance of the wireIess systems. Fadiag, multipath, and cochame1

interference are the main impainnents in Wireless channek Multipath arisa h m

scattering, reflection, and m o n of the transmitted signai of objectr that lie in the area

between the transmitting and receiving antmnas. Multipath causes intersymbol

interference @SI) in the receNed s ip i . The severity of the ISI depends on the deIay spread

ofthe chmneI. Fading describes the continuous change of the powa of the received signal

due to motion in combinaton with muitipath - induced signai stmgth variation with

location. The fading could be slow or fast depending on the Doppler spread of the channei,

where the Doppler spread depends on the velocity of the mobile or reflecting objects.

Cochanue1 i n t e r f i c e in CDMA cornes h m usas in the same ceU and h m adjacent

ceiis who use the same bandwidth but diffetent codes.

CDMA provides implicit diverSity protection agaht fiequency selactive niding because

different portions of the signal bandwdth fade independentlly. This protedon is achieved

by using the RAKE nceiver to combine the resolvable multipath signal camponmts.

In DS-CDMA, as we expIained before, the transmitted data is multipiied by a pnsssigned

code to spread the data over a wida bandwidth, A decoder which detects the signal by

passing it tbrough a matdieci tiiter, matched to the desmd u s a code, and a decision device

is h m as the comentionaI Smgie user detector or the comrmtionaI receiver-

A major problem with the m t i o n a l receiver is the near-fàr pmblem, whae nearby

strong osas dominate the transmission. As a resdt the feceiver f8üs to detect the si@

h m the fàr weak uset This probIem is due to the multiple access interference (&U.Q

where the comentiond receiver ignores or d e s no effort to estimate the MAI when

detecting the desirrd user. A practical solution to the near far problem is the use of power

contd to keep a i I users with the same power at the receiver.

MuIti-user detection is another solution to the narr fin ptoblem. Recently interest in

improving the DS-CDMA pafommce against the near far problem has increased, In

multi-user detection the Monnation of aU user's MAI are used to detect each user. Verdu in

[103] and [l W] deàves the optimum rndtiuser detector for DS-CDMA systcms, and shows

that tht near fiP problem is not an inherent probIem for DS-CDMA, but is due to the

inabiiity of the comentional receEver to exploit the MAI dimension. ûtha mdtiuser

detectors are proposecl in the literature which we wil l discuss in more detail in chapter 2.

Adaptive array processing and miart mtennas, is another class of useful techniques to

mitigate the cochannel interference, kdhg, and multipath in the wireless charme1 1701

[113]. The adaptive array can =ce1 the cochmmeI intaference by placing nulls in the

antenna beam pattern in the direction of the interfice. It dm can combine the muitipath

components of the desmd user by placing beams in the anteana becmi pattern in the

direction of that paths, R d î s reported in [q and [79] show that evai a reduction in the

&ect ofdelay sprred can be achieved by employing the adaptfve array proCeSSi11g.

Adaptive antenna awys may be combineci with a xnultiuser detector or interference

canceIIer to enhance the pe&ormrulce of the wkeiess cornmUIlZcaîion system. An adaptive

antenna smiy in combinaton with an interference cancelIer for CDMA systems is shown

subsequently to have improved @ormance and increased capacity that can't be achieved

by each technique done. frobIans ofthe comrentional aatama anay and the comentional

d e r such as low signal to interference ratio, the near fhr problem, and intafaaice with

same angle of amval (AOA) as the desireci iisa can be solved by the combined techniqyes.

In this thesis we focus on combineci adaptive apay antenna and eqtializaton to inmase the

wireless CDMA systems capacity and improve the paformance.

1.2 Thesis Contributions

We have d e s e v d contnLbutions in this thesis to the body of digital communication

fiterature. These conbnibutions are:

1) The development of the adaptive combined bear space-time receiver for CDMA.

-Analysis of the optimum tap weights and the MMSE in synchronous and

asynchronous CDMA systems.

- Closed form expressions for the auto-correiation matrix and the cross-conelation

vector.

- CornpanSon study between the Iinear space-tirne receEva and the time ody receiver

in synchronous and asynchronous CDMA systems.

2) The development of the adaptive combined decision feedback space-tirne receiver.

- AnaIysis ofthe optimum tap weights and the MMSE in MAI and ISI channels.

- Closed form expressions for the auto-c~felation ma& and the cross-correlation

vector.

- Cornparison shuiy between the kear and decision feedback structures in MAI, ISI,

and AWGN channeis

3) The deveIopment of the adaptive combîned centralized decision feedback receiver.

- Analysis of the optimum tap weights and the MMSE in MAI and ISI channeis.

- Ciosed form expressions for the auto-correlation matcix and the cross-correlation

vector.

- Comparison study between the lin= and decision feedback structures in MAI, ISI,

and AWGN channeis

4) Design and anaiysis of the lin- space-tmie receiver m âequency selective fadTng

chluinel,

- Analysis ofthe optham tap weights and the MMSE.

- Ciosed form expressions for the mto-cmelation mat& anci the cross-correlation

vectar.

- Comparison saidy between the h e m space&me structure and the time only structure

5) Studying some practical issues related to the space-the sûuctms in cellular Wgeless

CDMA system

- Studying the transient behavior of the linear sp8ce-the receiver

- Shidying the e f f i oftime m g user popdation

- Studying the e E i of packet trammm 0 *

on

- Studying the e f f i of out-offd interf ice with the same spreading code as the

desired user

- Studying the effect of out-oEcell interference and shadowing on the outage

probability

S e v d of the main ideas of this work have already been published in several papas. The

Adeptive linear combined space-tlme receiver is published in [54]. The Adaptive decision

f i a & combhed space-time structure is published in 1501. In [49] we present the

Adaptive centralized decision feedback combined spacatime receiver. The performance of

the hem adaptive combined space-time receiver in fnsuency selective fading chamel is

published in [SI]. The effect of t h e v-g user population and packet tnuisnission is

presented in [SOI. The transient behavior of the iinear adaptive cornbineci space-time

receiver is pubiished m [Ml. An inviteci tutonal papa about space-the prucessing is

published in [17].

13 Thesis Structure

ûverview of recent research in the area of muitiuser detection, adaptive array processing,

and combhed spatial and temporal techniques for DS-CDMA systems are &en in chapter

2. In chapter 3 we develop the Adaptive iinear comhed spacetime structure. Adysis of

the M E end the optEnm tap weights for the linear structure is also given in chapter 3.

Ui chaptex 4 we propose and analyze the adaptive d&on feedback combined space-thne

meha, A camparison study between the lin- and the decision f d a c k structures is

aiso given m chapter 4. The adaptive cmîdhd decision feedback receiver is proposed in

chapter 5. Adysis ofthe optimum tap weights and the MMSE of the wntdzed structure

is preseated in chapter 5. A cornparison study among the lin=, decision feedback, and the

centdized shuctraes m MAI and ISI charme1s is also @en in chapter 5.

-ter 6 is devoted to the fkquenq selective fbding channeis. In chapta 6 we study the

performance of the Iinear space-time structure in fieqyency selective fading channeis. In

chapter 7 we shidy some practicd issues reIated to the space-the structures in cellular

wireless CDMA systems

Chapter 2

Spread Spectrum Multiple Access

For swaai decades spread spectnim communication was

considered oniy for milita^^ applications. The neson is for its ability to deal with anti-

jamming and low probability of intercept problems which have a major concem in military

communication [92]. Recently, spread spectnrm communication has been shown to have

many mteresting applications in civili811, comm&cation systems. Spread spectmm is one

of the shongest candidats for ceiiuiar comxnunication systems and third generation

WUeIess communication [83, [7q, [77], [108].

2.1 Related Research on Spread Spectrnm Systems

Conventional Receiver A b d of K comentiod recehen for Rcaving each of K intafixing DS-CDMA si@s

is shown in Fig-(2.l), and it consists of a bank of matched fhrs foiiowed by decision

devices. Due to the cross..cone1ation between the spreading codes and since the conven-

tional &ver d e s no effort to estimate the mdtip1e access interfierence (MAI), cochan-

ne1 interference Win affect the paformance of the comnntional receiva if the spreachg

gain is N, and there are K equal power mterfering CDMA sigds with different codes, the

signal to total interfmce ratio at the input is rouptiiy N K Moreoveq if the received

powa of the Mirent usas are not the same, the strongest risa will daminate the trans-

mission, and the coment io I receiver wiii faü to detect the si& for the weakest users.

This is hown as the neapfin prob1em. The oniy practical solution to this problem is the

use of power controL Also by designhg spreading codes with good cross-comIation

properties and the use ofpowerfid forward em>r correction codes the problem ofnem-far

reception can be mmmi;7lri

Optimum MPltLUser Deteetor Verdu shows that the ncsr fae problem Îs not an inherent problem for the CDMA systems,

but is due to the inability of the comrentio~ meiver to exploit the MAI dimension, In

[1û4], and [IO51 he dweloped the muitiuser maximum Iikelihood detector, or sirnply the

optimum mdtiusa detector. It is optimurn m the sense thaî it minimi;rres the pmbrobaty of

error. The optimum recenfer is shown in Figœ(2.2) and it consists of a bank of matched fil-

ters foUowed by a mdti-uset Viterbi processur. Vdu shows that there U no p d o m c e

degradation due to the MAI. Udortunately the optimum muitiuser detector has complex-

ity that gmws eqonentially 6th the number of usas and it is not practical for cellular

systems where the nimiber of users is hi& Another probIem of the optimum multiuser

detectors that it requins the knowledge of the received signals powers, delays, and the

spteadmg codes for di asas.

Suboptimum Mnlti-User Detector Due to the complexity of the optimum receiver and the near fer problem of the cornen-

tional receiver maay mearchers sought a suboptimal receiver that has paforrnance com-

parable to the optimum receiver Va complexity does not depends on the nimiber of users

or grows linearly with the nimiber of users. In the following paragraphs we wiU survey

some of the suboptmial rnultniser detectors fomd in the üterahm.

Linear MuIti-User Detector One of the important multiaser is the deconelaihg receiver which is shown in

Fig-(23). It was proposeci in 1591 end 1601. The deco~elaîhg receiver applies the imrerse

of the correlation matrix R of the sprPadmg codes to the output of the bank of matched fil-

te=,

ReceEved signal I Matchad Filter for

p s a K

\ Sampling rate = bit rate

FiH2.1) The block diagram of the conventional menter.

Received signai

Matchai Filta for

osa1 - Matdzai Filter for

usa2

Sampling rate = bit rate

Fig(2.2) The block dhgmm of the optimum multmser receiver.

whexe h the case of synhnous mterference,

and

where Tb is the symbol period, N is the n u m k of usas, ci(t) is the spreading code of the

ith , The decomlating receiver comple&eiy eImiinates the MAI and it is analogous to the zen,

forcing equalwr used to eiiminate the intersymbol intaference (ISI). It is fond that the

decorrelating &ver has many interesting properties such as: 1) signifiant capacity gain

over the commtiond feceiver; 2) it is not requïred to estimate the energy of the users; 3)

its complexity is Iinear with the n m b a of usas, so it is less cornplex than the optimum

muitiuser receivex The drawback of the decodahg &ver is that thae is a noise

enhancement at the output in a smDilar way as the zero forcing equaüza does.

The minixntmi mean square error (MMSE) muitiuser detexmr is another cornmon

approach which takes into account the background noise. This type of detector was pro-

posed in [6 11, [62], and [I 141. This stmctme applies the hear traasfonmiion, T, to the

output of a b m k of matched nIters, see Fig-(2.4), where in the synchnous case T is equal

to

descision block

Received signal 8 I I I

Matchcd FiItcl

for user K

t \ Sarnpiing rate = bit rate

F%(2.3) The bluck diagram of the deeorrelatmg receive~;

decision block

Received signal

Matchcd Filter

l for usa 3

I I I I I

- Smpfhg rate = bit rate

1

Fi2 .4 ) The block diagram of the MMSE receiver.

Matched Filter

for user K I

where R is the cross-correMon matrix @en in Eq.(2.1), A is a diagonal matrix contain-

hg the received amplitudes ofthe users, and - is the two sided powa spectral density (3 of the background noise. As an be seen, the MMSE detector takes into account the back-

ground noise. If the background noise goes to zero the MMSE detector wiil converge to

the deconelating receiver. The MMSE multiuser detector is anaiogous to the MMSE

equalizer used to combat the ISI. From the Iinear sansformation T the MMSE multniser

d-or rquires estimation of the recéived amplitudes of the aü usas. Also its perfor-

mance depends on the power of the MAL Thaefore, comparai to the decorrelating

receiver that is some l o s in the &stance to the near far problem. Howewer the important

advantages of the MMSE muitiuser detedor is the adaptive implanentation of this detec-

Subtractive Interference Canceiiation Subtractive interference multiuser detectors are another important group of muitiuser

detectors. They are classined as successive interference canceilation and parailel interfia-

ence caceflation.

Sliccessive Interference Canceilation (SIC) In successive interference canceiiation techniques [4q, a conventionai &ver detects the

strongest interfêrence, makm hard data decisions, respreads the data, and caricels it h m

the received signal. This process is repeateâ mtil di intaferers are canceUed The p b -

Iems with this muitiuser detector are: 1) the strongest usa wiU not bene& h m the MAI

cancellaîion; 2) The sigds should be ordmd in a descendmg orda accordhg to their

received powers, so any change in the MAI powa profle req@res reordering the signais

again; 3) one bit delay is required per stage; 4) if the fmt stage estimate is not reliable,

then the powa ofthe interférence will quadruple.

ParaIlel interference cancellation (PIC) Parde1 interference canceIIation (PIC) was proposed in [451, [q, [lW] and [IO11 where ail the MAI for each usa art estimated and subtracted in p8f8Uel. The first stage of the

PIC

. -

-

I

LStage O - b Stage 1 'tL

Fi24 The bloelr &gram of the parailel mteHerence crneehtion receiver.

h )( Delay Tb Delay Tb Delay Tb

Stage

n

muitiwer detector is shown in Fig-(2.5) and Fig-(2.6). Tht outptrts of a ban, of matched

Bters are passeci thmugh a hard decision deviœ to get an estbmte of the detected bits,

these bits are multipliexi by the estimate of the amplitnde of the received signais, respread

by the spreading codes for each user, and subtracted âom the received signal. A good

mriew of the muitiuser detection can be foimd in 1141, [47], 1711, [105'Jand [107].

Adaptive Muiti-User Deteetor nie opthmm and the suboptirum mdtiuser detectors discussed above generaiiy lequire

side informâtion such as the spreeduig codes, powers, and delays for each user, which

mcrease the complaaty of the muitiuser detectors. Firrthermore, if they operate in multi-

peth cbannels the lmowlcdge of each path gain and delay is also reqiiireb

Adaptive stmtures for muitiuser detection which do not reqaire explicit knowledge of any

of the variables mmtioned above are proposed in [l], [64], and [87$ Even the spreading

code for the desired user is not mpired. The only nquirement is that the spreedaig code

for each usa shouid be repeated each transmi#ed bit The adaptive naturr for these struc-

tures recpires ovahead bits to train the coefficients. These detectors have many interesthg

featrrns besides not nquiring side mformation. They havt much better pafonnance than

the comrentional receiver, and are much less cornplex than the optimum receiver. The5

complexïty does not depend on the number of usas. Even more, [Il and [64] showed that

these structures combmc the hction of RAKE reception and MAI cancedlation. Also

these detectors are found to be near far resistant, so they do not require strict power con-

m l . The SHnpEcity of these structures make them a#ractive not only for the base station

but for the mobile too. The structure in Figo(2.7) is d e d adaptive hear multiuser detec-

ter. and the stnictnre in Fig-(2.8) is d e d adaptive decision feedback multiuser detector.

These detectors are pmposed in [l] and [64]. Another structure proposeci În [87] is calied

centrdized adaptive decision feedback mdtiuser daector, where the receiver has access to

the detected symhls of aii usas.

The adaptive multiuser detectors discussed above recph transmission of a seqyence of

training bits to adapt the receiver co&cients, and the spreading code shodd be repeated

eech bit (short code).

Fïg(2.7) The bloeL diagram of the ulnpüve lia- muitiuser receiver.

Fi-(2.8) The block diagram of the decision feedback mdtiuser meiver,

Smct in CDMA users staa and end th& tnmsmission mdomiy, the training opedon of

the adaptive mdtiuser detection may be a dif]ti:cuIt task in such a emrironment, because the

detecbr needs to be continuousIy trained es new intaferas enta and old interfi- Ieave

the systern at randorn tintesC Honig et al [32] propose a blind multiuser detector that does

not req@re a training sequence- Study of the convergence behavior of the LMS adaptive

muitiuser detector is done in [65] and [55]. Review of ment metuch on adaptive mul-

tniser detection couid be fomd in [106] and the neferences therein.

Adaptive Antenna Array (AAA)

AU the adaptive and non adaptive structures mentioned above can be considemi as tempo-

ral filtering- Firrther increase in the abiiity of any d v e r to cane1 cochannel interference

can be achieved by the exploitaîion of the spatial domia. Adaptive array processing can

d i s c r b b k among users according to th& gecmietticai location or theh angie of amval

(AOA) to the receMng antenna. Adaptive may processing is useful in canceling cuchan-

ne1 intderence by placing nuiis in the anterma btem pattem in the direction of that inta-

ference* [q, [26], [29], [37l, [70] and [79] are good fere en ces in adaptive array

processing. The comentiod adaptive antema m y structure is shown in Fig(2.9).

Adaptive Antenna Array in wire1ess Systems

In [I 121 Wmters studies the use of adaptive antenna a m y (wiîh optimum combining) to

innease the capacity of radio CornmutLication systems with multipIe usas. In [II21 Wm-

ters used space diversity not ody to combat RayIeigh fading of the desireci si@ but dso

to d u c e the power of the c o c ~ e I interfefence at the feceiver. By optimum combining,

the reçeived signai at the antenuas are weighted and combined to maxhhe the output sig-

nai to interference plus noise &o. In [log] Wmters studies the use of optimum cambining

for indoor radio systems with multipIe users. R d t s show tbat the system with a L-de-

ment adaptive recehhg antenna can achieve up to a L-foId increase in the number o f

users- [42] stadys the capacity improvement ushg antenna axray in third generation

CDMAwireIess systems.

0 is angle of a d (AOA) of s(t)

Fii(23) The conventionaï adaptive antema array.

Combmed Techniques Combination of the spatial Qmam (adaptive array processing) and the temporal domain

(mulher detection) can f i d e r mcrease the capacity of the CDMA ceîidar system and

enbance the interférence cancdation capabilities of the CDMA muitiuser detectors.

Combmed Diversity & EquaIization For non CDMA systems the wodr in [67], [68], 1691, [99] and 1581 combine diversity (va-

tid) and cqualization to combat cochame1 interference and fhding. Faiconer et al in [18]

swey recent advances in equalization and diversity combinhg in digital portable wireless

systems. For CMDA systems the combination of diversity and DFE aqualuaton is pro-

posed in [9q.

Combined AAA & Conventional CDMA Receiver In CDMA systems the total power of interference and noise is much larger than the

desired user's power at the chip levet The prob1em with conventional beam fomiing

(adaptive antenna anay) is that it can't cornrage in a situation where the signal to interfer-

ence plus noise ratio is small. Kohno in [44] cambined the adaptive awy processing and

the mmentioriai r&er. He uses the inherent prucessing gain for the CDMA system to

update the weights ofthe adapadaptive antenna array. The structure @es some improvement in

capacity but has m q disadvantages. Due to the use of a coxxventiond receiver this struc-

ture is still not nm-fb resis&nt, A h ifthae is a high htmfkence level and many usas

have the same angle of amval (AOA) as the desired us- the adaptive antenna array will

not be effective in separating interferers.

Combined AAA & Subtractive Interference CanceIlation To solve these problems Ohno proposed in [q a combination of an adaptive array and a

mulîher in ter f ice canceller. Kohno fomd that this system can achieve high pafor-

mance and stable acquisition in heavy M c or when many users have the same AOA as

the desireci usert where the conventional antama array can't However pafonnmce of

both structures, [44] and [q, can be degded by the delay in the f d a c k control bop.

Aiso the structure pmposed m [46] nquires meny adapiations and the cenceIIer ne& to

be adapted first before the anay adapts. Moreova. in wireless cellular systems where the

namber of mterferers is in the order of 60 or more the Kohno stmchm is not usefd.

Beause the antenna enay can canceI only L-1 h k r f i e ~ e f ~ in non mPltipath and fiirling

channels and even fewer than L-1 in multipaîh and fiâhg, where L is the nurnber of

antenna elancnts. A review of his work can be foimd in [47].

2.2 The Proposed Structures

From the above m e y of ment research in Mdti-User Detection, AAA, and Combmed

Space-Timie filtering, we introduœ novel strtlchnes that jointly combine the spatial and the

temporal domains to cancel cochannel interference and inmase the o v d capacity of the

CDMA systern.The proposed structures can be seen as extensions of the structures

proposed in [l], [64, and [87] to the spatial domain

h this thesis we propose a joint combination of AAA and adaptive multi-user structure.

AAA is used in this thesis to stea beams toward the desirrd user and his multipath corn-

ponmts, and place nuiis in the antenna beam pattern in the directions of interference to

reduce or eliminate their eff'ects.

