Chapter 9 Multi-Carrier CDMA · 2016. 5. 22. · ¾Orthogonality restoring combining ... ¾3.Equal...

NCCU Wireless Comm. Lab.

Chapter 9 Multi-Carrier CDMA

NCCU Wireless Comm. Lab.2

Chapter 99 Multi-carrier CDMA

9.1 Introduction9.2 Family of Multi-carrier CDMA Systems

9.2.1 MC-CDMA System9.2.2 Multi-carrier DS-CDMA System9.2.3 Multi-Tone CDMA System9.2.4 System features comparison

9.3 Differences between OFDM and MC-CDMA


9.1 IntroductionTransmit high data rate in a mobile environment

Multi-carrier Transmission

Multi-carrier transmission and OFDMMulti-carrier Transmission

Multi-carrier conceptOrthogonality

OFDMUsing FFT device without increasing the transmitter and receivercomplexitiesHigh spectral efficiency due to minimally densely subcarrier spacing


9.1 IntroductionMulticarrier Concept

1/T

(A)

(E)

(D)

(C)

(B)

1/T

fc+f1 fc+f2 fc+f3 fc+f5fc+f4

Orthogonal

Non-orthogonal

Orthogonal, n=2

Orthogonal, n=3

Orthogonal, n=1(OFDM)


9.1 IntroductionOrthogonality

Definition Time domain Frequency domain

Bandpass signal

where is the equivalent lowpass signal of xm(t)

if ,n is non-zero integer, i.e. , then .

*1 2( ) ( ) 0x t x t d t

∞

−∞

=∫ *1 2( ) ( ) 0X f X f df

∞

−∞

=∫

( ) ( )2 ( ) 2( ) cos(2 ( ) ) Re Re ( )c mi f f t i fctm c m lmx t f f t e x t eπ ππ += + = = ⋅

2( ) mi f tlmx t e π=

1 2 1 22 2 2 ( )*12

0 0

sin( )( )T T

i f t i f t i f f t i fTfTe e dt e dt ef

π π π ππγπ

− ∆∆= = =

∆∫ ∫fT n∆ = nf

T∆ = 12 0γ =

⇔


9.1 IntroductionThree types of multiple access schemes

Combination of CDMA and Multi-carrier modulation techniques

MC-CDMA SystemMulti-carrier DS-CDMA SystemMulti-Tone CDMA System


Frequency domain spreading + multi-carrier modulationMC-CDMA scheme

Time domain spreading + multi-carrier modulationMulti-carrier DS-CDMA schemeMT-CDMA scheme

9.2 Family of Multi-carrier CDMA Systems


9.2 Family of Multi-carrier CDMA SystemsFrequency domain and Time domain spreading

t1 3

2 4

5

6

7

8

9

10

ff1 f10f9f8f7f6f5f4f3f2

carriers

13

24

5

6

7

8

9

10

f1

f10

f9

f8

f7

f6

f5

f4

f3

f2

f

t

a1

a1

a1

a1

a1

a1

a1

a1

a1

a1


9.2.1 MC-CDMA SystemMC-CDMA System

Frequency domain spreadingThe resulting spectrum of each subcarrier can satisfy the orthogonality condition with the minimum frequency separation.In a (synchronous) down-link mobile radio communication channel, we can use Hadamard Walsh codes as an optimum orthogonal set.It can be implemented by OFDM technique.It’s a potential candidate for the 4th wireless communication system.



1

S

fT

∆ =


9.2.2 Multi-carrier DS-CDMA SystemMulti-carrier DS-CDMA system

Time domain spreadingThe resulting spectrum of each subcarrier can satisfy the orthogonality condition with the minimum frequency separation.This scheme can lower the data rate in each subcarrier so that a large chip time make it easier to synchronize the spreading sequences.The multicarrier DS-CDMA scheme is originally proposed for a up-link communication channel, because this characteristic is effective for establishment of a (quasi-) synchronous channel.

c MDN G=


9.2.2 Multi-carrier DS-CDMA SystemMulti-carrier DS-CDMA system

' 1

C

fT

∆ =

C SC

DS

N TTG

=


9.2.3 Multi-Tone CDMA SystemMulti-Tone CDMA (MT-CDMA) system

Time domain spreadingThe resulting spectrum of each subcarrier no longer satisfies the orthogonality condition.The MT-CDMA scheme uses longer spreading codes in proportion to the number of subcarriers, as compared with a normal DS-CDMA scheme.The MT-CDMA system can accommodate more users than the DS-CDMA system.

