Principles of 3G (CDMA), 4G (OFDMA) and 5G Part-A · PDF filePrinciples of 3G (CDMA), 4G...

Principles of3G (CDMA), 4G (OFDMA) and 5G

Part-A

Professor A. Manikas

Imperial College London

EE303

Prof. A. Manikas (Imperial College) EE303: 3G, 4G and 5G (Part A) v16 1 / 50

Table of Contents1 Introduction - 3G, 4G, 5G Mobile Cellular Systems

3GPPMobile EvolutionBasic Elements of Mobile Cellular SystemsFDD and TDDChannel Reuse and Reuse DistanceSignal Overlay

2 3G (CDMA)3G: Specifications3G: DS-QPSK implementation No.1 (QPSK1)3G: DS-QPSK implementation No.2 (QPSK2)3G: Channelisation Codes & Scrambling Codes3G: Base Station Tx3G: Handset TxSSS and CDMABasics of CDMABasic Properties of CDMA SystemsDS-CDMA: Synchronization

Analysis of a DS BPSK CDMA SystemModelling and AnalysisSNIRout as a function of EUE, Nc and KBER as a function of EUE, Nc and KBPSK Examples

Some Important CDMA System ComponentsPower ControlVoice Activity FactorSectorization


Introduction - 3G, 4G, 5G Mobile Cellular Systems 3GPP

Introduction - 3G, 4G, 5G Mobile Cellular SystemsThe 3GPP (3’d Generation Partnership Project )

I developed the 3G standarsI developed the 4G standardsI is developing standards towards next generation (5G)

3GPP expanded in 2011 to include 380 individual membercompanies and 5 Standards Development Organisations as shownbelow


Introduction - 3G, 4G, 5G Mobile Cellular Systems Mobile Evolution

Mobile Evolution - Motivation


Introduction - 3G, 4G, 5G Mobile Cellular Systems Mobile Evolution

Mobile Evolution - Motivation (cont.)


Introduction - 3G, 4G, 5G Mobile Cellular Systems Basic Elements of Mobile Cellular Systems

Basic Elements of Mobile Cellular Systems

A mobile cellular system consists ofI base stations ,I cells (a cell is the area serviced by a base station) andI mobiles (subscribers).

When a call originates, the base station negotiates with the mobile onvarious aspects (such as the channel used etc.), before establishingcommunications. After this, as the mobile moves from cell-to-cell, theservice is handed (hand-off or handover ) from one base station toanother.

Only one base station (BS) will service a mobile at any one time.

Note:I base station to mobile is known as FORWARD LINK (or Downlink)I mobile to base station is known as REVERSE LINK (or Uplink)


Introduction - 3G, 4G, 5G Mobile Cellular Systems Basic Elements of Mobile Cellular Systems

Type of channels:

Examples of uplink and downlink channels:

UPLINK DOWNLINKTraffi c Channel Traffi c ChannelAccess Channel Pilot Channel

Synchron.ChannelPaging Channel


Introduction - 3G, 4G, 5G Mobile Cellular Systems FDD and TDD

FDD and TDD

Based of the spectrum used in "uplink" and "downling", there aretwo systems

I FDD (Frequency Division Duplex), andI TDD (Time Division Duplex)I Definitions:

FDD: TDD:


Introduction - 3G, 4G, 5G Mobile Cellular Systems Channel Reuse and Reuse Distance

Channel Reuse and Reuse Distance

Most cellular systems use frequency division multiplex access (FDMA)and/or time division multiplex access (TDMA) technique to improvethe system capacity.

Example: 2G cellular systems = FDMA/TDMA

In these systems, each user is assigned one time/frequency-slot.I When the system gets larger,

time/frequency-slots 6= unique for each and every user

as this will limit the system capacity. Therefore these slots(time/frequency) have to be reused

I This implies that there is interference from other cells sharing thesame channels.



Cluster= the number of cells which have unique time/freq-slots foreach and every user.

I Available cluster sizes: 1,3,4,7,9,12,. . .

cluster size=3 cluster size=7

I The reuse distance D, in these systems, is determined by the worstcase interference situation.

I D is defined as the distance between the centers of two co-channelcells.

D = R√3C (1)

where C = cluster size

R = cell radiusProf. A. Manikas (Imperial College) EE303: 3G, 4G and 5G (Part A) v16 10 / 50


cluster size: 4 cluster size: 7



D is known as the reuse distance of the system.