In this thesis we introduce three novel structures for CDMA systems. The k t structure is

calleci the lin= adaptive combined space-time receiver, the secwd structure îs d e d the

decision feedback adaptive combined space-time d e r * and the third structure is d e d

the centralized decision feedback adeptive combhed space-time receiver. In ail structures

a L-dement linear array is used. Each element is foiiowed by a chip metched filter and a

FTR ater pcessing sampks at the chip rate. Where in the docision feedback adaptive

spacbtime receiver another FIR filter is used to feed back the detected symbols for the

desirrd wr to improw the perhmance. In the centralized stmctme the detected symbols

h m ail wers are fed back through a FIR filter to hprove the pafonnance. Detailed

description and d y s i s of ail structures are the subject ofthe thesis.

The proposed sarrctines have m n y interestMg f- as follows: (1) Large kcrease in

capacity compared to the conventional Rceiver and the dctcctors pmposed in [1] and [Ml; (2) Na-fkr &stance with no need for strict powa c0ntmI; (3) No side information is

reqriired such as deIays, powers, spnading codes, and chme1 impuIse responses. Even

the d e for the desired user is not needed The only informafion needed is the &lay of the

desired user and a trauiing sequence; (4) Compkity does not depend on the numba of

mers, so it is much l e s cornplex than the optimum mdti-usa d m , (5) It ain adapt in

heavy trafnc conditions with s d users hrrving the same angle of amval (AOA) as the

desired user, whiîe the conventional adaptive anay câtl't; (6) It jointly adapts the anay and

the detector. So, it solvs the pmblem of adapting the array and the canceIla separafe1y,

which is mentioned in [q; (7) Combines the fimction of multiuser detection, adaptive

antenna anay, MICE reception, and ISI eqyaher in a single structure-

Also considering the compkxity of our proposed structures we fond that it is comparable

to that of the comentional receiver with antenna array. Considering only the forward tap

coefficients, and assuming the length of each forward filter is N (the spreading gain) we

see that the total nmnber of multiplications per output bit is L W , which is the same as that

of a coherent conventional recelver in wwhich thae is a correlator at the output of each of

the L elanents. The main ciifference is that the mdtipIications in our receiver are cornplex,

while those of the comentiotlIiI receiver involve multiplying element outputs by chip val-

ues ( ti ), and our receiver r a p h s an adaptation algorithm.

Chapter 3

Linear Adaptive Combined Space- Time Detector for CDMA Systems

In uiis chapter the iinear adaptive combinecl space-time detector

for CDMA systans are descriied and discussed in more d d . FVst Section 3.1 describes

the received si@ and channe1 mode1 for the proposed sûuctare. In section 3.1 the signal

and channe1 mode1 for DS-CDMA system is described. In section 3 2 the block diagram of

the Linear Space-Time Receiver is givm and descri'bed The analysis of the optimum tap

weights and the MMSE in synchronous and asynchronous CDMA systems is given in sec-

tions 3.2.1 and 32.2. In sections 33 to 3.7 we d u a t e the performance of the hear

space-time structure in synchronous and asynchronous CDMA systems. Finally section

3.8 concludes the chapter.

3.1 Signal and Channel Mode1

Consida a CDMA basestation d e r with K users as shown m Fig-(3.1). The signal

transrmitted by the kthusa is denoted by Sk(t) , where k = 1,2, ..., K. Assume that

SI ( t ) is the signal for the desùed usa, and Sk(t), k = 5 3, ..., K are considemi as CO-

channel interference with respezt to the desired user. Each received signal cornes h m a

certain direction depending on the location ofeach user, see Fig43.2). We assimie that the

direction of a d (0k) is unSOtmly dishiuted in [-x/2, x / 2 ] . In this anaiysis we

assume two cases, the synchronous and the asynchronous mdtiusa channe1 to see the

resktance of our structures agah t the MAI and nefàr problem.

The receiver uses a L-elment hear d o m auay with inter-eIement distance separation

d. The received signai by the fh antenna elment is

where the cornplex baseband recebed si@ bxn the kth user is

in these equations, Tb is the symbol period, bLis the data symbol transmitted by user k in

the ith htervai, pk is the received power of the kth user, Nl(t) is a zero mean cornplex

ganssian random variab1e with power s p d density No at the antenna e h - 4 is the delay for the kth usa, we assumeci that t 1 = O for the desired usa, q ( t ) is the code

sequence of the kth usq whae

ak(m) E (1, -1) is the rnd chip of the kth user, and ' is the chip paiod. n(r) is a unit

rectangaiar pulse of duration Tc , Tc = Q I N is the chip perioâ, N is the processing

gain, AI(ek) is the steerhg coefficient of the kth usa at the lth ankana elment, whae

User k bransniitîer

Fw3.1) CDMA CeJi Site.

Fi33) DSCDMA transmîtter and rntenna elewnts stroctare

Where L is numba of elements, h is the wave Iength, I is the element nrmaber. @ is the

angle of atxival (AOA) of the kth usa with respect to the a m y normal.

3.2 Anaiysis of the Tap weights and the MMSE

The receiver structure is show in Fig-(33). The structure irtilizes a L-elemmt hear uni-

fonn array mtenna with elmient s e p d o n d. Each elanait is foilowed by a chip rnatched

filter (CMF) whose output is sampIed at the chip rate, and is pas& through a FIR nIta

which has a delay for each tap equal to the chip @od and the nmber of taps equd to the

processing gainM AU the L-adaptive FIR filter outputs are summed and sampIed at the bit

rate to fonn a soft decision output which is passeci through a decision device to fonn an

estanate of tht transmitted bits. The structure in Fig-(3.3) is called Iinear adaptive corn-

bined space-the muiti-usa dctector* Tht andysis of this structure is &en in the next sec-

tion.

3.2.1 In Synchronons CDMA Systems

In syncbronous CDMA systems whae al1 the spreading codes are synchronous zk = O h for ail users. The si& &ed by the If antenna element, x l ( t ) , is passed through a

h CMF and sampled at the chip rate* Then the output h m the CMF fkom the f mtenna

(a) ImpIemented with FIR filters

(b) Implemented with corzelator

F' ï33) Linear Adaptive Spaee-Tme Receiver.

= O for di k in the synchxr,nous CDMA system. Ako we assume that there is no

muütpath or Ming- If we sampte ul(t) at the chip rate and take N samples each symbol h intaval then at the ith symbol interval the content of the equdker at the f antenna ele-

ment is

th where c is the code vector for the R usa, whae k

and the superscript Tdenotes transposition, Now we can do the anaiysis in matRx and vec-

tor fomt, We wil l drop the superscript i fkom our equations, since it will not aècî our

analysis, and de.ûning,

w h m r isaKxNmatrix,end rk fork = 1,2, ..., K isthereceivedN-dmiensionalsignd

vector h m the kth user and is equal to

Let

be a R'ensiod vector containing the steering coefficients for the K users at the Ph

element Then the content of the FIR fùta of the lth antenna elanmt in vector fonn is

Let

w h wI is the N-dimensiod weight vector for the FIR filter of the antenna eIement.

Now we can define the contents of ail the FIR flters and th& tap weights in a vcctor f o m ~

Let

where W is a LN-dimensional vector which contains all the FIR filtas coefficients. Let

be a LN-ensional vector contaenng the contents of ai l the F R filters*

Now afta defining the vectors Wend Uwe can caidate the optimum tap weights and the

minimum main square mor (MMSE) of our stmctms.

The output of the structure shown in Fig-(3.3) is

MSE = 1- wHp-PHw+ WHRW (3.17)

where the supascript H denotes the Hermitian transposition, R is the autocorreiation

ma& of the input v e r U and is quai to,

and P is the cross-cordation vector between the input vector U and the desired output

b 1 ,where b 1 denotes the data h m the fmt usa,

P = ~ ( ~ b l ) (3.19)

To fmd the optimum coefncients, Wopt , h m the Weiner-Hopf solution

siibstituîe W 0 p mto Eq43.17) to nnd the minimum MSE (MMSE), then

In order to h d Wopt and the MMSE we need to caIculate R and P which are shown

~ [u;

whexe, d g the noise sequence is uncomlated,

and

where

In the above analysis we here found the MMSE and the optimum taps, Wopt , for a linear

space-time receiva as a fiinction of R and P in synchronous CDMA system, where R and

P depend on the nll~flber of us- and antema elements. From these r d t s we will see

later in this chapter the effe* of the number of users, and numba of antema elements on

the performance of ora proposed stnichrre in synchronous CDMA system. We wii i repeat

the same analysis in asyncfuonous CDMA system in the next section.

3.2.2 In Asynchronous CDMA Systems

In asynchro~~ous CDMA systems di users hase d B i t time delay whae

tk + O for k = 5 3 , . . . , K. We assume that r, = O for the desireci usa. If we sample

ur( t ) , Eq.(3.6), at the chip rate and take N samples each symboI intemai, then at the P symbol intaval the content of the equaher at the lfh antenna eiement is

where t6e code vector dk for the ph user is

and

whererk = p$c+&K,pkhmhteganumba, OSpbSN-landOSekcT,.

nien as in the prrvious analysis we wiiI drop the superscript i h m our eqpations. Let

be a N*K mûk that contains the received signal fiom aii K users, where rk for

k = 1,2, . . . , K is the received signal h m the kfh usa in vector form

Then the content of the FIR fdter of the nth anantenna eIement in vector fom is

Let

where ,,,[ is the weight for the FIR fdta of the lth antema element Now we can define

the contents of ai l the F R filters and th& tap weights in a vector form.

Let

where W is a LN..dimensiod vector which contains ai l the F R filters coefficients, and let

be a LNdimenSiod vector contaimng the contents ofthe ai i F R filters.

Now &a defining the vectors W and Uwe caa catdate the optimum tap weights and the

minimm mean square enor W S E ) of our stnic~tres.

The output of the receiver shown in Figœ(3.3) is

The mean sqriare errer is, [28]

MSE = I-W?P-~~W+W!RW (339)

where the superscript Hdenotes the Hermitian transposition, and whae R is the autocor-

relation matrix of the input vector U and it is equai to,

and P is the nws-corrclation vector betwexm the input vector U and the dtsind output

b1 ,where b denotes the data frorn the first user, then P is ecpai to,

To find the optmiimi coefficients, Wopt , from the Weiner--Hopf solution,

substitute Wopt into Eqb(3.39) to h d the minimum MSE (MMSE), then

MMSE = 1-@wOpt = 1 - f i 1 P (3.43)

In order to h d Wopt and the MMSE we need to calculate R and P which are shown

w h q assuming the noise sequence is uncorrelated,

and

and P is equal to

where

In the above analysis we found the MMSE and the optimum taps, Wopt, for our proposed

detector as a fimction of R and P in asynchronoos CDMA systems, where R and P depend

on the number of users, and an- elernents- F m these results we will see Iater in this

chapter the effect of the lll~~llber of users, and the number of antenna eIements on the pa-

formance of om proposed stmctine-

3 3 Performance Evalnation of the Linear Adapthre Space-Tie Stmcture

To d u a t e our structures we calculate the MMSE ushg the analysis done in the last sec-

tions. In this chapter we show the performance of the L,inear Space-Tie Receiver using

the MMSE d t a i a in synchronous end asynchronous CDMA systems.

3.3.1 Settings and System Parameters

For the evaluation we assumed that the system has the folIowing parameters unless otfier-

wise rnentioned

1- Spnading Gain N=8

2- Syncbronous and asynchronous DS-CDMA

3- M y AWGN and cochannel interfkmce, i.e. no fkding, multipath, or ISI.

4- The rrcaved E b W l 8 dB for each risa, so we asmme pedect power conml

5- The number of adaptive filter coefficients for each antenna element is equd to the

proc=msgain.

6- The distance between the mtenna eIements d=k/2.

7- Angle of amval (AOA) is a random variab1e uniformly distnIbuted in the m t d

[rd2, -1t/2] unless other wise mmtioned.

8- Random spreading codes.

Where m the following resuits the MMSE is averaged ova aU the random parameters:

noise, data, AOA, spreading codes, end dehys. 300 are used to get the average

MMSEC

3.4 The effect of the number of antenna elements on the MMSE

The e f f i of the number of antema eIements on the MMSE performance is shom in Fig-

(3.4). This figure is Qawn by caidating the MMSE equation derived in previous Secfions

as a fmction of the number of rnitc?mir elments used at the receiver. Fig-(3-4) also shows

the effed of numbg of usas accessing the wireless charme1 at the same the by drawing

different m e s , each m e represents d number of usas.

From Figi3.4) we see that an inmase in the number of mtenna elements increase the pa-

f-ce (mimmi7e the MMSE) and the capacity without the need for extra bandwidth

(bandvuidth is kept fixeci).

By adding morc mtenna elements the structure is able to cancel more interfirence. If the

n m b a of users is much less than L*PG then interference effects are essentialiy elimi-

nate& leaving only e f f i h m noise. Thm by fiitther doubiing the number of antema

dements we could &ce the MMSE by 3dB. Also increasing the number of usas does

not affect the MMSE if enough elements are nsed, which twislate directly to more capac-

ity. For a large number of users, doubling the numba of antenna elernents decreares the

MMSE by more than 7dB.

Also camparing the synchro11ous and asynchronous results we see that there is some deg-

radation in perfonnsulce due to the asynchronous channels and this is bexause each user

wii i give two intediering vectors.

(a) in asynchronoas CDMA system

@) m synchronoas CDMA sgstem Fig-(3.4) The effect of antenna elements on the MMSE.

3.5 Capacity Cornparison

In Fig-(3.5) we d U 8 f e the capatity (numbef of simultaneous equal power users) of the

pioposed structure by calculating the W E as a fûnction of number of users. In this fig-

ure we also compare the capacity of the proposed sfructines with the structura proposed

in [64]. We did not compare it with the comrentionai receiver because the structures in 1641

were f o d to outperfom the conventional receiver.

We set PG=8 for ail structures. Also we choose two sets of mtennas for the proposed

ftnichne fm compariso~~ These sets have 4 and 9 antema elements respectiveiy.

The cwcs are drawn by caldating the MMSE as a fimction of the numba of usas

eccessing the Channel. For example, in synchtonous CDMA sysîem with 4 antenna ele-

ments we am let 30 usas access the Channel with a MMSE approxhately equal to -15dB.

For the structures in[64] only 3 users cen acccss the c h e l with MMSE equal -15dB.

The signiscant increase in cepatity by using the proposed structures is clear fiom the fig-

iae and we can accommodate more users than the proceshg gain.

For more antexma elements the structure outperform the t h e only hear MMSE receiver

and become more robust c o c b e l interference. For example with 9 antenna ele-

maits we cm let 30 users access the charnel with MMSE eqrtal to -25dB, where for the

time only hear MMSE receiver we can't get this level of performance even with one user

(i.e. no intafaaice at all).

Also cornparhg the synchronous and asynchnous fesults we see that there is some deg-

dation in pafoanance due to the asynchnous channeis and this is because each usa

wiU give two interferef~. For examp1e the proposed structure with 4 mtenna elements and

20 users am be used in the system with MMSE equd to -20dB in synchrowus chgnne1,

but in asynchronous Channel oniy 12 users with -2OdB.

(a) in uynchronous CDMA system

@) m synchronous CDMA system Fi39 Capadtg Cornparison.

3.6 Near-Far Resistance

One ofthe common pb1ems m wire1ess CDMA systems is the nea~firr problem, where

the powet of the received signais for different users are not the same. In conseQuence the

conventional receiver wil l fail to detect the signal fhm weakest usez, and the usas with

the highest power wül dominate the transmissio~~ As mentioned =lier in this thesis, the

oniy practid remedy to this problem for the co1zventional &ver is the employment of

strict power contrd [23].

One of the most desirable fatures of any CDMA &ver is to be near-f'ar resistant, such

that the paformance of the receiver does not depend on the &ed powers of the differ-

ent usas. In this section we wili d u a t e the performance of the proposai structures in a

near-fhr emrirO~llllent

Figi3.6) shows the MMSE of the pposed structure as a fimction of the intaference to

desired signai ratio. Figœ(3.6) shows 3 different cases with diffaent number of users

accessing the channe1. In this remit we calculate the MMSE for 2,16, and 20 users. We set

the nimiber of the forward FIR flter taps equal to the procesSmg gain whidi is equal to 8,

and the nimber of antenna elements equal to 4.

In the k e e ceses we change the powa of ody one interfierex keeping the other interfer-

ence powm k e d at the samc value as that of the desired signal and caldate the MMSE

as a fimction of interference to desired user power ratio. The feSiSf8nce of the proposed

structure to the mm-f'ar tesistance is clear Erom this figure and thne is no need for strict

powa control, which wiU minimize the complexîty of the receiver.

(a) in asynchronous CDMA system

@) hi qmchronous CDMA system F i i . 6 ) The &kt of the Nesr-Far problem on the m.

A h we see hm Fig-(3.6) that the &xt of M interfisren with the same power as the

desimi usa is worse that the &ect of one interfe~er with power equal to M h e s the

powerof the desired user, which oontradict, [64] due to the use of antenna anay, and this is

Wcely due to the nict that the antema amiy cm place a null m the direction of one inter-

ferer much &y than to piace nnlls in the directions of many interfezer~. So no matter

what is the power of the interfice, the sûucture di a place nuii in space and time to

null out that interfierence.

3.7 The effect of AOA on the MMSE

Adaptive antaina amys diffefentiate between usas according to their AOA by placing

nds in the direction of interfice and placing a beam towards the direction of the

desind user. A common problem in AAA is that it can't différentiate between users with

the same AOA or e ~ n caa't converge toward the MMSE solution. This problem is usually

found when the interference and the desired user are close to or at the same AOA When

the nirmba of usas is smali the pmbabiiity that two users have the same AOA is srnail.

However in high WC channeIs(i.e. large nianber of usas) like celiular systems this

probability wilI increase and wil i affect the performance of the AAA.

The structure proposed m this chapter combines the spatial and the temporal domains

jointiy to cancel c o c ~ e l m t d e ~ e ~ l c e and increase the capacity of the system. However

if the mtafaace has the same AOA as the desired usa then these structures may cane1

the intafetence ody in the tene domain, Also ifthe interference code has high crosscom

lation with the desirrd usa then the structure may mceI the interference in the space

domain O&

In this section we wül show the affect of the AOA of the pafomance of the proposed

structures. Fig-(3.7") shows the &ect of one or many interferers having the same AOA as

the desired user.

Fii3.7) The effeet of interferences with same AOA as the desired user in synchro-

nous CDMA system.

Figi3.7) Shows that there is some degmdaüon m performance when interfisence has the

same AOA as the desired user. However the structure can stül work and give good pafor-

mance compared to the AAA. This is b-e the structure has the ability to cancel inter-

fefe~lce m the spatiaI and temporal domains jointly, when eva interference has same AOA

the structure will try to cancei tiiis interference in the time damain.

3.8 Conclusion

The Linear sp8ce-tirne stnichnt has been descnied in details. A h , the d y s i s of the

optimum tep weighîs and the minimum mean squarr e m r in synchronous and asynchro-

nous CDMA systems are givm. A closed form for the correlation matrix R and the cross

cordation vecbr P are derived, R and P found to be fimction in the number of antema

elements, numba of usezs? spreriding codes? AOA, and the noise power spectrai density.

The numerical dua t ion of the sûucture based on these anaiyses is also @en in this

chapter. In the next chapta we wil l propose another novel structure which is called the

adaptive decision feedback oombined spacetime detector.

Chapter 4

Decision Feedback Adaptive Combined Space-Time Detector for CDMA Systems

In this chapter we dmlop the decision feedback adaptive

combined space-time detectoz The structure uses the knowledge of the detected symbols

of the desired user and feeds them back through a FIR filter with a tap delay eqrials to the

bit duration. This detectur does not raqiiire any side information atcept the detected

symbds and the delay of the desired user. Even the code of the desired user is not required

4.1 introduction

In this chapter we pmpose a modif~ed version of the linem adapthe combined space-tmie

detector proposed in the prevïous chapter. The new detector uses the fcedback bguaüzer

structure. It feeds back the detected symbols ta a FIR fiiter with tap delay equai to a bit

dttration and whose Iength depends on the chmue1 delay spread. The output of the all feed

f o d FIR filtas and the feed backward F R filter is added and sampled at the bit rate to

fonn a soft decision output, then passed through a decision device to form an estimate for

the transmitted bits, Figb(4.1) shows the block diagram of this nceiver.

4.2 Signai and Channel Mode1

Consider a CDMA receiver with K asers. The signal trcinsmitted by the kth user i s denoted

by Sk(t) , whae k = 1, î, . . ., K. Assrmie thai SI ( t ) is the signal h m the desired usa,

49

and Sk( t ) , k = 2,3, ..., K are considered as co-channel interference with respect to the

desir#l user. Each signai received cornes fkom a certain direction. We assimie that the

direction of arrivai (Bk) " d o d y distnebuted in r?, y. To see the mistance of our t L LJ

structure against co-chmmel interfaence and ISI, in this analysis we assinne multiple users

and inta symbol interference ISI (no fhding) Channel,.