c MDN G<


9.2.4 System Features ComparisonSystem features comparison


9.2.4 System Features ComparisonSystem features comparison

MC-CDMA Bw=1/Ts , B=(Nc+1)/Ts

Multi-carrier DS-CDMA Bw= GDS/(NcTs), B= (Nc+1) × GDS/(NcTs)

MT-CDMA Bw= GDS/Ts , B=2 GDS/Ts +(Nc-1)/NcTs

f1 f2 fNcBw f1 f2 fNcBw


9.2.4 System Features ComparisonReceiver structures

DS-CDMA schemeMC-CDMA scheme

9 different structuresMulticarrier DS-CDMA

Compose of Nc normal coherent (Non-Rake) receiversMT-CDMA

Compose of Nc Rake combinersDetection strategies comparison


9.2.4 System Features ComparisonDiversity and combining schemes

Diversity

CombinerSelection diversityEqual gain combiningmaximum ratio combiningOrthogonality restoring combiningMMSE combining……….

q1

Combiner

r(t)q2

q3

qn



MC-CDMA system 1.Orthogonality Restoring Combining (ORC)

choosing the gain qm as

Noise enhancement

, zm is the complex envelope of the m-th subcarrier



MC-CDMA system 2.Controlled Equalization (CE)

(ORC)



MC-CDMA system 3.Equal Gain Combining (EGC) and Maximum Ratio Combining (MRC)

MRC maximizes the output SNR



MC-CDMA system 4.Minimum Mean Square Error Combining (MMSEC)



MC-CDMA system 5.Maximum Likelihood Multi-User Detection (MLMUD)

Find a set of aj,(j=1,2,….,J) to minimize the following likelihood function Λ



MC-CDMA system 6.EGC-EGC Multi-User Detection

First using EGC to estimate a set of aj,(j=1,2,….,J,j≠j’)And then estimates the aj’ by EGC after removing the MAI from the received signal



MC-CDMA system

7.ORC-MRC Multi-User Detection (O-M MUD)

8. CE-ML Multi-User Detection (C-M MUD)

9.Decorrelating and MMSE interference Canceller (DIC and MMSEIC)



MT-CDMA system

DFE - Decision Feedback Equalizer

LE - Linear Equalizer

JEIC - Joint multiple access Interference Canceller/Equlizer


9.2.4 System Features ComparisonDetection strategies comparison


9.2.4 System Features ComparisonBit error rate comparison

BER lower bound

BER of an MC-FDMA


9.2.4 System Features ComparisonBER performance


9.3 Differences between OFDM and MC-CDMAMC-CDMA system is a CDMA based on a combination of CDMA and OFDM signaling.

OFDM-CDMAMC-CDMA can be implemented by OFDM technique.


9.3 Differences between OFDM and MC-CDMAMC-CDMA spreads the signal in the frequency domain according to the code.MC-CDMA can view as employing the frequency diversity method.MC-CDMA performs better than DS-CDMA in Downlink level. But performs even worse in Uplink level.MC-CDMA has gained much attention, because the signal can b easily transmitted and received using the fast Fourier transform (FFT) device without increasing the transmitter and receiver complexities and it is potentially robust to channel frequency selectivity with a good frequency use efficiency.


[1]Richard van Nee Ramjee Prasad, OFDM Wireless Multimedia Communication, Artech House Boston London [2]Ahmad R.S.Bahai and Burton R. Saltzberg Multi-carrier Digital Communications Theory and Applications of OFDM, Kluwer Academic/Plenum Publishers.[3] L. Hanzo, W. Webb and T. Keller, Single- and multi-carrier quadrature amplitude modulation –Principles and applications for personal communications, WLANs and broadcasting, John Wiley & Sons, Ltd, 2000. [4] rasad, R.; Hara, SP. “An overview of multi-carrier CDMA,” Spread Spectrum Techniques and Applications Proceedings, 1996., IEEE 4th International Symposium on , Volume: 1 , 1996 Page(s): 107 -114 vol.1

Chapter 9 Multi-Carrier CDMA · 2016. 5. 22. · ¾Orthogonality restoring combining ... ¾3.Equal...

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