Remember: The system capacity could be increased by increasingthe number of channels available in a single cell, i.e. reducing thereuse distance D.

But this reduction is limited by the co-channel interference, (i.e. theinterference from other cells sharing the same channels).

In a CDMA system , the available spectrum and time are not splitinto distinct slots. Instead the whole (available) spectrum is used byeach user.

I Since the same frequency channel could be used by all theusers/subscribers in a cell, the cluster size could be reduced to 1, i.e.

if CDMA then cluster = 1 cell and D = R√3 (2)


Introduction - 3G, 4G, 5G Mobile Cellular Systems Signal Overlay

Signal OverlayThe spread spectrum signal, from a CDMA system, has a very low powerspectral density and, therefore, a CDMA system can overlay on topof 2G narrow-band mobile cellular systems (of the same frequency band).This is because the interference (due to CDMA signals), added to anarrow-band mobile system channel, is very low and, therefore,the presence of CDMA signal will hardly affect the performance of thenarrow-band mobile system.The CDMA system, however, needs to perform some extra processingto reject the narrowband interference due to the presence of thenarrow-band signals.Comment:The capacity and performance of a mobile cellular system could besignificantly improved by using CDMA techniques. In the paper “On theCapacity of a Cellular CDMA”, IEEE Transactions on VehicularTechnology, Vol.40, 1991 (by Gilhousen et al) the improvement in thecapacity is discussed and it is stated that “no other proposed schemeappears to even approach this (CDMA) performance”.


3G (CDMA) 3G: Specifications

3G: Specifications

IS-95 UMTS

Generation 2G (USA) 3G (EU)

Type CDMA W-CDMA

Frame duration 20msec 10msec

Uplink

25MHz︷︸︸︷824− 849MHz︸︷︷︸

mod:QPSK

60MHz︷︸︸︷1920− 1980MHz︸︷︷︸

mod:balanced QPSK, i.e. QPSK2

Downlink

25MHz︷︸︸︷869− 894MHz︸︷︷︸

mod:oQPSK

60MHz︷︸︸︷2110− 2170MHz︸︷︷︸

mod:dual QPSK, i.e. QPSK1

Bss 1.23MHz 5MHz,10MHz,20MHz

Tc 813.8ns 244ns,122ns, 61ns

rc = 1Tc

1.2288Mchipssec 4.096Mchipssec (reduced to 3.084Mchipssec )

roll-off-factor 0.22;Note: Bss = 1Tc(1+roll-off-factor)=4.99712MHz'5MHz

Receiver’s type RAKE RAKE


3G (CDMA) 3G: DS-QPSK implementation No.1 (QPSK1)

3G: DS-QPSK implementation No.1 (QPSK1)

(Note: if =Tc2 delay is inserted then OQPSK1− offset QPSK1)

Transmitter:

m(t)S/P

m (t)I

m (t)Q

b (t)I

b (t)Q

h(t)

h(t)

P . (2 F t)s ccos π

P . (2 F t)s csin π

s (t)I

s (t)Q

s(t)

s(t) =√2Ps . cos

(2πFc t + tan−1

bQ (t).mQ (t)bI (t).mI (t)

)Prof. A. Manikas (Imperial College) EE303: 3G, 4G and 5G (Part A) v16 15 / 50


Receiver:

r(t)

2cos(2 F t)π c

2sin(2 F t)π c

b (t)I

(n )T1 cs

nTcs

h(t)

h(t) b (t)Q

P/S

(n )T1 cs

nTcs



3G: DS-QPSK implementation No.2 (QPSK2)

(Note: if =Tc2 delay is inserted then OQPSK2 )

Transmitter:

m(t) b (t)I

b (t)Q

h(t)

h(t)

P . (2 F t)s ccos π

P . (2 F t)s csin π

s (t)I

s (t)Q

s(t)

s(t) =√2Ps . cos

(2πFc t + tan−1

bQ (t).m(t)bI (t).m(t)

)



Receiver:

(n )T1 cs

nTcsr(t)

2cos(2 F t)π c

2sin(2 F t)π c

b (t)Ih(t)

h(t) b (t)Q


3G (CDMA) 3G: Channelisation Codes & Scrambling Codes

3G: Channelisation Codes & Scrambling Codes

In 3G WCDMA:I PN-codes = channelisation codes + scrambling codes

Channelisation codes :I are used to differentiate between users/channels within a cellI also known as OVSF (Orthogonal Variable Spreading Factor) codesI these are the Walsh codes (see next page)