The receiver uses a L-dernent d o m -y with distance sepamtion d

The cornplex baseband transmitted signal k m the kth user is

The total received signai at the 2th eIement h m al usas including AWGN is

interval, hh is the -plex channel coefficient as seen by user k ctriring the mth path, pk

is the power ttansmitteù by the kth user, Nl(t) is a zero mean complex gaussian mdom

process with No is the cornplex noise power spectmm at the ph antenna element, zk is the

delay for the k%ser, t h of the rnfh path of the kth tli, M M the nurnber of the

mdtipath components, and ck(t) is the code seqpence ofthe user, wherp

r n - O

ak(m) E ( 1, -1) is the m-th chip of the k*lr user, and Tc is the chip paid n(t) is a mit

rectangular pulse ofduration F , Tc = Z& /N is the chip period, N is the w g gain,

A , ( B h ) is the ststeag coefficient of the kth user in the mth path at the 1th anternia

element, where

Where d is the distance between adjacent elements, L is number of elements, k is the

wave length, 1 is the element index. Oh is the angle of arrivai (AOA) of the kfh user in the

m" path with respect to the m . normal.

4 3 The Proposed Structure

The new detector is shown m Fig.(4.1). It uses the decision feedback eqwdker structure.

the stmctme u t i b s a L-elemaits anay antema. Each element is followed by a chip

mtched i5iter (0 whose output is sampIed at the chip rate and p H through a

hctionally spaced adaptive F R filter which has a delay for each tap equal to the ch@

period and lem equal to the processing gain. The detected symbois are fed back ttirough

a FDR filter with a tap delay equais to a bit duration and whoep lai@ depends on the delay

spread of the chamel. AU the L-FR fdters output and the fced backward FIR Mter are

summed and sampled at the bit rate to form a sofk decision output which is passeci through

a decision device to foxm an estimate of the transmitted bits. The structure in Fig.(4.l) is

d e d an a d a p h combinexi qmc&he decision feedback mdtiuser receiver- In the

fbUoWmg d y s k we wül assume MAI and ISI channel only. Mdtipath fâding will be

oonsidged in chapter 6.

The signal rrceived by the ph 8ntenna elment, x,(t), is pased hugh a chip matched

filter and sampled at the chip rate, the. at the ith symbol Etaval the content of the FïR

nIta of the ph element wi l l be represmted by an Kdimensiond vector asniming the

numba of the fmd taps is equal to N for the analysis.

In this analysis we assirme that all the ISI components have the same gain, so hh = 1 /M

for ai l k and m, where the code vector Ch, in the f" sampling interval is,

and

whexe the Ndimensional vector r h is the received signal h m the kth , during the

nrth path. Then the received signai h m the kfh user fimm di paths is the matrix Xk of

dimension (N x M) where X' is eqaal to,

We droppeù the supescript i from om eqoations, since it Win not affect om analysis.Then

the received signal from di usas fbm the a i l M paths is the m&ix X of

La the steering vector of the of the k*h user for the ai i Mpaths at the lth eIement be equd

to

where A, a Maensional row veçtor. Let

T T T A, = [A,, An O.. AL]

be a KWdimensional row vector containing the st&g coefficients of ai i usas for ail

paths, then the contents of the FIR filter of the [th antama elnnent in vector form is

Then let

be LN- dimemional vmor contains the content vectors for al1 the L forward F R flters.

and

be the taps weight for the FIR mter of the lth e1ement, then

L

be a LN-dimensional vector containhg the weights for the ail F R fltas of the ail ante~na

e~ement

Let the contents of the feedback FIR f l t a be

whm Q is the number of taps of the feed back filter- Let the tap weight vector for the

feedback FIR .filter be

Now we can defme the contents of the ail FR filters and th& tap weights in two vectors.

Let

where Wis a (LN+Q)iiimensional vector containhg the ai l FIR fiitem coefficients, and let

be a @U+Q)-dimensional vector containing the contents of the di FIR filters.

Now a f k defining the vectors W and U we can caiculate the ophum taps and the

minimum meen square e m r (MMSE) of our structure-

The output of the structure show11 in Figure l is equal to

y = w% The mean square enor is, [28]

MSE = I - # P - ~ ~ w + W%W (4-35)

where R is the autoconelation mahix of the input vector U and it is equal m,

R = E(UU~) (4.26)

and P is the cross-reaction vector between the input vector U and the desired out b 1 , w h a

b l daiotesthedataWtheWusa,UiaiPisaqualto,

P = E(Ub1) (4-27)

To h d the optimum coefficients, Wopt, fhm the Werner-Hopf solution, [28]

Wopt = R ~ P

substitute Wqt in Eq.(428) into Eq44.25) to find the MMSE,

MMSE = l-flwVt= l - f l ~ % (4-29)

In order to fmd Wopt and the MMSE we need to calculate Eq(4.26) and Eq(4.27) which

is shown below.

where

and

mi

where op^^ i is a Q&l veztor with dI its elements quai zeros. In the above d y s i s we fond the MMSE and the optimum tap weights for the proposed

structure in a multipath fading chameh as a fimction of R and P, where R and P depend on

the number of uses, and the nPmber of antemia elanents. From these resuits we wiü see

later the effect of the numba of usas, nurnber of antema elements, and the nimiber of

m e r taps on the pediomiance ofthe proposeci structure.

4.4 Nnmerical Results

In this &on we caldate the MMSE using the squations daiveci in the previous section

as a finiction of the number of users and nzrmber of antenna eIemenk Here in these resuits

we wili see the e f f e of ISI on the pediomance of the centraIized decision feedback space-

time reCenrer. The impuise response of the channel is assmned to have thne independent

paths, and dI the paths are assumai to have the same gain and th- is no f i g e The deIay

for each path are assimieci to be &ody disfriied in the mtervajs [OJ), [2TJT), and

[3T,4T) for the h t , second, end third p h respectiveiy

The DS-CDMA signal used in the a n d . has a processing gain(PG) equai to eight, and

uses BPSK moddation. We use d o m codes fw spreading our data with eight chips in

each code. The samphg nepriency of the signai before the chip matched filter is

equd three thes the chip rate, so there are three samp1es p a chip. The angle of arrivel

(AOA) is modeIed as a randam process unifiormly distnied in the intaval [-x/2, z/2] . E,/N, is set to 18dB for ail usas dess otherwise mmtioned, where Eb = pRTb. W e

slso assame plane m e propagation, and the sepamtion distance between antenna

elanents is set to u 2 . In these d t s 300 runs are used to average the MMSE around aU

the d m parmeters: noise, data, AOA, spl.eadi.g codes, and Channel impulse

respo--

The effect of the nimiber of mtenna elements on the MMSE is shown in Fig.(42) for

différent numbers of usas accessing the channeI. W e set the number of the forward taps

egnai to 8 and the nurnba of bac- taps equal to 5. F m Figme 2 we see that by

increasing the numba of antenna element the structure becornes more robust against the

co-channeI interference, as a result the capacity is increased while the bmdwidth is

nmains mchanged.

In Fig.(4.3) we d u a t e the capacity (number of sllnuitaneous equal power users) o f the

proposed structures. In this figare we also compare the capaciw of the proposed structures

with the structures proposed in [Il. We set PG=8 for ail structures. The cwes are & a m by

caiculating the MMSE as a function of the number of users accessing the c h e l . The

sigmSc811t increase m capacity by ushg the proposed structures is clear h m the figure and

we can accommodate more usas than the processing gaia

For more antenna elements the stnictrrns outpaform the structure in [l] and becorne more

robust against cochaMe1 interference.

In Fig.(4.4) we compareci the stnicaae proposed in this chapter with the hear space-the

structure and the structures proposed m [l] and [64]. In this cornparison we essume that the

AOA for ai i paths ofail usas are d d y disûiiuted in [-IV& z/2] . From Fig.(4.4) we

see a huge increase in capacity far our structures compand to the one elexnent structures

proposeci m [l] and [64]. Also comparing the decision feed back and the hear space-tmie

stmctms there k no sigMcmt impvemeat in pafomuuice f8r the decision feed back

sauctme cornparrd to the h e m spac&me s t r u e , i-e. there is no need for the decision

feedback filter for the case where the angle of anival of the various ISI components are

widely d i s t r i i And this is due to the abiiity of the space thne structure to reduce the ISI

compnents in the space domam. From Fig.(4.4) we conclude that sp-thne stnrcntre can

reduce the delay spread ofthe Channel and cana1 seva ISI by piaüng nuJis in the direction

of the late amval paths. The abiIity of the proposed structures to suppress rnultipath

components (which act iike intafaace having the same spreading code as that of the

desired signai), also extends to nrppressing out of c d interferers which for shoa spreading

code, may aIso have the saxne spnading codes as the desired signal. This wüi be seen lata

in chapter 7.

However ifthe al i ISI components of the desired risa have close or same AOA then we see

signInca~t gain m capacity and pedorrnance for the decision feed back structure proposeci

in this chapter compareci to the linear space-time structure proposed in the previous

chapta. Fig.(4.4) shows the effkct off all the paths of the desired user have same AOA.

Comparing Fig.(4.4) and Fig.(4.5) there is no degrdation Ïn perfommce for the decision

f i back space+time structure, howmr there is simcant degradation for the linear

space-time structure. This results &es an admtage to the decision feed back over the

hear space time meber in situation where the angle spread is very small or zero, Le. at

the base station with the antenna is located bigh above the roof top.

4.5 Conclusion

Tn this chapter we propose the decision f d a c k combmed space-time receiva. We

developed a formula for the MMSE and the optimum taps. Analyiical results based on the

MMSE d e r i a showed that the decision feed back space-time stmtme has the ability to

cancel cochrmnel interférence and ISL Also we found that in very miall angle spreack the

decision feed back structure out pcrform the lin- sp-time stnicture. However in large

angle spreads thgt is no Avantage of the decision feed back over the linear, the bar

spacetime &er will be preferable for its simplicity

Fis(4.2) The effwt of the No. of antema elements and the No. of mers

on the MMSE-

2 3 4 5 6 7 8 9 Number of antenna elements

Fig.(43) Capa- c u v e for the decision feed back combined space-time

structure

Number of users

m4.4) Cipacity cornparison between the proposed structures where

AOA of the various ISI wmponents are assumed to be widely distiributed.

m(4.5) Capaeitg cornpuison betwecn the proposed structures where

AOA of the variotu ISI components are assumed to be the onme.

Chapter 5

Centralized Decision Feedback Adaptive Combined

Space-Time Detector for CDMA Systems

previous chpters in this thesis we propose and analyze the linear

and decision feedback adaptive space-time detectors. We assumai that the receiver has no

knowledge of the detected syrnbols of aii osas in the case of the iinear space-time receiva,

and we assumed that the receiver has lrnowledge only about the desirrd usa syrnbois and

feed them back through a F R filter to enhance the paformance as in the decision feedback

space-tmie detector. As a naturai extenson to these two cases we propose a receiver which

we assimie has access to the detected symbols of ai i wrs, so that we can fad than to the

detector through a F R fdter. This receiver is called the centraîïzed decision feedback

adaptive combined space-time detector

5.1 Introduction

Untii this point of the thesis we considered two structures only. In the lin- spacetime

smicnne we asmme that the receiver has no imowledge of the de- symbois of ail

users, whaeas in the decision feedback spae+tEne ncaVer we assume that the receiver

feeds back the deteckci symbols of the d e e d user only. The cenfraiÏzed structure which

we wiIl present in this chapter am be seen as an extension ofthe pmvious In the

centralizedstni~nneweassiimethatthet.eceiv~hasaccesstotéedeMedsymboIsofall

users and can feed them back h u & FIR fiIters to miprove the performance of the ovedi

CDMA system. This k h i of information are on& available at the base-station, so the

practicai application of the centralized daector is at the besestation. The complexity of

the centralized detector is hear with the nmber of users and it is more compIex than the

Iinear and the decision feedback detectors- But because it's application is Iimited only at

the base-station, so extra complexity am be acceptable.

The centralized idea in not new m the multiuser detectiort literature. Werent detectos

proposed in the Iitemture used simiiar ideas ofmtraiized decision feedback, but under

Werent names. However, it is worth mentionhg here before we start discussing these

detectors that the major difference between them and our structure is the joint utilization of

the spatial and temporal domains.

In [12] Duel H a e n proposed the decorrelating decision feedback multnisa detector, see

Fig.(S-1). In [12], forward and feedback filtas are chosm to elùninate aU mdtnisa

interference provided that the fe#lback data are correct. Decisions of users are made in the

orda of decreashg received powas. The teaiver for each user linearly combines sampled

outputs of the matrix matched filter with decisioas of ai I strongest interferhg users. Thus

the receiver for the strongest user does not benefit h m the interference cancellation

property and it is m e n t to the decomlating teceiver. In [13] Duel Hallen extended the

same idea to other mdtiuser detectors and c o m p d them.

Another receiver was proposeci by m j i c a al. in [84 called adeptive centralized

decision feedback detector, ssee Fig.(5.2). In this receiver a11 symbols h m aii wrs are

ess~med to be known. The adaptive centrabxi decision feedback detector consists of a

linear hctiondy spaced adaptive film7 a centralized feedback filter, and a decision device

as shown in Fig.(5.2). It was shown in [87] that this detector has the same timing recovery*

near-fm &ce and mdtipath combinecl properties as the adaptive linear muitiusa

detector proposed m [64] and [87l whereas MAI cancellation is sigdicantly improved.

H o w m r the complexïty of the adaptive centralized decision fe#Iback receEver increases

Iinear1y with nrmiber of users.

Subtractive interkence canceiîation is another techive that uses the decision feedback

Sync 1

1 Sync K

--

Matched FiIter K K

Matched Filterl

Matched Filter2

Fii(5.1) Deeordaüng Decision Feedback Muitiuser Detector [U].

Sync 2

Training &a

idea to amce1 the MAI [45),[100]. Thae are ~ W O types of subtractive interference

cancellation. The h t is called successive interference caucellation and the second is called

the paralle1 hterfezence canceilation. We &d these detecbrs in chapter 2.

In this chapter we propose another novel structure for DS-CDMA systems wnich is called

the centraiized decision feedback space-time receiver. This stmchirc is more complex than

the structures proposeci in the previous chapteas. The difference between this structure and

the other structures are; 1) no need to orda users 8çcording to th& received powas, 2) aü

usas will benefit h m the MAI ~8flcelIation, 3) jointly combine the antema array (spatial

domain) and the mdtiuser detector (temporal domain) to reduœ the &ect of MAI, ISI, and

near-fhr probIem, 4) no need to respread the detected symboIs of the users. In this chapter

we wil i discuss and d y z e the centraüzed decision feedback Teceiver.

5.2 Signal and Channel Mode1

It is the same signai and channe1 mode1 discussed previousiy but we include it hae for

clarity. Consider a CDMA receiva with K usas. The signai transmitted by the @h user is

denoted by Sk( t ) , where k = 1,2, . .., K. Assume that Sl ( t ) is the si@ h m the

desired user* and Sk(t) , k = 2,3, ..., K are considered as co-chaime1 interference with

respect to the desind usa. Each t i g d receEved cornes h m a certain direction, We assume -R X; that the direction of arrivai (Bk) U uniformly distniuted in [T . h thir d y s i r wc

assume a mdtiuer and inter symbol interference ISI h m multipath (no fading) channeI,

to see the resistance of our stnictu~ aga& c4-channel intaference and ISI.

The r&er uses a L-eIement d o m =y with distance sepamion d.

The compla baseband trmsmied signai h m the kth user is

The chamiel impulse response at the lth elexnent is

The recavcd signal at the f h element h m the kth user is then

The total d v e d signal at the nth element h m aU users incfudhg AWGN is

In these equatiom, Tb is the symbol @od, b# is the data trammitted by user k in the ith

intaval, hh is the complex Channel coeficient as seen by usa k during the mfh path, pk

is the powa transmxtted by the kth usa, Nl(t) is a zem mean complex gaussian random

process with No is the compiex noise power qectmm at the antemi element, TR is the

delay for the kfhusa, t h of the rnth path of the kth user, M is the nimiber of the

mdtipaih components, and ck( t ) is the code sequeme ofthe Kfh user, where

rectangular puise of dmation T, , Tc = /N is the chip paiod, N is the processing gain,

A 1 ( B h ) is the steering coefficient of the kth user in the rnth path at the [th antenna

Where d is the distance betftreen adjacent elements, L Lis nmnba of elements, h is the

wave length, I is the dement mda Oh is the angle of- (AOA) of the k*h user m the

&mthwithrespectto t h e a r r a y n d .

5.3 The Proposed Structure

The new detector is shown m Fig.(5.3). The stnichrre atilizes a Lelement anay antenna

Each elemait is followed by a chip matched filter (CMF) whose output is sampled at the

chip rate end passed throt@ a h c t i o d y spaced adaptive FIR nltR which has a delay for

each tap equal to the chip paiod and length equal to the processing gain The detected

syrnboIs of di users are fed back through FIR filters with tap delay equal to a bit duration

and with length depeading on the deIay spread of the c h e l . AU the L-FR filter outputs

and di the feedback FIR filters outputs are summed and sampld at the bit rate to form a

soft decision output which is passed through a decision dwice to form an estimate of the

transmitted bits. The structure in Fig.(5.3) is called the adaptive centralized decision

feedback space-thne receivez In the following anaiysis we will assmne MAI and ISI only.

The fkquency selective fading channe1 wül be discussed in chapter 6.

The signal Teceived by the lth anteam element, q( t ) , is passexi b u g h a chip matched

filter and sampled at the chip rate, then at the ifh symbo1 intaval the content of the F R

filter of the 2th element w i l l be represented by an N-dimemionai vector assinning the

nmnba of the forward taps is e@ to N for the dys i s .

in this anaIysis we essrmie that there is no M g and all paths have same gain because we

want to s e t the effect of ISI, so hh = 1 / M for aIi k and m, the supersaipt T denotes

transposition, where the code vector Ch in the th sampling inteml is,

and

where the N-chnensional vector FR, is the receiwd signai h m the kfh user dining the

rnrh path. Then the received signal h m the krh wr h m di peths is the matrix Xk of

dimension (N x M) where X' is quai to,

W e dsopped the mpefscript i h m our equaîions, since it will not affect out anaiysis.Then

the received signal h m all users k m the aii M paths is the matrix X of

dimension(N x K M ) where X is eqpai to

Let the s t h g vector of the ofthe kth user for the di Mpaths at the lth element is equai

to

where A, is a Mdhensional row vecto~ Let

T T T

A, = [ A , , AD ... A,:] (5.19

be a KMaensioI181 row vector containing the steering coefficients of al l usas for ail

paths, then the contents of the FIR fiiter of the l fh antenna elanent in vector fomi is

Then let

U = T T 1 U I u2 O.. Y[ 1 L d

be a LN- dimensional vector contains the content vectors for ail the L forward F R filters,

and

(S. 19)

is a mdimensi~nal vector containing the weights for the all FIR mtem of the all mtenna

Let the content ofthe k? ffeedback F R filter be

where Q is the -ber oftaps of each fewi back filter. Let

end let the tap wtight wctor for the f d a c k FIR Hter be

Then the tap weight vector of the feedback FIR fdters is

Now we can define the contents of dl the FIR filtem and their tap weights in the proposai

structure as two Vectors-

Let

be a (LN+Q)-dmiensiod vector containhg the contents ofail FIR filters, and let

where Wis a (LN+Q)-dim&ond vector containing the F R filters coefficients-

Now afta dcfining the vectors W and U we can calculaie the optimum taps and the

minimum mean square emr (MMSE) of our structure9

The output of the structure shown in Fig.(53) is oqual to

y = WHu The mean square error is, [28]

MSE = ~-lyhl~-pHw++~w (5-m

whae the superscript H denotes the Hermitian treiisposition, and whae R is the

autocomlation maaU of the mput vector U and it is cqual to,

R = E(UU~) (5-28)

and P is the cross-reaction vvector between îhe @ut vector U and the desired out b 1 ,where

6 1 denotes the data k m the fbst user* then P is equal to,

P = E(Ub1) (5-29)

To h d the optimum coefficients, WWt, h m the Weiner-Hopf soIution, [28]

substitute Wopt in Eq.(530) into Eq-(5.27) to fmd the MMSE,

M M E = I - P ~ W ~ ~ = I - P ~ R - ~ P (5.3 1)

In ordes to fhd Wopt and the MMSE we need to caiculate Eq(5.28) and Eq.(5.29) wwhich

is shown betow.

where

and

where

@, = 0, = ... = a, = 'QXQ

whae I is the identity matrix, and

where OQKX 1 is a veztor with zms.

In the above analysis we foimd the MMSE and the optimum tap weights for the proposed

structure in multipath channels as a function of R and P, where R and P depend on the

number of users, and the number of antauia elemmts. F m these d t s we wiil see later

the effect of the number of usas end the number of anterma elements on the @ormance

of the proposed structure.

5.4 Numerical Results

In this section we calculate the MMSE using the equations d a m d in the previous section

as a hction of the nimiber of users and number of antenna elements. Here in these &îs

we wii i see the effect ofISI on the e e r f o ~ c e ofthe centraiized decision feedback spa-

time The impulse response of the chamel is assumeci to have three independent

paths, and ai l the patb are assumeci to have the same gain and there is no Ming. The delay

for each path are assumed to be d o r d y distriuted in the intends [O,T), [2T,3T), and

[3T,49 for the h t , second, and third paths respedvely.

The DS-CDMA signai used in the enalysis has a proceshg gain(PG) e@ to eighc and

uses BPSK moduiation. We use mdom codes for sprradmg our data with eight chips in

each code. The sampling fkquency of the received signai before the chip matched .filter is

t h e times the chip rate, so there are thee input samples pa chip. The angle of SIITival

(AOA) is modeied as a random process Uniformiy distniuted in the interval [-x/2, z/2] . E,/N, is set to 18dB for ail users dess ouierwise mentio~~ed, whae Eb = We

also assume plane wave propagation, and the separation distance betwem antenna

elements is set to U 2 . In these results the MMSE is averaged around all the random

pafameters: noise, data, AOA, spreading codes, and Channel impulse responses.

The efféb of the number of antenna elements on the MMSE is shown in Fig.(5.4) for

different n m k of users accessing the channeI. W e set the number of the fornard taps

eqtial to 8 and the numba of b8ckward taps equal to 5. From Fik(5.4) we see that by

inaeasing the number of antenna elements the structure becornes more robust a* the

co-channel interférence, as a result the capacity is increased and the bandwidth ranains

uuchanged.

In Fig.(5.5) we calculate the eff- ofnumber of users on the MMSE for Mirent number

of usas.From Fig.(5.5) we see the ab%@ of the proposed structure to totally cancel the

cochaanel interfkence and ISI by addmg more elements.

Comparison with the stmctum proposed in [2], [Ml, [87], and the spa-the structures

ptoposed in previous chapters are @en in Figo(5.6). We can see the si@cant increese in

pafo~mance and capacity of the proposed structure compared to the others. Comparing the

ail structures we see the sripaior ability of the ceatraüzed decision feedback space-thne

&a to ~8ncet MAI and ISI, which r e d t in much highex capacity without adding extra

bmdwidth,

Fig(S.4) The effect of the No. of antema eiements and the No. of users

on the MMSE.

Nranber of antenna elements change h m 1 to 9.

-16 - O C

3 -10 -

-20 7

Numberof mers

F0i.(5.5) Capacity c w e for the proposeci structure.

PIg.(5.6) CapacIty cornparison between the proposed structures.

In this cbapter we proposed the centrdized decision feedback combineci spae-time

teceiver. We d d o p e d a formuia for the MMSE and the optimum taps. Analyticai d t s

based on the MMSE criterion showed thet the centralized structure has the abiiity to

canceI cochannel kterfefence and ISI. The complexity of the centralized structure is iinear

with number of usas and am only be appIied at the base-mîon, so exira complexity wiiI

not be a pmblem at the basestation. In this chapter we also compare the centralized

receiver with the previoudy proposed receivas. It is found that the centralized structure

outpaforms the Iinear space-time structure and the decision feedback space-time

structure-

In the next chapter we wiii begsn a new stage of the thesis where we will see the effect of

the WireIess channe1 on the proposed structure- The hear space-time &ver will be only

tested because of its simplicity and pafomiance and we can use it as a bench mark for the

other structures.

Chapter 6

Performance of the Adaptive Linear Space-Tirne Stnictnre in Frequency

Selective Fading Channels

ali our previous mdysis and simulation we assinne that AWGN

and cochannel intederence are the only impairments in the channe1. Applying the proposai stnic-

tures to ceIlular systems &es rise to otha impaiments due to the wireIess channel. Wireless

commecation channeIs siiffa h m mmy impairments which place firnhental limitatioru on

the paform8flce of any wireless systan. The focus in this chapter is on the pdonnance of the

adaptive hem combineci sp-the structure in fkpency selective fading channeis.

6.1 Introduction In a wHeless communication h e i , the received si@ strength for a certain user is affecteci by

the physical channe1 in severaI ways. In this section we wiU stplaùi the channe1 mode1 wlnch wiiî

be used in the paformance anai* of the adaptive linear spa-thne receiver.

The trammittecl signai h m a wireiess system m k s at its destination dong a numba of different

paths r e f d to as muitipath. These mdtipath components mise h m scattering, reflection, and

difhction of the transmitted signal off objezts that lie in the area between the transmaM . .

g and receMng antainas m the wireless environment, Fig.(6.1).

The received signai strength for a givm user is dfiected by path Ioss, muitipath f h g , and shad-

Offfing. Path 1 0 s is a f'tmction of the distance b e ~ e e n the transmabn 0 . g and receMng ant-,

which is defixied as the ratio betweea the meived powa Pr and the transmitting powa Pr

Fig.(O.l) Muitipath Propagation in cellular systems.

where PL is the path Ioss. The immse path l o s for the Eee space model is given by

Where h is the wavelength, G , Gr are the powa gain of the transmit and d v e antemas,

nspectively, and d is the distance between the transmit and receive mtemas.

The main path is unially acmmpanied by a d c e dected path that may destructiively intafae

with the main p a k This is d e d the ground rdection model or the 2-ray modei. The iuverse path

loss for this model can be approximated by

where h, h, are the effective heights of the transmit and receive antemus respectEveIyC The effec-

tm path loss foIiows the humse fourth powa Iow (exponent equal to four) that results in a loss of

4ûdB/decade. In reai wireless emrironment the path l o s exponent Vanes h m 2.5 to 5.

In addition to detetmmistic path loss due to distance, there is a rmdom component due to the

location dependent, spatial diskiIbUtion of objects relative to the mobile. That is, the path 10%

experienced by the mobile depends on both the sepmition fbm the tmsm&er and on the particu-

lar placement of the smounding objects that may v e n t line-of-sght commUILication. This lat-

ter e f f i is calleci Wowing. From measurements, the random Vanation in path loss around the

distance dependent mean can be modeled as a log normal random miable. The path l o s and

Wowing is d e d slow fiiding or large d e path l o s [8q. In addition to path los and shalrowhg the received signal has another component wfüch wiii

cause its power to fluctuate with d spatial changes of the transmitîeir, meber, or scattering

objects in the emriroment, This componmt is d e d fast W g or d s d e mg, [8q.

The fiist m g is caused by the muItipIe signal refiections airiving at the base-station each with

i own phase. This multiple signai reflection is caused by scatterhg ofthe signai off objects near

the moving mobile. Whm the namba 0fmdtip8th components is large, each with d o m ampli-

tude and angle of amval &e at the receiva anth phases unifîormly distnLbated in the interval

[O, 2 ~ ) , then the in-phase and quadrature component of the rrceived signal level will be en

independent Gaussian process. The emreIope ofthe received signal is then a Rayleigh distrr'birted

random variable which probability d d t y fimction ghen by

Ifthae is a direct path, then the envelop of the r&ed si@ will be a Rician random variable,

which has a probability density fûnction as follows

where b is the mean power of the direct path and Io is the modined zero order Bessel function

of the first kind.

Mdtipath nrst Êding is descri'bed by its Doppler spread (the-selective tading), delay spread (fie-

Quency seledive fading) and angle spread (spaw s e l d e fading).

Doppla spread is due to the movement of the mobile usa or due to moving objects in the wireless

system envVOll~~lent. DoppIer spnad causes variation in the received signal amplitude with time.

The Doppler spread is inverseIy praportionai to the channel coherence time, whae the cohermce

thne repments the t h e sepmîtion for which the channef impulse response remains strongly cor-

related, and is a maisine of how fast the chamel changes in tirne.

Doppler spread and coherence the are panmeters which desmie the tbne myhg nature of the

wÎreIess channeL Howcva they do not offa mformation about the time dispersive nature of the

WneIess channeI. Delay spread and cohetence bandwidîh are panmeters which desmie the time

dispasive nature of the chamieI. Cohereace bandwidth is a statistical measrne of the range of fie-

quencies over which the channe1 c m be considered flat The coherence bandwidth is imrerseIy

pportionai to the delay qmad and is a measure of the chamel's frequeacy selectivity

In a CDMA system, we spnad the data signal over a mach wider bandwidth. The bandwidth of

the transmitted signal is u d y much greater than the coherence bdwidîh of the chaanei, and

fiesuency sel- ficihg c h e l mode1 is usually applied. So this chapter we wiU use the ik

qpency sc lech fading Channel model in the analysis and the evaiuation of the proposed

stntctme.

Angle spread refm to the spread of angles of the amvrù of the mdtipaths at the antenna array.

Angle spread causes space se1ective fadmg, which means that the Teceived signal b e l at different

antamas will have low or high correlation depending on coheraice distance. As for the coherence

the and the coherence bandwidth, the coherence distance represents the maximum spatial s e p

ration for which the chamel responses at two mtennhc remain strongiy comlated. The larga the

angle spread the shorter the cohemce distance.

At a high basestation antenna (above roof level) the agie spread is u d y vay small and the

signal exme1ope remains stmngly correlateci between two antemas sepafafed by several h, how-

ever at the mobile or at the basestation for indoor wireless systems the angle spread is usually

360' which causes the signai emrelope to be uncorrelated for distances greater than h/2 .So, In

the Channel mode1 which we used in this thesis we will assume zero angle spnad for each path, so

the signai 1-1 of the receEved signal at différent antenna wili be totdy wrrelated for each padi,

howcvcr the total angle spnad ofthe charme1 (ail resolvable paths) depends on the mtenna loca-

tion and heights and system type, Le. outdoor or indoor.

6.2 Channel model mduding AOA The chamel impulse response for the rtP' user at the I'h antenna element which will be used m tliis

chapter is giv= by, [75l,

In the above eqaations, hh ïs the anplex channe1 coeficient as seen by user k dining the m*

path, th is the May ofthe # path ofthe us- wtiere hh has a cornplex gaussian distriibu-

tion, Mis the number of resoIvabIe miiltpath component, is the steering coeilicieat of

the user in the m* path at the antema elernextt, where

where d is the distance between adjacent elements¶ L is number of elements, h is the wave

length, 2 is the elment nurnber. Oh is the angle of amval (AOA) of the usus in the rnh path

with respect to the anay nomial, where eh is a UIUformly distn'bnted in the interval

[rr/2, -ir/2]. In the above eqyations for the Channel mode1 including AOA we made the follow-

hg assumptions:

1) The bandwidth of the transmitted CDMA signai is much greater than the coherence bandwidth.

So, we assrmie a fiequency selective fiadmg chamel with Mresolvable paths.

2) Each path of the Mresolvable paths consisfs of many u~vesolvable paths that are approximateIy

of the same lm& and are wt resolvabIe at the receiver. Thus, they are combined into a single

path- 3) Each of the u~lfesolvable paths rnentioned above has its own time delay, and th& t h e delays

are approximately the same. Then they cumbined into a single path with time delay th.

4) Each of the unresoIvab1e p e b also has its own engle of amval, and th& angles of amval are A &ody d i W i in the in- [- 5 + e h , 4 + 0 h ] , where A and Oh are the angle

spread and AOA of the resoIvab1e patbs, nspectively. We assume that the angIe spread A is ne&-

gible and hk, is totdy mrrelated at each antenna element,

5) Each of the resoIvab1e paths consîsts of large nirmber of umesolvable paths where each has its

own amplitude. So the in-phase and quachme components of each ofthe xesolvable paths wii i be

an independent gaussim random process. Then the emrelope of each of the resoIvab1e paths will

be a Rayleigh t8tldo-m variable.

6) We assume a nmte numbs of resolvable peths, where Mis usuaUy between 3 and 6 each with

its o m angfe o f m AOA. In the npme~M results and simulations M d be eqaal to 3 each

WiIh its O W ~ AOA-

7) We assume tbat the fnspency rrsponse of each antenna is a p p r o ~ e I y 0at over the signal

bax~dwidth

8) W e also assume that the mobile is in the far field of the base-station anternia @oint source), and

the mobiie and the basastation awy are in the same plane. So plane wave propagation is used.

Fig.(6.2) 2) an th on of the &une1 mode1 descn'bed by the above eqiiatiom.

63 Recetved signal mode1 Considex a CDMA receiver with K usas. The signal transmitted by the kth user is denoted by

Sk(t) , where k = 1,2, . . ., K. Assinne that Sl ( t ) is the signal h m the desired user, and Sk(t) , k = 2,3, .. ., K are considaed as CO-chamel inWerence with respect to the desired usa. Each

si@ recehed cornes h m a certain direction. The complex baseband transmitted signai is

From (6.6) the Channel impdse nsponse for the k* user at the l* element is repeated here for

cl*

From (6.8) and (6.9) the received signal at the P fbm the wt is

Then the total feceived signal at the P elemeat due to all users including the AWGN is

where ail ofthe vgtiabIes above are defined befhe in this thesis.

f l 4 =Angle spread of the second resoLvable path

Base Staüon Recehhg Antenna

Fig.(6.2) Multipath Propagation Modd.

6.4 Analysis of the Linear Structure in Frequeney S e l d e Famg Channel Afkr d m g the chrinnei mode4 the tmmitted signai mode& and the received signal mode1

now we can d y z e the optimum coeficient vectors and the MMSE for the AdaptEve Lmear

Combined Sp8~e-?rie structure. In this Secfion we wül define the tap vectox modd and the coef-

ficient vector for the linear stmdme by usiag the same idea of representing the codes used in

chapta 3, whidi is d e d the ro-d wdk vudor, In this chapter we wïU change the rofated CO&

veeior, the tap vector, and the coefficient vector to inciude the fiequency selective channel moâel

desmied in this chapte'

First let us d h m chagter 3 that the signai received by the 2th antenna elment, xI ( t ) , is

passed through a chip mched fiiter and sampled at the chip rate, then at the ifh syrnbol i n t d

the contents of the FIR mtex of the lth elemait wiii be represented by an wdimensional vector

assimimg the number of the f o m d taps is equal to N for the anaiysis.

whae t h = phTc + e h , where pkm is an integer number, and O 5 &km < Tc . Let

be the vector the received signal h m the kth usa over the mth fading path. Thm the receXved

signal h m the kth user firom ail paths is the rmttrk Xk of dimension (N x M) where Xk is

asuai to,

We dropped the superscript i h m our equations, since it d l not effect our analysis.Then the

nceived signal h m aii uses h m dl Mpaths is the matrix X of dimeaSion(N x K M ) where X is

equalto

Letthe steaing vectorof the of the kd user for dl Mpaths at the element is eqPal to

where A, is a M-dimensional vector. Let

be a KM-eaSionaL vector contahhg the steering coefficients of aU usas for ai i paths, then the

contents of the F R ater of the l fh antenna elexnent in vector form is

Let

be a Ndimensi~nai vector containing the weights for the FTR filta of the antenna elment

Now we am d&e the contents of the all FIR film and th& tap weights in vector form.

Let

where Wis a mdimemi~nal vector containhg the all FIR filtas coefficients, and let

be a LN-dimaisiond vector containmg the contents of the ail F R flters.

Now a f k defimng the vectors W and U we am caldate the optimum taps and the minMium

mean sqoerr aror (MJMSE) of our stntctiaLe.

The optppt ofthe Iinear structure is equal to

The mean scpre ermr is

where R is the autocorrelation matrix of the @ut vector U and it is equal to,

and P is the crossimrrlation vector bmeen the input vector U and the desired output bl ,where

b1 denotes the data h m the fbt usa, then P is equal to,

P = E(Ub1)

To find the optimum coefficients, Wopt, fiom the Weiner-Hopf solution

substitute Wopt in Eq.(629) into Q(6.26) to fmd the MMSE,

In order to h d Wopt and the MMSE we need to caldate Eq.(6.27) and Eq.(628) which is

shown below.

where, assuming the noise and data secpences are independent then,

K M

and

and P isequelîo

P = E

where

and

In the above d y s i s we fomd the MMSE d the optimum tap weights for the proposed stnic-

ture in a multipath fhpency selective fiidmg channeis as a fimction of R and P, where R and P

depend on the number of users, and the number of antenna elements. F m these results we wiU

see later the e f f i of the nlltllber of users, number of antesma elements, and the number of for-

ward taps on the @ormance of the proposexi stnicture.

6.5 Channel Mode1 and System Parameters In the foliowing sections we will d u a t e the proposexi structure in a fnsuency selednte fàding

chamel. The numerid evaiuation is based on the adysis in section 5.4 where we will see the

effect of man- parameters on the MMSE criteria In the next subsecfions the chamel rnodel and

the system parameters used m the evaidon wil i be descricbed in detaiî. These d t s are also

used to show the advantages of the pposed space-thne stmcrmes compared to other adaptive

muIber structures. The simulation mode1 used for the fkpency sefective fibding channel which

mc1udes the AOA panmeter is shown kt Fig.(63).

6.5.1 Channel Modd Parsmeters In this section the charnel parametas WU be expiaîned. These channe1 parameters wÎiî be used to

evaiuate thepafonnanœ of tht space+time stmctma

m o n (6.6) shows the charme1 impulse response mode1 for the aepUency sdective fadmg

incIuding angle of arrivai. In the eV8h12Lfion process we have chosen the number of the resolvable

multipatb components M for each usa to be equai to 3, and the totai received average power for

each mobile (user) is same for a l l mobiles. Only in mistance measu~ements wilî thïs

8ssumption will be changed to see the pafofipaace of the proposed structure in a near-far envi-

romnent Each path is modeleci as a single planas mvehnt with zero angle spread,The three

paths m d d y distncbirted within the first 6 chips of a bit period. So the delay spread of the

Channel WEI be 6Tc. where Tc is the chip period.

hh was chosai to be a cornplex Gaiissian random variable. The in-phase and quadrature compo-

nmt ofhh an thm a Gaussian =dom variable with equal variance and zero mean. So the m g -

nitude of hk, wül be a Rayleigh random miable. Aiso we w c e that Oh is a UILiform random

variable in the interval [-ir/2, +x/2 ] . AU 3 independent resolvabIe paths for eacb user have the

m e variance. This model is an extreme scenario of fiequency selective fading charme1 where ail

the paths have the same VaTiance. Howmr. this chanael model wil l help in evaluating the perfor-

mance ofthe space-time structure m worstase channel s d o s . The multipath components and

delays of aIi interfaing received signais are independent.

- User K

delay tkl

I

I I I I I I

h , I I l

delay tm

Fig.(6.3) Channel modei used in the simulation.

6.5.2 Systern Parameters For the cvalPation of the paformmce of the sp-time structures, we consida the following sys-

tem setting dess o t h d e mmtioned.

1) We essume a base-station with d o r m linear anay of one to nine onmi-directionai antennas.

2) Antenna spacing equd to b/2.

3) We assume a DS-CDMA system where the data and the spreading codes are with BPSK modu-

lation.

4) AU iwrs have the same data nite and the spreading gain PG is e q d to 8; ia 8 chips p a code.

Longer spreading codes could be used, also the complexity of the structure depends on the length

of the spreading codes.

5) The codes used are random spreading d e s . Later in the next chapter we will compare Mer-

ent spreading codes.

6) The received Eb/N, for each usa is sqrial to 18dB. So we assume eerfect power control,

where No is the powa spectd density of the additive noise.

7) The nmber of adaptive f m a d filter taps for each antenna is equal to the processing gain.

8) We average the MMSE ova 300 m. Where in each run we select different codes, delays, fad-

mg coefficients for each user.

9) One eqaker input sample per chip and the data symbols have a variance of dty.

6.6 The effect of the number of forward taps Because we are applying the proposai structure in a fkpency selective fading channel, then the

structure needs to ance1 cochaMe1 interfefence, mce1 ISI, and combine multipath components.

In Fig.(6.4) we display the caldated M E weraged over 300 mm, as a hction of the nimiber

of forward taps per element, and 4 mtenna elements are used. This calculaiion is b d on the

andytical &ts d a m d in section 6.4. Fig.(6.4) dispIays the d t s for ciiffixent number of usas

d g the Channel. F m Fig.(6.4) it is clear that the MMSE is improved by increasing the

numba offorward taps ap to mughly 14, Le. the srmi of the delay spread and the numba of chips

p a symbol. This is nasonable because the lengîh ofthe forward Mta should be mough to collect

the energy m al l the 3 paths.

2 4 6 8 10 12 14 Nurnber of fomrard taps

Fig.(6.4) The effea ofnumber of forward tabs on the MMSE

However a Iowa nmber of forward taps can be used with good pafo~natlce and nduced s t ~ ~ c -

trrn mmplexïty. For example with namba offmard taps equal to 4 (haifthe processing gain)

the MMSE is less than -7.5dB for 6 users which is stül a very good perfomance result compared

to 1641.

In the following d t s the nurnber of forward taps is set to 8 d e s s othemïse mentioned. From

Fig.(6.4) the degadation in @ormance between the 2 usas cese and the 18 usas case is approx-

imstely 7dB with the numba of forward taps equai to 14. This degradation can be finther reduced

by Usmg more antenna elements as we will see in the ncxt sections. Table 6.1 shows some partic-

ular resuits at Mixent number of taps and different number of users to show the signifiant

capacity mcreese of the spacc+time stnictrn:es. ûutage probability of the MMSE is calculateci in

section 6. 1 1.

6.7 The eEeet of the number of resolvable paths To set the abilty of the space-time stmctiire to perfomi the job of a RAKE receiver we conducted

the following test. In these fesults we set the -ber of interferers epual to 4 and the number of

811te~18 elements eqaal to 4. Fmm Fig.(6J) it is clear that the space-thne stniçtrrrr has the ab-

to co11ect the energy in the resoIvab1e multipath component. Hence it paforms the fùnction of

RAKE reception. This is because the adaptive forward filter is a tapped dehy h e with delay

equai to the chip period, so any paths that are separatecl by more than a chip can be resolved by the

mter and th& en- can be coilected. We conclude h m these r d t s that the sp-time struc-

ture @ i the hction ofmuItimer daection and RAKE reaption in one stmcture.