Scambling codes:I to differentiate between cellsI these are m-sequence or gold-codes



IS95 - Walsh Function of order-64


3G (CDMA) 3G: Base Station Tx

3G: Base Station Tx


3G (CDMA) 3G: Handset Tx

3G: Handset Tx


3G (CDMA) SSS and CDMA

(a) SSS: (b) CDMA (K users):



The PN signal b(t) is a function of a PN sequence of ±1’s {α[n]}

I The sequences {α[n]} must agreed upon in advance by Tx and Rx andthey have status of password.

I This implies that :F knowledge of {α[n]}⇒demodulation=possibleF without knowledge of {α[n]}⇒demod.=very diffi cult

I If {α[n]} (i.e. “password”) is purely random, with no mathematicalstructure, then

F without knowledge of {α[n]}⇒demodulation=impossible

I However all practical random sequences have some periodic structure.This means:

α[n] = α[n+Nc ] (3)

where Nc =period of sequencei.e. pseudo-random sequence (PN-sequence)



Remember

DS-SSS (Examples: DS-BPSK, DS-QPSK):

b(t) = ∑n

α[n].c(t − nTc ) (4)

where {α[n]} is a sequence of ±1’s;c(t) is an energy signal of duration Tc

FH-SSS (Examples: FH-FSK)

b(t) = ∑nexp {j(2πk[n]F1t + φ[n])} .c(t − nTc ) (5)

where {k[n]} is a sequence of integers such that

{α[n]} 7→ {k[n]} (6)

and {α[n]} is a sequence of ±1’s;c(t) is an energy signal of duration Tc


3G (CDMA) Basics of CDMA

Basics of CDMABLOCK DIAGRAM



Example: DS-BPSK CDMA System

SISO = Scalar-Input Scalar-Output Channel



SISO Multipath Channel

SISO Multipath channel of the i-th user

In the absence of multipaths the above diagram has only τi1 and βi1terms.

For the simplicity we will drop the second subscript and we will use τiand βi ,and thus the BPSK/DS-CDMA in the absence of multipathsmay be represented as follows:↘



Basic Properties of CDMA Systems

CDMA is one of the applications of spread spectrum communicationswhich is used in civilian, commercial and military communication.

Two systems: DS-CDMA (i.e averaging system) and FH-CDMA (i.e.avoidance system).In this course only DS-CDMA will be considered.

Assign a specific PN-code to each user

PN-code (having the status of ‘password’) acts like a ‘channel’

DS-CDMA: two main casesI PN-signal period = NcTc = Tcs (known as ‘short codes ’CDMA)I PN-signal period = NcTc � Tcs (known as ‘long codes ’CDMA)



DS-CDMA: two main types1 synchronous DS-CDMA

i-th user

j-th user2 non-synchronous DS-CDMA

i-th user

j-th user



DS-CDMA: SynchronizationThe Rx requires a replica of the PN code, with the correct clockphase, in order to despread the signal.Therefore, Rx =“synchronization circuits”+ “demod. circuits”The process of synchronizing the receiver to the transmitter’s PNcode consists of two stages:

I Acquisition (coarse synchronization).I Tracking (fine synchronization).

Operation: acquisition ; tracking + demodulation; loose tracking ;acquisition ; tracking+demodulation; ......etc..........


3G (CDMA) Analysis of a DS BPSK CDMA System

Analysis of a DS BPSK CDMA Systemobjective: to relate the BER pe with the total number of users K aswell as with the EUEequ at the receiver.i.e.

pe = f{EUEequ,K} (7)

Main AssumptionsI single cell system of K users,I @ multipathsI PN code period = Nc =PG=TcsTcI System=perfectly power-controlled(all SS signals arrive at the receiver with the same power)

F NB: power control can often be implemented in practice with greataccuracy.

I System = totally asynchronous (there is no common timing referencefor the transmitters/users)

F NB: This is actually an advantage of CDMA over other multiple accesstechniques, because all users can transmit independently and nosignalling information is required.