From Fig.(65) here is approximately 6dB miprovernent in the M E between the one and two

resoIvabIe path cases, and the improvement is appmximately 9dB between the one and three

resoIvabIe path cases*

2 US=

4 U S ~ ~ S

6 US=

8 US-

4 taps

-1 1 dB

-10 dB

-8 dB

6.5 dB

6 taps

-15 dB

-14 dB

-12.5 dB

-12 dB

8-

-17.5 dB

-17 dB

-16.5 dB

-16 dB

10 taps

-19 dl3

-18 dB

-17.5 dB

-16.5 dB

-2 I I I I I ! 1

O 2 4 6 8 10 12 14 16 No. of forward taps

Fig.(65) The e f f i of number of resoIvable p a h on the MMSE.

6.8 The effkct of the number of antenna elements It was show11 in [64] that the adaptive linear multiuser detector with a number of taps eqyd to the

pmcessing gain (PG) has the ability to canal up to K=PG-I interferers. By using an antenna

array with L e1emeats one can incfease the capacity ofthe system by L fold without adding extra

bdwidth.

Tbe proposeci adaptive sp-time stmctme jointiy adapts the spatial and temporai domains. By

using L anterma elements, the number degnes of M o m of the adaptive multiuser detection is

mcreased h m PG to LSPG.

Fig.(6.6) shows the effêct of the nimiber ofantenna elements on the MMSE in kpency selective

fâding channe1. Comparing these d t s with the r d t s in chapter 3, we see that in fieqyency

seIective fkhg channeis the numba of degrees of âeedom is reduced due to the multipath com-

ponents since the forward Elter length only spans one symbol. In Fig.(6.6) we set the nmnber of

fornard taps equal to 14. F m Fig.(6.6) we see that by addmg more elements the structure is able

to cancd more interfikence. If the -ber of interfèrena is less than P P G then in te r f ice

effects can be essentially eliminat#l leaving the effects h m noise only. Then by m e r doubling

the nimiba of elements we can Teduce the MMSE by rorighiy 3dB. Also the space-time structme

makes pedormance less sensitive to the number of interferers. If the numba of usas is pater

than the degrees ofhedom then doubling the nirmba of elements decreases the MMSE by more

than 7dB. For example, with two antema eIernents and 16 users the MMSE is equal approxi-

mately -6dB, but for 4 antenna eIements the MMSE is approximatdy - lMdB with the same num-

ber of users which is a gain of 8.8dB.

Number of Antenna Elements

Fig.(6.6) The effect of numba of elements on the MMSE.

6.9 The e f f i of nnmber of asers, In this atpannent we compare the proposed space time stnichae with the non-array stmctms

proposed in [64]. The cornparison wil i be based on the MMSE aiteria Ui this srperiment we will

see how the pposed spaoethe receiva impves the capacity of the DS-CDMA system with-

out adding extra bandwidth. Capaciw hem meam the numba of usas with the samt power that

can access the CDMA wireless channe1 in a one cell system, So* here we assume only one d

Out-offceU intderence will be considered in the next chapter. in I n s test we set the num-

k of f m m d taps equal to 14 for dl structures in cornparison.

Fig.(6.7) compares the proposed space-the stnicture capacity with the structures proposed m

[Ml. A signiscant inmase in the number of interferhg signais which can be tolerated in fie-

quency s e l d e channe1 for the space-thne receNa without the need for extra bandwidth is clear

h m the figure. For example with 4 elanmts we can let 16 users access the channel with MMSE

approximately equal to -14.5dB, however with the structure in [64] with same sprtading gain only

2 risas can access the channel with MMSE epual to -13.5dB. For more antenna elanents the

structure outpafonxis the time-only receiver and become more robust against interference. For

m p I e with 9 elements we can Iet 16 usas access the chaiinel with MMSE l e s than -20dB,

where for the sûucture in[64]we can't get this paformance even with one usa, Le. no intaference

at all. From this test we see the power of the space-time structure to accommodate more interfer-

ence to enhance the capacity withoin adduig extni bandwidth.

6.10 Near-Far resistance As disclissed eariier* one of the conmion problems for the conventional CDMA receiver is the

near-fhr pmblem. One of the most deswble featpres ofany CDMA receiva is to be near-far resis-

tant, sach thaî the performance ofthe receiver does not depend on the received powas of different

users. In the foLiowing numerical r d t s we evaIuate the pafonnance ofthe pposed structure in

emrieonment.

Interference to Desired power ratio in dB

Fig.(6.8) The &kt of near-fkr problem.

Fig.(6.8) shows the MMSE of the h e m adaptive combineci space-time receïver as a fimction of

the aiterference to desired signal ratio* In this figure we assume oniy 2 users in the systems (i.e.

only one interfiefence. We diange the power of the intafere~lce and calculate the MMSE at each

time. In the figure we plot the results for 4 entama elanents. The &siance of the proposed

reœiver to the near-fiu p b l e m is clear fimn the figure and thde is no need for strict powa con-

mi, which wül m h b b the complexity ofthe * 6.11 The Outage probability of the MMSE As rnentioned before the MMSE was calculateci by avemging ova 300 nms. In each rtm we ran-

domly pick the spreading codes7 delays, AOA, and the Rayleigh fâding coeficients for each usa.

To get more insights on the behavior of the space-time receiver* one should look at the outage

probability ofthe MMSE. Outage probability here means the numba ofnms the MMSE is below

a certain level out of the total mm.

From Fig.(6.9) to Fig.(6.12) we campare the outage probabiiity for the adaptive linear space-the

feceiver with 2,4, and 8 elanents with the outage probabüity of the smcture proposed in [64] for

diffkrent oucage Cntezia (-6 dB, -IO dB, -14 dB, and -18 dB).

These figures w a e drawn for diffkxent numbn of usas and Maent number of elanents to see

the perfomuuce of the spmtime receiver compared to thne only receiver,

In Fig.(6.9) we draw the outage probabiiity of the MMSE below -6âB. Comparing the differeat

stnrcttires it is cl= that the more elements we add the lower outage we get For example with 4 - elemmts the probability of an outage is almost zero up to 16 users.

Figures fhm (6.13) to (6.15) compare the cumulative distriution finictions (CDF) of the MMSE

for the space time detector with 2,4, and 8 elements and the time only d e r . It it very clear

h m the 6grnes that the CDF for the space-time structure is less sensitive to number of uses than

the t h e ody daector and it becornes less sensitive by adding more eIements.

6.12 Conclusion Chapter 6 is b o t e d to study the @ormance of the iinear space-time teceiver in frequency

selective fàdhg charnels, where mdtipath and h i h g can degrade the performance of any sys-

tem. The 8I181ysi.s of the optimum tap weights and the MMSE of the linear spacetime sûucture in

kqyency se1ectrve fadmg chmeis are gnrai m chapter 6. Adytkai d t s are also presented in

this chapta. Fmrn the resuits we found that the linear spacwbe r& is not or@ capable of

ancehg MAI and ISI bat &O has the ability to collect the magy ofthe diffaait mrimpath corn-

ponents. W e found that these sûuctms combine the hction of muitiuser detection, antenna

arrays, ISI equalùer, and and reception in one sngle structure. Also h m the cornparison

study between the linear MMSE muitiuser receiver (temporal only) and the linan space-time

receiver we conclude that the space-the receiver has the abiiity to improve the paformance of

the CDMA system signifimtly.

Fig.(6.9) Oucage probability on the MMSE P(MMSD-6 dB).

i Note: No cktage O& for E1=8 beiow 14 u s e 1 0 ~ f

I I 1

2 4 6 8 10 12 14 16 Number of users

Fig.(6.1 O) Outage probabiiity on the MMSE P(MMSD-1 O dB).

Fig.(6.11) Outage probabüity on the MMSE P(MMSD-14 dB).

Fig.(6.12) ûutage probability on the MMSE P(MMSD -1 8 dB).

Fig(6.13) C i a n d e distniution fimction of the MMSE with 2 users.

Fig.(6.14) C u m u I m distn'bution hction of the MMSE with 4 usas.

Fig.(6.15) Cumulative d i s tn ion f'rniction of the MMSE with 14 users.

Chapter 7

Practical Issues Related to the Space-Time Structures in Cellular CDMA Systems

7.1 Introduction

In this thesis we propose several space-the structures for DS-CDMA systems. In previous chap-

t a s we demonstrate th& advantages compared to other stnictmes. From these d t s we see that

these structures have the ability to combine mdtipath components, cancei muitiuser intexference,

and mcel intasymbol intafiremce. We fond that these space-time &ers provide a signifi-

cant improvement of the pafomuuce of the CDMA systan compared to other structures.

in this chapter we consider some practical issues. The linear adaptive space-time receiver wül be

testeû m this chapter to give some insights into the potentiai advantages of the space-tirne struc-

ture in ceUW CDMA systans.

In this chapta we mn many simuIations. The adaptive algorithm technique used in these simuia-

tion is the normaüzed LMS for its SimpIicity. But before we present these simuIations we shidy

the transient behanor of the hear space-time stnicture.

In section 7.2 die transient behavior of the lin= space-the receiva wiil be analyzed and dis-

cussed. The efféct of time Varymg user population and packet transmission wil l be discussed and

simuiated in section 7.3. In section 7.4 we consider the efféct ofthe spreading codes. ûut-of cell

interference is considemi in section 75 . Section 7.6 studies the capacity implovement oEued by

the iinear space-time rroeiver incIuding the &éct of shadowing and distance r e M attenuafion

BER results is discussed in &on 7.7. F i d y the conclusion is in section 7.8.

7.2 'kansient behavior

Many adaptive aigoridm techniques are weii established and their pmpaties are weii known in

the literatnre, [28]. 01P concem in this thesis is not on the adeptive technique itsekfbut on the per-

formance of the lin- space-thne detector in cellular systems and the study of some related

aspects that meai the advanages of the receivet. So the normaked lest mean square algonthm

wiU be adopted in this thesis for its simplicity. However, hding the best suitable adaptation aigo-

rithm thas fi& the hear spacetmne structure is an intereshg area ofresearch and it is outside the

scope of this thesis. Before simulatmg the Iinear spae-time detector using the NLMS algorithm

we wül study nrst its transient behavior using the LMS algorithm.

In [65J, Miller studies the transient behavior of the adaptive MMSE receiver for DS-CDMA sys-

tems. Miller examines the lm@ of the training period as a fimction of the number of interferhg

usas and the e&ct of the near4hr probIem. R d t s m [65] show that as the nimiber of users is

i n m e d , the eigen-value spread of the autocorrelation matrix is increased, thus requirhg longer

ûahhg period. A h resuits in [65] state that as the stmgth ofthe interferhg usas i n m e s , the

length ofthe traimng paiod is also signiscatltiy mcreased.

In this d o n we wili sbady the transient behavior of the lin= spacetime receiveir. Also a corn-

paris011 stady between the adaptive MMSE time only receiver rmd the adaptive combined space-

thne receiver is d e d in this section.

7.2.1 Transient behavior analysis of the MSE

The dy t i ca l presentation ofthe transient behavior of the MSE of the lin= s p a c e - e structure

pposed in tbis thesis is presmted in this section. Using the analysis gisen in [28] and assriming

the independence assumption used m [28], the MSE at time n is d&ed as

where A, i = 1,2, ..., NL is the eigen-value of the correlation matrix R d&ed in the previous

chapter; end xi(n), i = 1,2, . . ., NL are the diagonal elements of the matrix X(n) , whae x(n) is

caldateci using the itaative equation:

x(n + 1 ) = BX(n) + p2~min&

where B is an NL-by-NZ xnatrix with elements

and is the step size of the adaptation algorithm. An initiai setting corresponding to di tap coef-

ficients at zao is needed for the vector X(n) to calculate J,. This was found to be quai to

~ ( 0 ) = f l n ~ ~ ~ ~ ~ v (7.5)

where V is the matrix of eigen-vectors and Wop, is the optimum tsp weights which are given in

chapter 6. The MSE at t h e n couId now be calcuiated using the above equations to obseme ana-

iytically its behavior in 6equency s e l d e fading channeIs.

In this section we calculate the MSE using the eqyations d a m d in the previous section as a h c -

tion of the namba ofusers and mimber of menna eIements. The impalse response ofthe chsuine1

is seme as in chapter 6 and it is assumeci to have three independent paths, and all the paths are

assumeci to be Rayleigh ~~ with the same variance. The path deIays are d i s t r i i imi-

f o d y within the nrst six chips of a bit in- ie. the delay spread is equal to six chip periods-

The DS-CDMA s i p i used in the d y s i s has a processing g W G ) e q d to eight, and uses

BPSK modulation. We use random codes for spreadmg our data with eight chips in each code.

The sampling hpency of the received signal before the chip matched filter is equal to t h

thes the chip rate, so thae are tbree input samples per chip. The angle of amvaI (AOA) is mod-

elad as a random process uaiformly dkbibuted in the intend [-W2, x / 2 ] . E,/N, is set to

l8dB for dl users unless otherwiSe mentioneà, where Eb = pkTb. and No is the PSD of the back-

ground t h 4 noise. We also assinne plane wave propagation, and the s e p d o n distance

between antenna elaments is set to k/2. In these d t s 300 nms are used to get the average MSE

ova ai l the random parameters: noise, data, AOA, spreadhg codes, and charme1 impulse

responses (fading and mdtipath).

In Figi7.l) we compare the MSE cwes for the adaptive Iinear space-the receiver with the

adaptive Iinw time oniy d v e r . We set the step-size equal to 0.0007, n m b a of forward taps

eguai to 8, and the number of elements for the space-time recciva quai to 4. From Figœ(7.1) we

see that the space time receiver converges faster than the the only receiva, for example with a

four etement spacetime &er the MSE reaches -10 dB Ievei der 55 trainmg bits wbile the

tjme oniy receiverreaches the same level a&r 230 training bits, as a result it wiil reqiiùr a shorter

training period, Moreover, with more psas the space-time stmctm becornes e v e ~ fhster than the

time only structure. The convergence time is less sensitive to the number of usas and the steady

state MSE is much Iowa for the space-time structure than for the t h e only structure. The most

like1y explaaaton of the TeSults is that the space-he structure &ces the comlation among

codes by exploiting theu spatial characteristic (thm angle of arrivai). So wm though the space-

t h e receiver has more coefficients to adapt, its training paiod is much l e s thm that of the time

only receiver.

In Fig-(7.1) we compare both structures with the same stegsize. But because the maximum stable

stepsiP shouid be isversely proportional to the numba ofthe adepted parmeters [28], we npeat

the analysis m Figq.2) with diaerent stepsizes to set the e f f i on the MSE transient m e - Fig-

(7.2) shows the tt8nSient crwe for the adeptive time onIy teceiver with two different stepsizes,

(0.0007,0.0028), and compares these two cws with the adaptive space-time receiver with step-

- --

100 200 300 400 500 600 7OO 800 900 1ûûû Number of training b i i

FiM.1) 'It.ruisient behavior cpms of the space-tirne and t h e oniy receivers.

400 500 600 Number of training bits

Fï7.2) Effkt of the stepsize on the training c m e .

F i 3 ) Cornparhg the space-time receiver with the t h e on& receiver

size oqual to 0.0007. W e assume that thae are 4 usw in the system end a space-time d v e r

with 4 antenna elaents. From Figœ(7.2) we see that evm with the smaller stepsize the adaptive

space-time receiver stül comerges fista For example with four elements space-time receiver the

MSE maches -8 dB I m l afta 50 iterations whüe the tirne only rember with smaller step size

reaches the same Ievel after 600 iterations.

In the finai nsult we compare both skuctiaes with Metent chip rates. nie nason for this is to

keep the total numba of taps for both stmdum the same, so both structure wül have the same

number of multiplications per bit In the foRoWmg resuIts we assume that the space-the &er

has pniassing gain equaI to eight and has 4 anternia elements. The thne only stmcture has pro-

cessing gain equal to 3 5 where both stnictures have the same bit rate and step size is set to

0.0007. We plot the ames for 1 and 4 users case. Figœ(7.3) shows that the space-thne structure

has better CoIllvergence time and les steady state MSE with Iess bandwidth repuired.

7.23 The adaptive NLMS algorithm

Many adaptive algorithm techniques are well established and theg properties are well lmown in

the litercihae* Our concan in this thesis is not on the adaptive technigue itseIfbut on the perfior-

mmce of the structure in a time varying user popdation. So the nomialwd least mean square

algorithm, NLMS, wül be adopted in this work for its simplicity* However, finding the best adap-

tive algorithm which fits the adaptive combineci space-thne detecior is outside the scope of this

thesis*

The NLMs adaptive algorithm is as follows [BI,

where in the above eqiiations, a is a positive constant, p is adaptation constant whae O < p < 2 ,

e(n) is the mor signal, and n is the itaation mmiber.

7.2.4 Remarks on the transient behavior

In this section we conducted some expeWents to see the transient behavior of the lin= space-

time structure and compared it with the t h e oniy receiver. From the results we conclude that the

adaptive spawtime structure improves the performance and reduces the training paiod length of

the CDMA system compared to the time only receiva Also the results show thet with much less

bandwidth the spacetime receiver still has b&er cornergence and steady state MSE than the time

ody receiver. The most likely exp1anation to these restxits is because the space-trme structure has

more degcees of fieedorn than that of the thne only structure and it reduces the correlations among

the spading codes by exploiting not only the temporal characteristics but dso their spatial char-

acîeristics too. In conclusion, even though the space-time receiver has more coefficients to adapt.

its aaining period is much less than that of the t h e only receivex These results make the pro-

posed adaptive space-time structure more attractive for CDMA systems.

7.3 Effect of time varying user population and packet transmission

MMSE multiuser detection (the only) Ïs one of the mdtniser techniques proposeci in the lit--

ture [Ml, [114]. One of the important feetrnes of the MMSE detector is that it can be implemented

using a hctionally spaced squalurr [Ml, [2] and can be adapted using any of the popuiar adap

tiw aigoxitimis [28]. in [64] and [2] it is shown that the adaptive MMSE detector is near-fkr resis-

tance and has higher capacity than the conventional &ver. Although the adaptive MMSE

detector has many interesting propaties it has some problans.

ûne ofproblems ofthe adaptive MhilSE mdtiuser detector is the effect of t h e v q b g user pop

dation (effect of sudden death and birth). In cellular systems csnying voice, packet data, mdti-

media etc., usas start ûamdtthg and terminate th& transmMon at différent tnmes, as show in

Fig-(7.4). Also in packet tmumhion users start and terminate their t d s i o n x.nany times

daring one session. Packet tnmsmission in ceiialar systems cause transient problems for the adap

tive MMSE detector due to the anisai and departrire of interferers' packets; an adapadaptive MMSE

receiva needs to readapt each time a new interferhg packa arrive.

Iii [2], A b d m a al. fomd that the sudden birth of interfefe~~ce can degrade the MSE by

more than 10 dB. The BER WU be hi& at that thef and the d-r may have to be retrained.

But the detector perfomance doesn't degrade due to sudden death, hskd [2] found the pdor-

mance Hnprove when interférez lemes. In [351, Honig proposed a rescue dgoridmi that detects the

appearance of new mterferers; if a new i n t e r f i is detected then the decision-directed mode is

suspend& and a b l ï d algoriUrni, simitar to the one proposed in [32], is used to readapt the filter

coeEtient,

In this section we study the effect of the sudden birth and death of interferers on the paformance

of the praposed Iinear adaptive spacetime mdtiusa detector, for a fiequency selective fading

channel. The e f f i of packet transmission will be studied too.

7.3.1 Simulation Resnlts

In this section we simdate the adaptive combined spiice-thne detcctor in a frequency selective

niding channd to study the effkct of sudden de& and birth. The NLMS algorithm with a = 1

and j~ = 025 to update the filter coefficients* The impulse response of the channel is assumed to

have tbne independent resoIvab1e pathst and the gain of d resolvable paths are assimied to be

Rayleigh distncbuîed random variables with the same variance. The path delays are independent

and distriied dormly within the fmt six chips of a bit intend. So, the delay spread of the

channel is equal to six chip periods.

The DS-CDMA signai used in the simulation has a p w s h g gain QG) equd to eight, and uses

BPSK rnodukion. Randam codes with eight chips are used to spread the data The sampling rate

ofthe received signal before the chip matched filter is oqPal three times the chip rate, so there me

k e e samp1es per chip. The AOA of each resolvable paîh is modeled as a random process iim-

f o d y districbuted in the interval [-n. XI. Eb/N, = 18dB for ail users* We assrmie plane wave

propagation and the distance between elements, cl, is set to M2. In these fesults the m e is aver-

aged over 100 nms.

Fig-(75) shows the &kt of the sudden birth of hterférers on the MSE. In Fig-(7 5) we assmne

that the system starts with six iises and a new user is added every 300 bits pntil the total nwnba

of iisers reaches ten users. Fig-(75) is d r a . for systems with one and foin antenna eiements.

Three conclusions are daim fhm this figure. The first conclusion is that the steady state response

a k each new iisa of the adaptive iinear MMSE detedor is degraded sevexely, whüe for the

adaptive space-time detector this d-on is vay smalI and it can be reduced by ddmg more

elements. So with the spacetime detector the steedy state MSE is less semitive to nimiber of

usas. The second conciusion is thaî it is not critical for the adaptive spac&me detector to be

retmined after each new user, and this is because the MSE does not rise above below -10 dB,

which is stül a very Iow d u e . The finai conclusion is that tht spa-time detector converge fister

than the adaptive linear MMSE detector which reduces the overhead bits rrquired to train the nIter

coefficients. F r m Fig-(7.5) 5) is clear that the MSE reacts fàster to lower values afta each new

intderer. For example with four antema elemeats the MSE naches -10 dB, which is a very gwd

threshold to start the decision direaed mode, with a training seqyence four times shorter than the

adaptive linear MMSE detectar. Also note that with PG quai to 8 we am accommodate more

than 10 psas with very low MSE which is more than the processing gain. This d t s make thÏs

spacatime detector suitable for high capacifl wire1ess network with packet transmission.

Fig-(7.6) shows the efféct of packet transmission, In Figo(7.6) we assume h t the systern starts

with tkree users. These k e e usas are 8ssumed to ûansmît continuousIy A ncw user is added to

the systan which is assumeci to trammit two packet of length 200 bits at the itaation nimibas

400 and 1200, remahhg dent at otha times. From Figœ(7.6) we sa that the spacetime detector

does not deteriorate at the second padret likt the adaptive Iinear MMSE detector. From this figure

it is cl= that the space time structure better retains some memory of departed intdefers, p b a -

bly as a result of a lazger number of d e g m of M o m than the lin- MMSE dm Also h m

Fig-(7.6) we see that sudden death does not affect the MSE, but it may enhnnce the pafarmance

for systems with fewa degrees of needom.

From Fig(7.6) we conclude thaî what we have to tare about is the &st packa of any new inta-

faer. So we consider a RAMP POWER ACCESS technique to m e r d u c e the e f f i of the

sudden biah of interferers. This technique hcreases the over head bits nqiiirrd m each packet, but

with the spm t h e daectar the RAMP POWER ACCESS technique wi l l be used ody in the fmt

packet of& user, or aRa long periods of-

Fig-(7.7) compares the RAMP POWER ACCESS teCanique with the fiùl power access for Four

antenna dementS. In the RAMP POWER ACCESS technique the hterfefe~~ mcrease th& powa

h e d y over 200 bits mtii they get fidi powa. From Fig-('Iî) the RAMP POWER ACCESS tech-

nique reduces the degradation in the M E due to new interférer by mmd 5 dB somehes by

more than 7 dB. From this resulf by using the RAMP POWER ACCESS technique we make m e

that the MSE &es not jirmp above the -16 dB level.

73.2 Remarks on sndden birth and death

Adaptive MMSE t h e only multiuser detectm proposed in [2] and [64] have large capacity and

paformance compareci to the comentionai receXvez. Th& complexity arcept for the cornplex

multiplications and the *the algorithm is similin to the conventional receiver. Sudden births

degrade the performance of the adaptive MMSE detector, the BER will be hi& at that tirne, and

we may need to retniin the filter coeBcients again. In this section we stuied the effect of time

varying user population and packa transmission (sudden birth and death) on the perfiormance of

the combined space-the muitiuser detector proposed in this thesis. From the lcesults we made

timx important points. The first point is that the combined space-the structure deviates the

&ect of sudden birth and thee is no need to ntrain the detector because the detected symbols are

sti i i reliable and the MSE is stiU Iow. The second point, at start-up or after a new interferer arrives

the filter coefficients converge fasta than thaî of the adaptive MMSE detector, thus reducing the

reqyked training period. F i d y , the space-time structure has some memory of the packets of

departed interferers, so nnew packets of the semt interferers will cause las hami to the pafor-

mance. Aiso we use the RAMP POWER ACESS techniques with the space-time structure to fur-

tha d u c e the effect of the appearance of the first packet of the new intdaem. As a resuit, this

detector is an amaaive solution for high capacity wire1ess systems with packet transmission.

Intaference birth Interference death

Fi74 Packet transmission in d u i a r systems.

ne system &ts with 3 $sas continuousIy New usa is added whicb is a~sumed30 transmit two pckeets oflagth 200 bi@ at iteratioe nimibas 400 and 1200. i

F i i . 6 ) Effect of packet transmission.

F i i . 7 ) h p power access technique W. fiin power access for the Iinear

spiadme receivu.

7.4 Effect of spreading codes on the performance

In ail our previous resuits we bave assinned that random codes were used for spreading the data

~ e ~ ~ e ~ l c e of the CDMA users. We don't claim that =dom codes are the best suitable d e s for

the spaa+time receIYers proposed in tbis thesis. However random codes give as many codes that

wecansuppottmoreusers~theprocesSmg~Previ~~~reSultsslmwthatspacbtmitstm~-

trins can support many more ppas than the processing gain.

Finding the best spreading code setpence for the sp-time stnictures is an interesting area of

research, but it is out of the sape of this thesis. However in this section we show some simulation

results to see the effect of the spreading code set on the pafomance of the linear space-the

receive' Random codes and Hadamard code are selocted for the cornpanSon, The enect of ushg

the same sp-g code Win be diswsed in the next section. The following simulations are con-

ducteci in bcpency sel* fading chamiel deScnieci in chapter 6.

Figg(7.8) shows a comparison betwan the performance of the Iinear MMSE d e r (time only)

with random and Hadamard codes. It is found that Hadamard codes gives somewhat better d t s

on the average t'an mdom codes, which shows the e t of spreading codes selection of the pa-

formsvlce of the linear MMSE muitiuset: detection d e r (the ody). This r e d t is in consistent

with the d t s in [2].

Figq.9) shows that with the space time recenter there is no diffkrence in pafonnance betweea

the different code sets. This is because the space-he structure does not depend on the correlation

between codes oniy to separate the intedérace, but on diflerences mong theP spatial signatures

tm. So this important featme of the spacmtime nCemr can reduce the complexity assochted

with spreeding cudes planning aud assigment.

From Fig47.9) we see that there is no difference in the perfionn~ulce of the space time receiver

with =dom codes or Hadamard codes. To look more closely at these resuits we plot the outage

probabiIity of the MSE for the spacatime structure with Hadamard and =dom codes. Fig-(7. IO)

shows the outages probability of the lin= spa-time structm with Hadamard and raridom

codes. The results show no différence in oatage between the Hadamad and the random codes.

F0i(7m8) EEect of the spreading codes on the performance of the heu MMSE receivv ( the oniy ncehnr).

Fi(7.9) Effkct of the spreading codes on the performance of the hear spacetime

O 1 I I 1 1 1 I I 1

jNumber of tapsi=8 j

4

jNnmherof used i inambc! of an*a eïeqents= 4 i

......... ........ ........ ........ ........ ....... ........ . . . . . . . JZbllYt~J8 .dB. .. .; .:. .:. .i.. .;. .;. -

\ -10 - S.. - .................................... ...........................................................

-25 I I I I 1 1 1 1 I

O 100 260 300 400 500 600 700 800 900 1CKK Number of bits transmitted

Figî7.10) En'ect of spreading code seleetion on the outage probrbiüîy of the lin- ear sp-time receiver.

7.5 Ontsff-ceii interference

Until this point, we have sssumed that a l i htaf-ce came fhm the same ceii as the desired user.

Since we consider the spacetime structure for celinlar CDMA systems, it is necessary to look a .

the &ect of out of ce11 interfiérence (hteraiI interference) and the intaference h m the same

ceii (mtra-ceil interfice) on the paformance.

There are two types of out of ceii interference. The first one is intafaaice h m spreading codes

différent h m the desired user code, and the second type is interference with the same spreading

code as the desimi usa. The fmt type of interference has a War &éct as the interfierierice

the same ceU as the desiteci usa, but with l e s power due to distance separation. This type of inter-

ference was considered in prwious chaptas ofthis thesis.

The second type of interference, wwhich is the subject of this section is the inter-ceU interference

with the same code as the desind user, One of the important feature ofthe CDMA system is that

the same fresuency bandwidt. could be reused m the adjacent ceh. So ifwe consider hexagonal

cells, then there could be up to six adjacentceii interferers with the same code as the desired user.

in [2], Abdiilrahman studied the Hect ofthe tmie only structure with decision feedback filtex He

f m d that one i n t e r f i with same code as the desired usa could degrade the pedormaflce

severely, specidy if it is located on the ceil boundary. AbduIrahman did not study the case whae

more than one interference with the same code as the desireci usa, but it is clear that it wi i i be

worst He concluded h m his study that neighboring ceiis shodd not assigned the same spreading

codes over the same d a hpency. The same spreading codes couid be assigned ove the same

carrier h p n c y in ceh that are Mer a p e He aiso came with a plan for the spreadhg band-

width end spreading code assigismen& of neighboring ceIlS.

h Figq.11) to Figo(7.20) we examine the &kt of outof& interference hwing the same

spreading code as the desmd usa within the celï of interest on the perform811ce of the iînear

space-time receiveL In the fo1Iowing simdations the outof-ceii intafeer wii i be considered as

the intederer who has the same code as the desired user.

In Fig-(7.11) and Figœ(7.13) we assume that there are 4 usas in the system. Two cases are consid-

end, in the f b t case we amme di users have different spreading codes and in the second case

we assume that the fourth risa has the same sprradmg code as the desired user, this foinih user is

considend as àaerfera h m the adja~e~lt-celI. In these smmlations we esSmne that the impulse

response ofthe chamiel is same as in chapter 6 and it is assmned to have tbree idependent paths,

and ail the paths are essumeci to be Rayleigh distriied with the same variance. The path delays

are distrbuted unifbrmly within the &st six chips of a bit intaual; i-e. the delay spnad is equal to

six chip pexiods. The DS-CDMA signal used in the analysis has a processing gain(PG) equal to

Rghf and uses BPSK modulation. W e use random codes for spreading oip data with eight chips

in each code. The sampiing fieqyency of the received si& before the chip matched filter is equal

k e e times the chip rate, so there are three mput samples per chip. The angle of e v a l (AOA) is

modelexi as a mdom process uniforxniy distn'birted in the interval [-x/2, n/2]. E@, is set to

18dB for dI users udess otherwise mentioned, whae Eb = pkTb. and No is the PSD of the back-

ground t h 4 noise.

In Fig-(7.11) we assume oniy one element antenna and we see that thae is about 2dB degradation

in the MSE when the out-of-ceii intdefence has the same code as the desired user. The intderer

with the same spreading code differs fimm the desired si& in that it arrives with a different, ran-

dom, delays and through Merenf random, rndtipath. However in Fig47.13) with four antema

elanents we get the same result in the two cases and thae is no degradation in the MSE. We

repeat the same a<paimmt in Fig(7.15) and Figi7.17) with diffèrent namber of taps for the FIR

fiiter to see the &ect of number of taps on the me. From these r d t s we dmw the sasne conchi-

sion, but with 14 taps FIR flters we see better MSE r d & .

In [2] it was found that the capacity of the DF-MMSE reCaver is about 4 with processing gain

e t p i to 8. In Figures numbered Fig-(7.12), Fig-(7. M), Fig-(7.16), and Fig47.18) we assume

there are four users in the ceii of interest and the fifth user is in the neighbormg cell. Also we

assume that all users have the same received pow=

In Figœ(7.12) and Figi7.14) we assume that thexe an 5 usas in the system with the 5th usa hav-

hg the same spreadmg code as the desind uset In FigC12) we assume one anteana elemenf

and fkom the d t s we sa that there is about 6dB degradation m the mse due to the 5th risc That

is why [2] conclude that neighboring c d shotdd not assign the same spnadmg codes ova the

sarie kqyency band and the same spreadmg code shorrld be assigneci oniy to cells that are fiirther

apart. Howewa with the h e m spact+time structure we can reduce the efféct of out-of-celi inter-

ference. F m Fig-(7.14) the degradaïon due to the 5th user is about 1 dB, which suggest that

neighboring celis can be essi@ the same qxeadïng codes.

We repeat the samt previous simulation with différent nirmba of the ETR filter taps. In Fig-(7.16)

and Fig-(7.18) we assmne that the nmdwr o f fornard FIR fiIter taps eqyd to 14. We carne to the

same conc1usion as the pnvious results but with the 14 es FIR mters. We see better MSE results

comprmd to 8 taps FIR fiRa Also comparing Figo(7.14) and Fig-(7.16) we see that with the 14

taps FIR filters we &ce the effect of the 5th user with the same spreadbg code as the desired

usa. In Fig47.14) the degradation is around 1 dB, however in Fig47.18) with 14 taps the d m -

tion is mund 0.5 dB,

In Fig-(7.19) and Figw(7.20) we oonduct another eirpe8ment to see the effect of out of ce11 inter-

fefence with the same spreading code as the dcsind user but with different power.

In this enpairnent we assume that there are 4 users m the desired user ceii and the 5th user is in

the neighbor cd. The 5th usa which is in another di is assumed to start transmission with - 2ûdB Iess reœived power than the desired usa. Then this interference start increasing its ampli-

tude by 0.1 mry 500 bits, simulaihg the effm of the 5th user moving towards the ceii bomdary

of the desired user.

From Fig-(7.19) and with one dement antenna we see that the MSE degrades as the 5th usa gets

c1oser to the ceii bomâary of the desiffd usa until it reaches an unacceptable Ievd From this

nsult it is c k that with one mtenna element we can not assign the same spreadmg code set to

the neighboring tells, but oniy to ceils that arr far qart. But with the four antema elernents we

see that the Iinear space-time receiver can handle this kmd of intafkmce and make it possiile for

the system designer to assign the same code to neighboring ceus, which wii i reduce the compIex-

ity 8SSOCiated with code planning and assipnment, In Figœ(7.20) we nptat the same experiment

with Mirent nimibers of taps and we &O reach the same conclusion. However the MSE is better

with the 14 taps sûuchae.

In this section we studied the effect of out o E d intafaaice with same spreading code as the

desired user on the performance of the iinear adaptive space-time receiver pposed in the thesis.

R d t s show îhat the Iinear space-tmie iecava couid p a f d y hande sevaal interfierence h g

the same code es the desireci usa, and elimuiate the need for spredhg bandwidth and spreading

1 .

Ail nw3: have Werent 'spkdmg codei

4 0 0 5 0 0 6 0 0 Numbetr of bits tmsmilted

PIï(7.11) Effect of out-of-celi interference wah same spmding d e as the

dcshrd iwr on the hear MMSE neeiver (thne only receiver).

L I !+ 5th user with d e spkdhg code as ihe desired user i ...... : . . . . . . S . . : . . . S . . . . . : ..............................:.........:.........:......... : ........

Pï. î2) Effkct of out-f4ell interference rsith same spreading coàe as the desired wer on the bear MMSE receiver (tirne oniy @et).

r r . . r .

Number of bits ttansmitted

Fii7.13) Effcet of out-of-ceII interference with same spreading code u the

desired user on the iineiar space-time liecetver,

Fii.14) Effect of out-of~ceil interference with same qreading code as the desired user on the linear space-time r e c e h ~

FEgî7.15) EEect of o u t - o f d mterfhnce with srme spreading code as the desirecl user on the hear MMSE receiver (time oniy receher).

Fïï7.16) Effkt of oat-ofeeii interfierace with same spreading d e as the

desired user on the b e a r MMSE receivv ( t h e on& receivu).

_ .................... ........*......... I........... . . . . . . . . . . . . . . . . . . . . .......-.........*..

Number of bits transmitted

Fiîfl.17) E&ct of out-of+ell. interference with same spreadïng code as the desired user on the hear space-time receivec

F i i . 1 8 ) Effkct of out-of+eil interference with same spreading CO& u the desYed user on the hear sp-tfme neenter,

1 1 I 1 1 I 1 L 1 :Niimberoftaps=û : INnmhekofuse~=S f

..... ...... .-... ....-... -.--...- . . . . ........ ....... W . . .;Eb/.No=f8dB. .:. .:. .:-.- ..:. .*...:. 1. .;.. - f Desired and 5th user have the sime spreading code :

f 5t user $tart w@h receh@i powe of -2O;d.B beï@w ail us&s ......... :Amgütadeofthe5&.asu.hcr~-bg@;l-ewry5QO~~-..-.- - - i - - - - - - - -

FiM.19) Effkct of otat-of~ceiI hiterference with s ~ m a spmding code as the desired user and dlnerent pweR

Desjredsind 5th user hrve the +me spreaàing code : St us-= siPrt wIth receivedpower of d b dB Wou aR &ers i . .----.*-..-. -..-...-................*.., .....-...............*............-.....--.-........ Ampiitn'de of the 5th iwr merirJes bg0.1 every 500 bits i

-20) Effkct of orit-~flcen interference wîth same spmdhig d e as the

desired user and dinerent power,

codrs planning. R d t s also suggest the posscbility of h g the same code within the same WU.

Such a featme is important in systems used the same spreading codes within the same c d . In the

IEEE 802.1 1 stan- the Barka code with 1 I chips is nsed for spreading the data sequence m

wireless L M . AU usns use the same code but with differe~lt sMb, beauw the Barker code has

very good auto-coneIation proper@ Howsier in kpency selective fading channek, delay

spread cm ceuse many users to have the same code shift, in this situation the pafoxmance will

deteriorate. So we suggst the application of the space-time &ers in the IEEE 802.1 1 stan-

dard,

7.6 Effect of Shadowing on the outage probability

In a wirdess chamel the meiveci signai strength depends on two effects: 1) s m d scde fading

and 2) large s d e path 10s. In d sale fading the receiyed signai strength wili fluctuate with

s d spatial changes of the transmitter, &a, or scattering objects mimd the receiving

antema This type of Ming is discussed in chapter 6.

The large d e path los depends on two effects: 1) the distance between the d v i n g and traas-

mitting antemas, which is d e d path los, and 2) the spatial distriiution of objects around the

recàving antenna which wiiI block line-of-sight or shadowing.

Path loss is a fimction of the distance between the traasmitting and feceiving antennas, which is

d&ed as the ratio between the nceived and the transmitted powers [86].

Howeva, due to the spatial diSnnion of objects mund the mobile, the avaage nceived power

at two dBerent locations having the srmie distance k m the traiasmitting antema may change

wideiy. This effect is d e d shadowing. Measurements have shown that the path loss a partidm

location is randorn and distriiuîed log-normdy [8q. The power received at distance d h m the

transnùtting mtenna is

where p(do) is the benchmark Tecenred powa at distance do, n is the propagation exponent, and

Z is a log-normal gaussian miable; Le- its logaxithm has a garissian densi@

with zero mean and variance a* is in dB. A typical value of a is 8 dB.

In the foliowing SimiIlations we wiU carry s cornparison study between the linear space-thne

nceiver and the tempord only receiver to see the capacity improvement supported by the hear

space-tirne structure relative to the t h e only structure. In this study we include the effects of

shadowing and distance related attenuation.

In this study we assume 3 &roups of users and they are assigned Ietters A, B, and C respectively.

AU users in p a p A are a m e d to be in the same ceII as the desired user and ail have the same

average received powet. Group B users are out-off-ce11 interference and have average received

power of20 dB below the desind Group C usas are also out-offeli interference with 40

dB average nceived power below the desired user.

The cornparison saidy is conducted in k e e sanarios. In the first sCenano we assume 2 u s a in

p u p A, 2 usas in group B, and 2 users m p u p C, for a total of six usas in the system. In the

second s c e d o we assume 4 users in group A, 4 usas in group B, and 4 usas in grroup C, for a

total of 12 users in the systm. This makes the system in the second scenario have double the

capacity of the system m the W sscenerio. Tii the thùd scenario we assume that 8 usas in grop

A, 8 users in group B, and 8 risers in group C, for total of24 usas in the system. This d e s that

system in the third scenaio have four times the capa* of the system in the first scenario. AU

these scenarios include the e f f a of shadowing with a = 8dB and k p e n c y selective m g .

In Fig-(711) we compare the Iinear space-the stnichire with the t h e only structure in scenraio

number one. In this d t s we assume that the numba of forward taps=8, stcp size equal to 02,

and n m b a ofeiements-4. From Figœ(7.21) we see that the linear space-time stiuctme has more

than 6dB gain in MSE mpared to time only stnicture. To see how much capacity improvement

that caa the lin- space-thne receiw offa campareci b the time onIy receiva we conduct the

same simulation m scemrios n . 2 and 3,

In Figi7.22) we simidate the Iinear space-the recenter in scenarios nuinber 1,2, and 3. W e

assmne that the lin- spacetime receiver has 4 elements. Comparing scenarios nmnber 1 and 2

we set that by donbling the nmnber ofasas the sbachae stiII has tht srmie steady state MSE, bat

it requires longer time to adapt, Also comparing the hem space-time structure in scemaio num-

ber 3 with the time oniy shucnire in d o number one, we see that even with four times the

capacity of the time ody structure the Iinear space-time receiver still maintab better MSE level,

about 4 dB bnta. From this compaasOn we conclude that the h e m spacatime structure with 4

elements has more than four times the capacity of the thne ody receiver.

In Fig-(7.23)and Figœ(7.24) we study the outage probabiiity of the MSE. Fig47.23) shows the out-

age probability of the MSE below ddB and Fig47.24) shows the outage probability of the MSE

below -12 dB. Both r d t s confh that the iinear space-time recRver offa 4 times the capacity of

the t h e ody &ver

In the prewious TeSnZts we assumed that the out-offkeil interference is a structured interference,

i.e. eech interrférence has it is own spreading code. In [U], the authors assumai that the out-off-

celi mtafamce is a pussian random variable. They found tbat for a l o g - n o d shadowing with

standard dwiation of 8 dB and propagation exponent of 4 the out-off-ceU interference cm be

modeled as a gaussim randorn variable with mean less than or quai 0.247& and variance less

than or eqiial 0.O78Nw where Ns is the number of usas per sector.