DS/BPSK CDMA System: Modelling and Analysis

Note that the carrier of i th transmitter is√Pi . exp (j(2πFc t + ψi ))



i-th user’s data signal mi (t) and PN-signal bi (t):mi (t) ≡ ∑

nai [n]. c1 (t − n.Tcs ) ; nTcs < t ≤ (n+ 1)Tcs

bi (t) = ∑k

αi [k ]. c2 (t − kTc) ; kTc < t ≤ (k + 1)Tc

with Tcs = NcTc ;⌊kNc

⌋= n;

(8)

The period of each user’s PN-sequence is selected as Nc = TcsTc, and

therefore there is one code period per data bit (or Nc chips per bit).Thus, for the BPSK case, the processing gain PG is:

PG = Nc =TcsTc

(9)

The transmitted signal si (t) of the i-th user is therefore

si (t) =√Pi .mi (t).bi (t). exp (j (2πFc t + ψi )) (10)

where Fc is assumed common for all carriers.



SNIRout as a function of EUE, Nc and KFor a perfectly synchronised system , at the ouput of the correlator(i.e. G1) we have:

SNIRout ≈ 2.EUEequ = 2Eb

Nj +N0(11)

However,

Nj = (K − 1).PBss

= (K − 1).P.Tc =(K − 1).

=Eb︷︸︸︷PTcs

Nc=(K − 1).Eb

Nc(12)

Therefore

SNIRout ≈ 2.EUEequ

= 2Eb

(K−1).EbNc

+N0=

1K−12Nc

+ 12 EUE

(13)



BER as a function of EUE, Nc and K

We have seen that

SNIRout ≈{K − 12Nc

+1

2 EUE

}−1(14)

However,

pe =T{√SNIRout

}(15)

which implies that

pe = T{√

2EUEequ}= T

1√K−12Nc

+ 12 EUE

(16)



BPSK Examples


3G (CDMA) Some Important CDMA System Components

Power Control

In order to achieve the full benefits of using CDMA, the transmittedsignal powers Psi should be controlled in such a way that receivedsignal power, from all the users at a cell, are the same.This makes power control a key feature of CDMA mobile systems.



Voice Activity Factor

Human speech contains a lot of pauses where there is no data totransmit.Thus a speaker is active for about half the time due to listening andpauses in speech.

The fraction of time that a speaker is active is known asthe voice activity factor aExtensive studies have shown that 0.35 < a < 0.5.A popular value used is a = 3/8 = 0.375The voice activity feature can be taken into account in acommunication system by suppressing the transmission when voice isabsent.Assuming that we have a scheme where the carrier is turned-offduring the speech idle periods then a reduction in interference (bya factor of the voice activity) can be achieved.



implementation of voice activity:

{TDMA/FDMA: very diffi cult

CDMA: very easy

For a large number of users the capacity increases by a factor 1/a.Therefore, using the voice activity monitoring approach the capacityand the performance of a CDMA system will be improved(this improvement cannot be obtained in FDMA/TDMA systems)

In particular the power of a user’s signal at a specific time instant canbe expressed 1× Puser with probability a and 0× Puser withprobability (1− a).Using voice activity the performance can be improved even more.

BPSK : SNIRout = 2.EUEequ = 2Eb

Nj +N0(17)

where Nj =(K − 1).Ps .a

Bss(18)



Therefore we can model the “on-off”activity of each user a binomialdistribution, which implies that the probability that k user (out of K )are active is given as follows:

Pr(k users are active) =(kK

)ak (1− a)K−k (19)

where K is the number of users per cell

Note that as K =↑⇒spread of distribution=↓Set a threshold Kth , εth such that:

Pr(number of active users > Kth) < εth (20)



Sectorization

It is used in TDMA/FDMA and CDMA systems



Sectorization

Sectorization is achieved by using directional antennas instead ofomnidirectional antennas.

Each cell is divided to three sectors using three directionalantennas each having 120◦ beamwidth.Using sectorization the performance can be improved even more.(The expected value of the total interference is reduced by a factor ofs = 3wrt single omnidirectional antenna case)

BPSK : SNIRout = 2.EUEequ = 2Eb

Nj +N0(21)

where Nj =(K − 1).Ps .s

Bss(22)

In practice: 3 dB<SNIRout < 15dB



A better approach is to use three linear antenna arrays (smartantennas)

antenna array switched beam array system adaptive array system


Principles of 3G (CDMA), 4G (OFDMA) and 5G Part-A · PDF filePrinciples of 3G (CDMA), 4G...

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