In the foliowing feSults we use the s a m t interfîîce modd as m 1231. In Figi7.25) and Fig-

(7.26) we compare the capacity of the Iinear space-thne receher with the time oniy receMr where

out-offcdl interfkmce is modeled as gaussim mdom variable with meanSO247Nc and

varain~O.O7Wc, whae Ni is the number of usas per ceU. These resuits is a h CO&XII that the

lin- space-the nceivei with four anteruia elanents offw more than four times the capacity of

the time oniy receiva, This conclusion is in complete consistency with the pmious resuits.

7.7 Bit error rate resnlts

MMSE criteria was used in evaiuating the p a f i i c e of the sp-time structures proposed in

this thesis in ail previous d t s . M E is easy to dcalcte and it is less time c o d g to sim-

date. Evm thoiigh thae is no iinear relationship between the MMSE mi the BER, MMSE &es

us the ability b understand and chanicterize the space-the structures proposed in this thesis.

Smce (WSrna n (SNR)& at the output ofthe receiver, where SNR=EbNo, then low MMSE

4 0 0 5 0 0 6 0 0 Number d transmitted bits

F'i.21) Effeet of shrdowing and distance related attenmtion on the MSE.

Fgi-(7.22) E f f i of shadowiug and distance reïated attenuation on the !in- space-time reeeker.

Fi7.23) Outage pmbability of the m e ni shndowing enviromnent

............................ : Four eTements:"""" '

SœMri0#2 :

pis-<7.25) Capacity cornparison, out-offd inMerence ù modeleci as gaussian

random variable Normai(0.247Ns, 0.078Ns).

Fw7.26) Outage pmbabüity of the MSE whve o a t - o f f d intufwence is modeled as gausîan -dom varïabk Nor-

d(0.247Ns, 0.078Ns).

values means Iow BER. Eb/No belween 3 to 6 dB is considered a good perfiormance for DS-

CDMq [21-

To fmd the daiwd BER f o d a for the MMSE feceiva is a very dïEcuit task. Revious efforts to

derive an exact enor probhility fomiua for the MMSE receiver is tirrned out to be a very compü-

caîed proCCdure [2].

Due to tht non availability of closeâ analfical formula for the probability of error in CDMA sys-

tans with interfiice the BER rates perfomance is usually obtained by assuming that the inter-

f m c e is gaussian. A simple method was proposed in [3 11. Tins method is based on the gaussian

modd for interférence to dculate the bit error rate.

In Figœ(7.27) we CaIdate the BER using the above fonnulz~ The BER is caiculated by averaging

Pe over 500 nms. This results is cunducted m hpency selective fading chamel discussed in

chapter 6. In each run we d o d y pick the spreading codes, delays, AOA, Rayleigh fadmg coef-

ficients, and the channe1 impulse fesponse for each user In the following r d t s we assumed that

E@=18dB, nPmba of forward taps is set to 14.

In Fig-(727) we dmw the BER m e as a fitnction of number of usas. Up to 16 users are

assuneci m the system, this gives us the abiiity to accommodate more usas thet than processing

gain which is assumeci to be equal to 8. Fnmi Fig47.27) we see that with one antenna element we

can accommodate Iess than 4 users with BERCIO-~, however with the space-time receiver we can

accommodate 8 users with 2 elements and 16 users with 4 elements. From this results it is clear

that the space-time structure offéred a signiscant miprovernent in the DS-CDMA system capacity

by edding more elemmts. Ais0 with the spacetime stnictrrre we can support systems mpires

good @ty of service (QOS). For arampIe with for elements and 6 usas we can get B E R ~ O ' ~

wiuiprocessinggaineqpal8.

In Figœ(7.28) to Figœ(7.31) we sttidy the outage probgbility of the sp-time structum and com-

pare it with the outage probabfity of the lin- MMSE mnltiuser meiver (time ody receiver).

The outage probability ofX is dehed as the ntmber ofmns such that the pbability ofBER Iess

than X out ofthe 500 nms we conducfed in this sminlation. Fig-(728) to Fig47.31) shows the

Figq.27) Bit error rate performance of the hear space-time receiver.

eu- I s â W 9 P e e

/ ................... ..*...-.................*.--.......- ........................................ o . . . . . . . . . ; . . . . . . . ............. ; . . . ..................... .;.-.*. ........ ................................................................................................ I / 4

c r

i m Y

P e e

..............................................................................................

...............................................-.........-......--------.-..-................. / .................................................................................................

No outage occpr rot two elements structure and nwbber of user& - - - - - -NaOlit.ge.occiu lr-fetvtVele@eats &mit- and.Mtmber o f ~ s e m 4 4 . . .

Fig47.29) Outage probability of x=loJ.

No outage oeeur foi four elemek stmctuk and nuniber of a s e k l 2

F i 3 0 ) Outage probability of x=lo4.

outage probability of X= 1o02, 1 o-~, 1 04, and IO-' respectively.

From Figœ(7.28) to Fig47.3 1) we see the -or performance of the linear space-thne receiver

compared to the linear MMSE &a (tirne ody receiver) in tams of BER. For example with

P(BER<IO-~) in Fig-(729) we have an outage probability of 0 2 for one elexnent receiver with 6

users, however with the space-time &er we have 6x10~~ oidage with two elements and zero

outage with 4 elements. Also notice that we have zao outage with 4 antenna elanents up to 12

usas in the system and the processing gain aqrial to 8. We conclude h m the BER r d t s that the

space-the structure offcrs large miprovernent m the capacity and shodd be considered as a seri-

ous candidate for any DS-CDMA &ver design in the future.

7.8 Conclusion

In this @ter we discussed several issues that effect the application ofthe spacetime structures

in celiuiar systans. The linear adaptnre space-time &ver was selected mong the other space-

time stnxctures for the study for its simpIicity and can be considemi as a bench mark for di the

space-the receivers deweloped in this thesis.

In section 7.2 we shidy the transient behavior of the linear spacetixne structures with the LMS

algorithm. From the shidy we conclude that by using that space-time receiver we can reduœ the

&kt ofnumbn of users and th& powers on the training paiod

in section 73 we discuss the effect of t h e varying user population and packet hansmwi O O

011,

R d t s shows that with the lincar spa-timc d e r we are not only able to reduce the effêct of

suddm death and bkth of interfice on the system but also we can elimmMe the effect of sud-

den birlli of pack- of departexi interference by retaining some memory of deparieci intderence.

Effect ofpeadhg d e s codes studied in section 7.4 and redis shows that the space+time structure

not only depends of the correlation propaties among the codes but aIso on the spatial propaties

too. The d y shows rhet there are no Merence in per6omiance between the mdom and the

orthogonel COQ for the space-time Teceiver. This reduces the cumplexity associated with code

planing andmanagement

Efféct of outi)fi:eU mterference is studies m section 7.5, the study shows that the sp-time

structure can hande i n t e r f i with the same code as the desired user and eliminate it, In section

7.6 we stdy the &kt ofshdowing and distance relaîed aîîeniiation in cellular system and show

that system employ the bear spacetime receiver can o f f i more that four times the capacity of

the system empioying the time ody receÏver

BER d t s is given in section 7.7, we found that the BER r d t s are in coqlete coI1SiSteflcy

with the MMSE d t s .

From this chapter we concIude that the space time receiver offamore than four times the capacity

of the time only receiver and it is a very a ü c h w stnictiire for high data rate systans with packet . bmsmsson.

Chapter 8

Conclusion and future research

8.1 Conchsion We have developed severai spatial-temporal processing dgonthms that can enhance and irnprove

the pafoxmance of the CDMA systems. These dgoritfmis showed that they have the ability to

substantially miprove the perfiormaxlce and capacity of the adaptive muitiuser structures (temporal

only) prevïously proposed in the Literaîure-

Chapter 2 gave an o v d review ofmdtiuser detection in CDMA systems and adaptive antema

array with the description of some muItÈuser stmctures.

In chapter 3 we developed the linear edaptive sp8ce-time multiuser meber. Andysis of the opti-

mum tap weights and the MMSE in synchronous and asyncbnous CDMA systuns is also gkem

in chapter 3. Closed fomi expesions for the autoaneiation mîrk and the cross-cumIation

vector are giwn m this chapterr- Nimerical r e d t s of the MMSE in synchmnonous and asynchro-

nous CDMA systems is caiculated and a com@son stady is carried out between the linear space-

t h e structure and the hem thne ody sûucture. The d t s show the inherent potentiai advantage

ofthe space-the receiva c o m p d to the time only receiyer.

The decision feedback space-the receiver is proposed in duper 4. Analysis of the optimum tap

weÏghts and the MMSE in MAI and ISI CDMA channeis is also @en in this chapta Comperison

study W e e n the linear and the decision feedback sp8ce-time and t h e only sîructures in MAI

and ISI CDMA diarmels is also @en in chapter 4- Aiaalytid d t s based on the MMSE criteria

showed that the decision feedback spacotime structure has the abiIity to mceI MAI and ISI.

Ais0 we fomd that in very d angle spread channels the decision feedback structure outper-

forms the linear space-thne structure. Howevcr in large angle sprrad there is no advantage of the

decision feedback over the 1i.m space-the structure; the hear space-time receiva wii i be pref-

erable for its simplicity.

Another new structure is pposed in chapter 5, which is d e d the centralized decision feedback

spacefime structure. The d y s i s for the optimum tap weights and the MMSE in MM and ISI

chsvrneis for the caitraiized tecave is &O fien in chapter 5. Analytical results based on the

MMSE for the centralized receiver is drawn in chapter 5. The results showed that the cmtrajized

receivet has the abiiity to cancel the MAI and the ISI. A cornparison study in MAI and ISI CDMA

channets for aü the proposeci sûuctms are done in chapter 5. It is found that the centralized stnic-

ture outpafonns the Iinm and the decision feedback structure, but extra complexity is needed for

the centraiized flters which makes the cen- stmture complexity grow linearly with nimi-

ber of users. Because ofits centmhed naîure, it am only be applied at the base station where the

symbols of all usas are adable.

In chapters 3.4, and 5 we consider the synchronous, asynchronous, MAT, and ISI CDMA chan-

neis. These ChanneIs wae selected to demonstrate the ability and the potential power behind the

spa-time proceshg to mcel MAI and ISI. Howeva, we proposed these structures for wireless

CDMA systems, where in the winless chamieh there an many other impainnents which may

E t the application of any system. Chapter 6 is devoted to shidy the p d o a n c e of the hear

spacetime recenter m fincrPency selective fsading channels, where multipaîh and W g can

degrde the paformance of any system. The anaiysis ofthe optimum tap weights and the MMSE

of the hear space-tmie stmtm in fiequency seiective fadmg chamiek is &en in chapter 6.

Analyticai resuits are ais0 presented in this chapter. F m the d t s we found that the linear

space-time receiva is not &y capab1e ofcanceliag MAI and ISI but aiso has the ability to coUect

the en- of the diffierent rmiltipath components. We found that these stnictures combine the

fimction ofmuitiuser detection, antenna a y s , ISI mer, and RAKE reception in one single

Also hm the cornparison study between the lin- MMSE muitiuser &a (temporal

oniy) and the Iinear spacetime &a we conclude that the space-the receiwr has the ability to

improve the performance of the CDMA system si@cantly.

in cbqtex 7 we st&y pfactid issus relateci to the application of the adqtive linear multiuser

detection to ceIIular wireless systems. We show how the hear spacetime feceiver ovemmes

many of the short comings ofthe lin= MMSE receicer (temporal oniy). First m chapter 7 we dis-

aiss m e issues relatai to the transient behavior of the iinear space-time W e r and compare it

with the hear temporal receiver. Even though the space-the sbmctwe has more coefficients to

adapt, h n the d t s we fiuud that it has a shorta traimng period, adapts Wa, has Iess steady

state MSE, and its t d h g paiod is less sensitive to the numba of interferers than that of the lin-

ear tem@ muitiuser detector. Also results show that with much less bandwidth the linear space-

thne receber stüi has better cornergence chacteristics and steady state MSE.

The effect of a time varying usa popdation and packet transmission is studied in chapter 7. From

these simulations we made three important obsenmtions. The h t is that the combmed space-

time receiver alleviates the e f f i of sudden birth of interferénce and there is no need to tetrain the

detector because the detected symbols are stiU reiiable and the MSE is stül low. The second obser-

vation is that, at start-up, or a f k a new mterference arrives, the filter coef6cÏents converge fastg

to Iowa MSE levels than that of the adaptive h e m MMSE receiver (tirne only), thus reducing the

rrqimed training @O6 The second observation is in complete cansistency with the tiranSient

behavior d t s . F W y , we foimd the space-thne receiver has some memory of the packets of

departed intafaas. New pack& of departed intafaaice wil l cause l e s hami than the first

packet, Fmm the iast observation we conclude that we have to adapt ody for the first packet of

newusas, sowensethewnpaccesspowatechMqaewiththesp~time~to~ucethe

effect of the sudden birth of interference. R d t s show that the enect of sudden birh of interfer-

mce on the MSE codd be redltced firxrther by aImost 7 5 dB by osing the ramp access powa tech-

nique.

In chapter 7 we srrnnine aiso the e f f i of spreading codes selection on the perfioxmmce ofthe lin-

air sp-time receiver.. We found that the h e m space-time receiver is less sensitive to spreading

code selection. This is bexmse the spacetime receÉver &es not depend oniy on the cross-coda-

tion properties among the codes, but on the spatial signature too. This fature can d u c e the corn-

plexity associated with the spreadmg codes plarming and assigrunent

Another issue we study in chapter 7 is the effeçt of out-of-ceil interference. In this experiment we

stuây the &kt of interfierence in neighboring tell that has the same spreadmg code as the desirecl

user. Srnidation results show that the hear space-time receiver can perfectly handle out-of~l i

intafereace having the same code as the desiffd user, whereas [2] fopnd that with the decision

feedback MMSE muitiuser rectiver, the CDMA system designer shouîd not assign the same

codes set to the neighboring cens; otherwise the o v d capacity of the system will be rnuch

lower. AIso with the spacetime structure we eliminate the need for the code planning suggested

by Pl-

ûut-of-cell interference d t s a i s ~ suggest the possi'biiity of using the same code withh the same

œli. Such a feetrire could be very important in systems such as wireless LAN (IEE 802.1 1). In

IEEE 802.1 1 standard the Barka code with 11 chips is used for spnading the data sequence in

wireless LAN. Ali usas use the same Barker code but with ciiffirent shüts, because the Barker

d e has very good auto-corzelation pro-. Howcva, in hpency selectnre fading channels,

dehy spread can crnise many users to have the same code shift. In this situation the performance

wi l l deteriorate. So with the space-thne structures we can reduce this effect

In chapter 7 we study the capacity miprovernent that can be achieved using the hear space-the

receiver. In section 7.6 we fomd that the hear spacatmie receiver with mtenna element offêrs

four h e s the capacity ofthe time ody rewhm. In these d t s we inchie the e f f i of shadow-

hg.

The BER pafonnance was aIso considend m chapter 7, and feSuIts showed consistency with the

MMSE resuIts. It was concludeci that the spaceAme receiver can provide very low BER resalts

for high deta rate transnissioon. This make the space-time structure very attractive for high data

rate systems with packets transrnissiou such as IEEE 802.1 1 or next generation standards.

8 3 Future work The areas of muftiuser detection and smart mtemias have gaineci a lot of attention in the la s few

ycas. These two mas an believed to provide the capacity needed for the futiae wireless systems

which must support di f f r t services such as wice, &ta, and v i b transmission In this thesis

we jomtly combineci the spatial aad temporal domains and proposeci several edaptive space-time

structures. These two area me not M y developed and require a lot ofresearch, h w g this thesis

we found many open problems that need more study in the firture. In this section we desaie

some ofthe these open probIems.

Joint space-time proeessing and power control

Sûict or tight power contrd is an essentid technique for the conventionai DS-CDMA receher to

ova corne the nm-fcar pmblem. The mdtiuser deteaion technique came to deviate the na-fitr

problem, which reduces the complexity associated with the power control design. Howeva,

power control has some f m that make it essential for any systexn. For ewnnpIe, powa control

provides a feedback to the mobile users to a d . th& transmittexi power to maintain equal

receNed powa at the base station. By doing so, each d e user will oniy tnmsmit the needed

power to maintain a certain SNR, which will nsult in extexuied battery life of the mobile and

&ced ovaall MAI in the ceMar system. Cornbined power control and space-time mdtiuser

detection is an intaesting area for futun m h .

Space-the receiver for mrrltirate reception

Next genenîtion mobile commlfnication systems are necessary to support several different com-

munication services, Le. mdtimedia, mice ûansmission, data traasmission, and v ida transmis-

sion. To support these, next grneration systems should handle mdtirate tnmmhion with

aiffkent qpalities of savice.

In maltirate ttansmission different users with ciBi î t data rates wiU anive at the receEver with

dïfFnent received power evai with strict power control. This is because high rate mers rire

assigned more power than low rate users. So m mdtirate ttansraission the near-fhr probIem is

inherent, and the commtionai receiver wii i fâii to handîe multirate transmissio~~

A suggested solution to this problem is the use of multiusa detection and adaptive antenna anay

techniques separately or combmed Om interesthg Miue research is the design and d y s i s of a

multi-rate combined space-time teceiver that can solve and handie Merait rate reception for next

Cornparison study between the Iong code design pWosophy and short code design philoso- P ~ Y

When the IS'95 standard appeared the ody available receNer was the matched filter receiver i-e.

aie comrentional &ver. The research cn multiuser detection techniqpes was just stârted at that

time [104]. The probIem was nicing the IS'95 group was that the comrentioml ncaVer is opti-

mum only in AWGN chamel, i.e. it is not optimum in MAI channe1. To ovamme this problem

the standards group «mie up with the idea of Iong codes to randomize the interference and d e it

look like Gaussian noise. A h many years ofmuitiuser detection research and dwe10pmenî, mul-

tiuser detection matureû, and was found to be a promising technique to hcrease the capacity of

the CDMA systan. But the probIem with IS'95 long codes is that it hinders the use of multiuser

ddon techigues- Due to this probIem the idea of short codes or pulse on code becornes d e -

vant. The long code design is calied random CDMA or R-CDMA and the short code design is

called deterministc CDMA or D-CDMA, [102]. Adaptive multiuser detection techniques is one

of the simplest multiasâ. detedon algonthms end it p v e s to be a pmmising technique [106].

But adaptke multiuser detection rtquues the that the spreading code should be npe8fed ~ n r y

transmitted bit, i.e. D-CDMA. In the fiterature there is a big debate about which design philo*

phy will lead to the Iarger capacity and better @ormance. For example, [102] daims that R-

CDMA has better capacity than D-CDMAu But many other papers prove the opposite [IO71 , [34]. Even though this debate stiiI gomg on thae is no complete standard design for CDMA sys-

tem arcept the one which irtilizes the long code idea. A oomparison stady between these two

designs is very important This study may change our view to any future standard for CDMA sys-

tem.

References

Abdulrahman Mi, A. U. H. Sheikh, D. D. Falconer, "Decision Feedback E q y h a - tion for CDMA in Indoor WmIess Communication," IEEE JSAC, vol. 12, no., 4, pp. 698-706, May 1994.

Abdulrahman M., A, 'PFE for Interfèrence and Multipath Suppression in a CDMA System," PhD thesis, 1994, Department of Systems and Cornputer Engineering, Carleton Universiity, Ottawa, Ontario, Canada

Abd- M., D. D. Falconer, "Cydostationary Crosstslk Suppression by Deci- Sion Feedbacic Equalization on Digital Subscrh Loops," IEEE JSAC, vol. 10, m. 3, ~p.640-649~ Apr 1992.

Alam M, "Pediommce of Muhirate CDMA Transmission in Cellular Environment," MSc. thesis, 1998, Department of Systems and Cornputer Engineering, Carleton University, Ottawa, Ontario, Canada

Anderson S., M. Milhert, M. Viberg, and B. Wahlberg, "An Adaptive Amy for Mobiie Communiication Systems," IEEE Trans. Corn, VOL 40, no. 1, pp. 230-236, Feb 1991.

Balaban P., J. Salz, "Optimum Diversity Combining and Equalization in Digital Transnission with Application to Cellular Mobile Radio -Part 1 Theoreticai Consid- erations," IEEE T m . Comm., vol. 40, no. 5, pp. 885-894, May 1992.

Baiaban P, J. Salz, "Optimum Diversity Combining and Equakation in Digital Tmsmission with Application to Cellular Mobile Radio -Part II Numerical Xesuits," EEE Trans. Coima, VOL 40, no. 5, pp. 885-894, May 1992.

Bang S. C., EL R Park, and Y. Han, cTaformance Anaiysis ofa Wideband CDMA System for FPLh!frS,? W C 97.

sanstein X. and A. M. Haimovich, "Space+T"mie ûptimum Combhhg for CDMA CommpniCSitio~~~," Wmless Personal Comm., VOL 3, Nos. 1-2, pp. 73-89,1996.

[IO] Cox D. C., "[iaiversai Digitai PortabIe Radio Communications," Proceedings of the IEEE, VOL 75, no. 4, pp. 436475, Apr 1987.

[ I l ] Despins C. L, D. D. Fdconer, and Mahmoud S. A., "Compopnd Strategies of Cod- ing, Eqmkation, and Space Diversity for Wide-Band TDMA Indoor Wneless ChanneIsF IEEE TmraEIS Comm., vol. 41, no. 4, pp. 369-379, Nov 1992.

[I2] DuéI-Hden A, "Decorze1atmg Deasion-Feedback Multiuser Detecbr for S y n b nous Code-Division Multiple-Access ChanneIr IEEE T m . Cornm., VOL 41, no. 2, pp. 285-290, Feb 1993.

[13] Duel-Haiim A., "A F d y of MuItiuser Decision-Feedback Detectors for Asyn- chronous Code-Division Multiple-Access m e r IEEE T m . C o n , vol. 43, no. 2/3/4, pp. 42 1-434, Feb/ Mar/ Apr 1993.

1141 Duel-Hallen A, J. Holtzmen, and 2. Zvonar, '"Multiuser Detection for CDMA Sys- tems," IEEE Pemnai Comm, pp. 46-58, Apr 1995.

[15] El-Ezabi A, A Duel-Hallen, "Combined Enor Comction and Multiuser Detection for Rayleigh Fading Synchronous CDMA ChanneIs,"

[ig El-Ezabi A., A Dwl-Hdm, "A Novel Interleaving Scheme for Muitiuser Deteciion of Coded CDMA Systems,"

[17] Fdcona DD, AU Legmah, S. Roy "Receiver Spatial-Temporal Signai Rocessing For Bmadband WmIess Systems", Wted papa to the IEEE PIMRC'2000, Lon- don, UK.

[18] Falconer D. D., M. A b d e b a n , N. W. K. Lo, B. R Petersen, And A. U. H. Sheikh, "Advances in Equalization and Divexsity for Portable W~eless Systems," Digital Si@ RoceSSing, vol. 3, pp. 148-162, 1993.

[19] French R C., The Effect of Fading and Shadowing on CbanneI Reuse in Mobile Radio," IEEE T m . Vehic. Te&, VOL VF28, no. 3, pp. 17 1- 1 8 1, Aug 1 979.

[20] Fulghum T., K. Molnac, A. Duel-Hallen, "The Jakes Fading Mode1 Incorporating Angular Sprrad,n

1211 Grag V. IL, J. E. Wilkes, 'Wueles & Personal Communicatio~s Systems," Prentice HaIl PTR, 1996.

[22] G d o t i s E. A, M. B. Pursley, "Error Probabüity for Direct-Sequence Spread- Spectnun Multiple-Access Communications- Part II: Approximations," IEEE Trans. Co- vol. 30, no. 5, pp. 985-995, May 1982.

[23] Giihousen K. S., L M. Jacobs, R Padowni, A. J. Viterbï, L. A We8veq Jr, and C. E. Wheatky III, "On the Capecity of a CeUttiar CDMA Systenqn IEEE Trams* Vehic. T d , MI. 40, no. 2, pp. 303-312, May 1991.

[24] GIisic S. G., P. k Leppanen, "Code Division Multiple Access Communications," KIpwer AcademÎc Puplishers, 1995.

[a Godara L. C., c ~ p p l i ~ t i ~ ~ of Antenna Arrays to Mobile Communications, Part 1: Perfiomiance Improvement, Fe8~1iîlîty, and Systern Consid~ons,"Proceedin&s of the IEEE, wI. 85, no. 7, pp. 1029-1060, Jd 1997.

[26] Godara L. C., "Applcations of Antenna h y s to MobiIe Communications, Part II: Beam-Fo&g and Direction-of-Anival Considerations," Proceedings of the IEEE, VOL 85, no. 8, pp. 1193-1245, Aug 1997.

[27l Goodman D. 3 , Trends in Cellular and Cordless Communications," IEEE Camm. Miqphe, pp. 31-40, Jme 1991.

1281 H a y h S., "Adaptive Filter Theory? 3rd Edition, b t i c e - H a & 1996.

[29] Haykin S., 'Ritder Amy Rocessing for Angle of Arrivai M o n , " A m y Signal Processing, Chapter 4, Rintice Hall, 1985.

[30] Heeswyk F. D. D. Falwner, and A. U. H. Sheïkh, "Decorrelating Detectors for Quasi-Synchronous CDMA," Wneless Personal Comm,, vol. 3, Nos. 1-2, pp. 129- 147,1996.

[3 IIHoltpnan, J.M., "A Simple, Accmate Method to Calculate Spread S m MdtipIe Access Error Robabiîities," IEEE Internatioal Conference on Conference on Com- munication, pp. 16334636,1991.

[32] Honig M., W. Madhow, and S. Vady "Biind Adaptive Interference Suppression for Nem-Far Resistance CDMA,'' IEEE Globecom 1994.

[33] Honig M. L., M. J. Shensa, S. L. Miller, and L. B. Miistein, 'Taformance of Adap tÏve Linear Interference Suppression for DS-CDMA in The Presence of Flat Ray- leigh Fading," VTC 97.

[34] Honig M. L., and W. Veerakachen, "Performance Vsriability of Linear Multiuser Detection for DS-CDMA," VTC 96.

[35]Honig M. L. "Rapid Detectîon and Suppression of Multi-User Interference in DS- CDMA," Proc. 1995, ICASSP, Detroit, MI, May 1995.

[36] Hosur S., A. H. TewfÏk, and V. G. Moghadam, "Adaptive Multiuser Receiver Schunes for Antenna Amys," IEEE Int. Symp. on Personal, Indoor, and Mobile Conrm, voL 3, pp. 940-944, Sep 1995.

[37 Hudson J. E., cbAdaptive Amy Principks," Peter Pereginus LTD, 198 1.

[38] Huang H., H. V i and GJ. Foschini, 'MidiipIe Antennas in Random Code CDMA Systems: Transmisson, Detection and Sp- Efficiency," WNCN'99, Sep 1999.

[39] Jakes W. C., "Mimwave Mobile Comm~cations," IEEE Press, 1974.

[40] Johason D. H., and D. E. DtIdgeon, "Array Signai Processing Concepts & Tech- niques," Rentice Hall PTR, 1993.

[41] Jmtti M., 'Multiuser Demodulation for DS-CDMA Systems in Fading ChanneIs," PLD. thesis, Department of Elecûicsl Enpsieerins Univetsty ofoula, FinIand.

[42] Kogiaan's A., ' V p W Capacity Studies for Adaptm Antmna Arrays in Thrm Gen- d o n CDMA Wrreless Systems," WCNC'99, Sep 1999.

& h o R., H. Imai, M. Haton, uDesign of Automatic Equaüza hc1uding a Decoder of Enar-Conecting Code,'' IEEE. Tm. Comm, vol. 33, no. 1% Oct 1985.

Kohno R., H. Wang, and H. Imai, "Adaptive Amy Antema Cambined with Tappeù Delay Line Using Rocessing Gain for Spread-Specûmm CDMA Systernsl Roc. 3rd EEE Int Sym. Personal, hdoor and Mobile Radio Cornm., PR 634.638, Oct 1992.

Kohno R., "Aii Aâaptm Cancelier of Cochsvtne1 Interference for Spread-Spectrum Multiple-Access Communkation Networks in a Power Line," IEEE BAC, vol. 8, no. 4, pp. 691499, May 1990.

Kohno R, H. Imai, M. Hatori, and S. Pasupathy, "Combination of an AdaptEve Axray Antenna and a Canceiier of Interference for Direct-Sequence Spread-Spectnim MultipleAccess System," IEEE JSAC, vd. 8, no. 4, pp. 675682, May 1990.

Kohno R, Tpatitial end Tempord Filtering for Co-Channel Interference in CDMA," Code Division MuitipIe Access Communications, S. G. Giisic and P. A. Leppanen, En&., pp. 1 17-146, The Netherlandx Kluwer, 1995.

L e p h M., D.D. Faicona, and AU. Sheikh, " T ' e n t Behavior of the Lin= Adaptive Combined Space-Time Receiver for CDMA Systems," IEEE PIMRC'2000, London, UK.

Legnain AM., DD. Falconer, and A.U. Sheikh, "Catralized Decision Feedback Adaptive Combined Space-rie Detector for CDMA Systems", IEEE International Confèrence on Communication ICC'99.

Legnain AM, D.D. Falmner, and A.U. Sheikh, "Effect of TunecVarying User Popu- lation on the Performance of the Adaptive Combined Sp-Tie Receiver for DS- CDMA sysams", IEEE WCNCt99.

Legnain AM.., D.D. Falconer, and A.U. Sheikh, "Performance of New Adaptive Combined Space-Tnne Receiver for Multiuser hterfefence Rejection in Multipath Slow Fadùig CDMA ChamieW, IEEE GLOBECOM 98.

L- AM., DD. Falconer, and AU. Sheikh, '"Decision Feedback Adsptive Com- bined Space-Tune Receiver for Mdtiuser Intederence Rejection in CDMA Sys- te&', The 10th Internaîional Cod. on Wm1ess Communication (wueIessr98), vol. 1, pp. 164-170, Jiily 1998.

LegnaÏn AU, DD. Falconq and AU. Sheikh, Wear Far Resistance ofNew Adap the Cambhed Space-Tmie R h for M d h Maence Rejection Ki Asyn- chronous CDMA Systems", 19th Bienniai Symp. on Comra, pp. 101-105, Jrm 1998.

Legnain AM., D.D. Falconer, and AU. Sheikh, "New Adaptive Cambined Space- T i e Receivet for MuItiuser Interference Rqection in Synchronous CDMA Sys- temsn, IEEE CCECF98, VOL 1, pp. 421-424, May 1998.

[SS] Lim T., Yu Gong, and B. Farhang-Bomujeny, "Coxmrgence Andysis of LMS Mul- tiuses CDMA Detectors," WCNC399, Sep 1999.

[56] Lin H. C., "Spatial Carrelation in Adaptive Arrays," IEEE T m . Co-, vol. 30, no. 2, Mar 1982,

[57] Lin S. C., "Optimum Diversity Combining with Finite-Tap Decision Feedback Equalizaton in Digital Ceilular Mobile Radio," ICC 97.

[58] Lo. N. W. K., D. D. Fdconer, and A. U. H. Sheikh, "Adaptive Eqyaikation and Diversity Combining for Mobile Radio using interpolated Channel Estimates," IEEE Trans. Vehic. Tah., VOL 40, no. 3, pp. 636-645, Aug 1991.

[59] Lupas R and S. Verdu, "Linear Multiuser Detectors for Synchronous Code Division Multiple Access Cbanneis:' IEEE. Inform. Theory., vol. IT35, no. 1, pp. 123-136, Jan. 1989.

[60] Lupas R and S. Verdu, 'Wear-far Resistance of Mdtiusa Detectors in Asynchro- nous Cha~ek," IEEE T m . Comra, vol. COM-38, no. 4, pp. 496-508, Apr 1990.

[61] Madhow U. and M. L. Honig, '%r Probability and Near-Far Resistance of Mini- mum Mean S q W Error Interference Suppression Schemes for CDMA, "IEEE Globecorn 1992.

[62] Madhow U. and M. L. Honig, "MMSE Interference Suppression for Direct Sequence Spread-Spectnnn CDMA," EEE Tram. Comm., vol. 42, no. 12 pp. 3 178-3 188, Dec 1994.

[63] Martone M., "An Adaptive Array for Wmeless Communication Systmis with Fre- quency Selective Muitipath Fading Using High Orda Statistics Rocessing," ICC 97.

[64] Miller S. L., "An Adaptive Direct-Sequence Code-Division Multiple- Access Receiver for M u l h e r Interference Rejection," TEEE Traa Comm., vol. 43, no. 2/31 4, pp. 17461755, FebJMazJApr. 1995.

[65]Miller S. L., Transient Behavior of the Minimum Meaa-Sqiiared Estor Receiver for Direct Seqgence S p m d Spectnrm CDMA," IEEE 1994.

[66] Monogioudis P., R. TathzoIlî, B. G. Evans, "LFSE interference Canceiiation in CDMA systems," ICC 94.

[67 Monsen P., 'Theoretical and Meamed Performance of DFE Modem on a Fading Muitipath ChaPmei," IEEE Tians. Comm_, MI. 25, no. IO, pp. Il44-LlS3, Ob 1977.

[68] Monsen P., 'Teedback Eqaalization for Fading Dispasive Ciwmeis,'' EEE T m . In£ Theory, pp. 56-64, Jan 1971.

1691 Monsen P., "Adaptive Equalization of the Slow Fading Channeln EEE T'ans. Coma, vol. 22, no. 8, pp. 1064-1075, Aug1974.

[70] Monzhgo R A., Thomas W. Miilet, "Intmktion to Adaptive Anays," John Wiley &Sons, 1980.

[71] Moshavi S., cMuiti-User Detection for DS-CDMA Comm~cations." IEEE Comm. Magazine, pp. 124-136, Oct 1996.

1721 Mydlow M. W., S. Basavaraju, and A. Duel-Hallen, cDecomlatmg Detector with DNersity Combining for Single Usa Ffe~uency-Selm Rayleigh Fading Muiti- path ChanneIs," Wdess Personal Coima, vol. 3, Nos. 1-2, pp. 175493,1996.

[73] Nagui A F., A Paulraj, cTerfbrmance of Wrreless CDMA with M-ary Orthogoaal Modulation and C d Site Anternia Anays," IEEE JSAC, vol. 16, no, 1995.

[74] Nagui A. F., "Spac6.Tnne Receivers for CDMA Multipath Signais," ICC 97.

[75l Naguiia A. F., "Adaptive Antennas for CDMA Wueless Networks,"PhD. thesis, Department of Eledcal Engineering, Stanford University, USA.

[76] ûjanpaa T., P. A. Ranta, S. Hamaben, and k Lappetaiainen, c'Analysis of CDMA and TDMA for 3rd Generation MobiIe M o Systems," W C 97.

[77l ûnoe S., K. Ohno, K. Yamagata, and T. Nakamura, '%deband-CDMA Radio Con- tro1 Techniques for 3rd Generation Systems," W C 97.

[78] Pados D. A., S. N. Batalama, "Low-Camplexity Blind Detection ofDS/CDMA Sig- nais: Awnfiary-Vaor Recekn," IEEE T m . Comm., vol. 45, no. 12, pp. 1586- 1594, Dec 1997.

[79] Pahnaj A. J., and C. B. Papadias, "Space-The Processhg for Wireless Communica- tious," IEEE Signai Proceshg Magazine, pp. 49-83, Nov 1997.

[80] Petasen B. Ry and D. D. Faiconer, ' W u m Mean S v Equalizaton in Cyclos- tationary and Stationary Interference- Andysis and Subscri'ber Line Calculations," lEEE JSAC, vol. 9, no. 6, pp. 931-940, Aug 1991.

[8 11 Pillai S. U., "Anay Signal Processmg," Springer-Virlag New York Inc, 1989.

[82] Proakis J. G., "Digital Commecation," 3rd Edition, MaGraw-HilI, 1995.

[83] Pursley M. B., cTgformance Evaluation for Phase-Coded S p d Spectrum Multi- ple-Access Communication- Part I: System Analysis," IEEE. Tnins. Cornia, vol. 25, no. 8, pp. 795-799, Aug 1977.

[84] Pursley M. B., 'Terfonnance E-on for Phase-Coded Spread Spectrum Mdti- pie-Access Communication- Part II: Coded Seqpence AnaIysis," IEEE. T m . colma, VOL 25, no. 8, pp. 800-803, Aug 1977.

[85] Pursîey M. B., D. Sarwate, and W. E. Stark, "Error Probability for Direct- Sequence Spread-Spectctm MaltipIe-Access Cornm~cations- Part 1: Upper and Lowa Bounds," IEEE Tm. Conrm, vol, 3 4 no. 5, pp. 975-984, May 1982.

[86] Rappapoa T. S., "WireIess CommUnicatio~~~ Rïncipks & Btce.'' Rentice HaiI PTR, 1996,

[871 Rapajic P. B., and B. S. Vucetic, "Adaptive Structures for Asynchronous CDMA Systems," IEEE JSAC, VOL 12, no. 4, pp. 685697, May 1994.

1881 Rong Z., T. S. Rappaport, P. Petrus, and J. H. Reed, "Simuiation of Multitarget Adaptive Awy Algorithms for Wdess CDMA Systems," VTC 97.

[89] Rong Z., P. Petrus, 'L S. Rqpaport, and J. H, Reed, "Despread-Respread Multi-Tm- get Constant M0d.tlius Array for CDMA System," IEEE Conm. Lenes, vol. 1, no. 4, pp. 1141 16, Jd 1997.

[go] Saleh A M., and R Valenniela, "A statisticai Model for Indoor Multipath Propaga- tion," IEEE JSAC, vol. 5, no. 2, pp. 128-137, Feb 1987.

[91] Shah A, A. M. Haimovich, "Bit Error Computations for Space-Tie Narrow-Band Interfe~enœ Suppression in CDMA Comm~catiox1s," ICC 97.

[92] Simon M. K, J. K Omiw, R A* SchoItz, and B. K. Levitt, "Spread Spectnmi Com- munications Handbook," RevÏsed Edition, McGraw-Hill Inc, 1994.

1931 Spencer Q., M. Rice, B. JeS , and M. Jensen, "Indoor Wideband Time/AngIe of Amval Multipath Propagation R d t s , " W C 97.

1941 Spencer Q., M. Rice, B. Jeffs, and M. Jensen, "A statistical Model for Angle of Arrivai in Indoor Multipath Ropagation,'' VTC 97.

[95] Stuber G. L., "Principles of Mobile CommUIiication," Uuwer Academic Pubîishem, 1996.

[96] Subramanian S ., "A Multiple-Anttnna-Multiple-Equalizer Sy stem for CDMA Indoor W~eless Systems," P U . thesis, Department of Electrical and Cornputer Engineering, University of -ria Britch Columbia, Canada

[97] Tan& S., M. Sasvahashi, and F. Adachi, ''Pilot Symbol-Assistai Decision-Directeci Coherent Adaptive Array Dntersity for DS-CDMA Mobile Radio Reverse Link,'' WrreIess 97.

[98] Ttain G. L., 'Inttoduction to Sm-Spectrum Antimuitipath Techniques and Theh Application to Urban Digital Radio," Roceedings of the IEEE, vol. 68, no. 3, pp. 328-353, Ma 1980.

[99] Va Etim W., "An optimum Lineou Receiver for MaltipIe Channel Digital Transmis- sion Systems," IEEE T m . Coinm, pp. 828-834, Aug 1975.

[ l O O ] V d M K and B. Aazhmg, "MuItistage Detedion in Asynchronous Code Divi- sion Multiple Access Communications," IEEE T m . Comm., voI. 38, no. 4, pp. 509-519, Apl 1990.

[lOl]Vanmasi M. K. and B. Aazhang, Wear-optimum Detection in Synchronous Code Dmsion Multiple Access Systems." IEEE Trsns. Cornia, VOL 39, no. 5, pp. 725- 736, May 1991.

[102]V'bu S. and A. J. Viterbi, 'Two Mirent Philosophies in CDMA - A Cornpari- son," W C 96.

[103]V& S., ''Minimm Probability of Error for Asynchronous Gaussain Multiple Acœss Chan neis," IEEE T m . 1Iiifonn. Theory, vol. IT-32, pp. 85-96, Jan. 1986.

[iW]V'erdu S., "Optimum Muitiuser Asymptotic Efficient$' IEEE T m . Comrn., vol. 34, no. 9, pp. 890-897, Sep 1986.

[105lVerdu S., "Muitiuser Deteztion," Ad~821ces in statistical signal ptocessing, vol. 2, pp 369-409, JAI Press 1993.

[IOq Vcrdu S., "Adaptive Multiuser Ddection" Code Division Miiltple Access Commu- nications, S. G. Giisic and P. A. Leppanen, Eds., pp. 97-1 16, The NetherIanà9: Klu- wer, 1995.

[10'I]V'du S., "Demodulation in the Resence of Multiuser Interfience: Rogress and Misconceptions," InteUigent Methods in Signal Processing and Communications, A Figueira-Vidai and F. Perez-Godez, pp. 15-44, BWuser, Bosten: 1997.

[los] Westman T., H. Holma, "CDMA System for üMTS High Bit Rate Services,'' VTC 97.

[109]Wmters J. H., " O p h u m Combining for Indoor Radio Systems with Multiple Users," IEEE T m . Conun., vol. 35, no. 11, pp. 1222-1230, Nov 1987

[I 101 Wmters J. H., "Signal AcquÏstion end Tracking with Adaptive Atrays in the Digitai Mobile Radio System IS-54 with Flat Fading," IEEE T m . Vehic. Tech., vol. 42, no. 4, pp. 377-384, Nov 1993.

[l il] Wmters J. H, The impact of Antenna Diversity on the Capacity of WveIess Com- munication Systems," IEEE T m . Comm., vol. 42, no. 2/314, pp. 1740-1750, Febl Mar/Apt 1994.

[I 12'JWintets J. H, "ûptmium Combining in Digital Mobile Radio with Cochaanel inter- ference," IEEE JSAC, MI. 2, no. 4, pp. 528-539, J d 1984.

[l 13]m~1ters J. H., "On the C a p e ofRadio Cornmupication Systans with Diversity in Rayleigh Fading Envh~nent," IEEE BAC, vol. 5, no. 5, pp. 871-878, Jun 1987.

[1 l4]Xie Z., R T. S h a and C. K. Rnshforth, "A Famüy of Suboptimmn Detaors for Coherent Multiusa Comxnunicatio11~," IEEE JSAC, VOL 8, no. 4, pp. 68349% May 1990.