18/5/00 p. 1 Postacademic Course on Telecommunications Module-3 Transmission Marc Moonen Lecture-9...

Module-3 Transmission Marc MoonenLecture-9 CDMA K.U.Leuven/ESAT-SISTA

18/5/00p. 1

Postacademic Course on Telecommunications

Module-3 : Transmission

Lecture-9 (18/5/00)

Marc Moonen

Dept. E.E./ESAT, K.U.Leuven

[email protected]

www.esat.kuleuven.ac.be/sista/~moonen/


Module-3 Transmission Marc MoonenLecture-9 CDMA K.U.Leuven-ESAT/SISTA

18/5/00p. 2

Lecture 9 : CDMA -Overview

• Multiple Access Techniques

TDMA

FDMA

CDMA : Frequency hopping vs. direct sequence CDMA

SDMA : See also Lecture-10• DS-CDMA

Single-user (matched filter) receiver, RAKE receiver

Code design, synchronization, near-far problem,.. • Multi-user detection based CDMA Receivers• CDMA/DMT combined schemes



18/5/00p. 3

CDMA Introduction

• Pioneer CDMA era:

1949-… : first contributions by Pierce, Shannon,…

1956 : `RAKE’ patent (Price & Green)

1970-… : military CDMA systems• Narrow-band CDMA era:

1978-… : CDMA for cellular applications

1993 : IS-95 standard (Qualcomm)

1996-… : IS-95 commercial operation (US, Korea, …)• Wide-band CDMA era:

1995-… : Research programs in Europe, USA, Japan

2000-… : Commercial operation UMTS/IMT2000



18/5/00p. 4

CDMA Introduction

• 1st generation cellular/mobile: -analog transmission -speech services -AMPS (US), TACS (UK), NMT (Scandinavia), NTT (Japan)• 2nd generation cellular/mobile: -digital transmission -speech & data services -IS-136/D-AMPS (US), GSM, PDC, IS-95• 3rd generation cellular/mobile: -higher bit-rates, multiple services, etc. -wide-band CDMA air interface

-UMTS (ETSI), IMT2000 (ITU)



18/5/00p. 5

Multiple Access Techniques

• Multiple Access Schemes : allow different users to access/share the same communication channel.

• Underlying principle = òrthogonality’

)( if 0

if zero-non)().(

ji

jitsts ji

sent signal for user j



18/5/00p. 6


Multiple Access Schemes :• Frequency Division (FDMA)• Time Division (TDMA)• Code Division (CDMA)

- Frequency Hopping (FH-CDMA)

- Direct Sequence (DS-CDMA)

• Space Division (SDMA) see Lecture-10



18/5/00p. 7


Frequency Division Multiple Access (FDMA)• Different users use passband modulation (Lecture-3) with

different carrier frequencies • Different frequency bands separated by guard bands

(-> òrthogonality’)

• PS: Compare with DMT/OFDM (Lecture-7/8). • PPS: DMT/OFDM sometimes also used as MA technique.

:

user-1

user-2

user-M

CHANNEL:

out-1

out-2

out-M



18/5/00p. 8


Time Division Multiple Access (TDMA)• Different users use different time slots in one and the same

communication channel • Different time slots separated by guard times

(->òrthogonality’)

:

user-1

user-2

user-M

CHANNEL

:

out-1

out-2

out-M



18/5/00p. 9


TDMA/FDMA• Example GSM :

- 125 frequency channels (in 900MHz band)

- 8 users time-multiplexed in each channel

- neighboring cells use different frequency bands to avoid inter-cell interference (hence actual number of frequency bands in a cell 7 times lower…)

frequency

time

FDMAfrequency

time

TDMA



18/5/00p. 10


Code Division Multiple Access (CDMA)• Frequency Hopping (FH-CDMA) :

- Carrier frequency for user-i is changed for each time slot,

based on periodic pseudo-random code sequence for user-i.

- `Fast’ versus `slow’ frequency hopped systems

`Fast’ = several hops over one symbol period

`Slow’ =several symbols transmitted during one hop

• Time Hopping (TH-CDMA) :

- Periodic pseudo-random code sequence for user-i defines

transmission moment for user-i.

FH-CDMA and TH-CDMA not further addressed here...



18/5/00p. 11


Code Division Multiple Access (CDMA)• Direct Sequence CDMA (DS-CDMA) :

• Relevance : Third Generation wide-band CDMA proposals

frequency

time

CDMA

code



18/5/00p. 12


Code Division Multiple Access (CDMA)• Direct Sequence CDMA (DS-CDMA) :

-Each user-i is assigned a periodic (period N) pseudo-random code sequence

-For each symbol (k-th symbol for user-i), a `chip’

sequence is transmitted

-Mostly binary codes ( ) with BPSK/QPSK symbols

-Multiple access based on code-orthogonality (see below)

ika

1,1 ilc

iN

iii cccc ,...,,, 321

iN

ik

iik

iik

iik cacacaca .,...,.,.,. 321



18/5/00p. 13


Direct Sequence CDMA (DS-CDMA)

• Transmission : Example :

transmitted symbols +1……… -1……… -1……… +1………

code sequence +1,+1,-1

transmitted chips +1,+1,-1, -1,-1,+1, -1,-1,+1, +1,+1,-1

• N = `spreading factor’ = # chips per symbol

• Symbol rate = fs, `chip rate’ = fc = N.fs



18/5/00p. 14


Direct Sequence CDMA (DS-CDMA)• Transmission block scheme:

• code-multiplication may be viewed as (digital) filtering operation, with FIR transmit filter

• Chip sequence is fed into (analog) transmit filter p(t) for transmission (not shown) (see Lecture-3)

1121 .....)( Ni

Nii zczcczC

iiii cccc 4321 ,,,

,..., 1ik

ik aa

,....,. ,.,.,.,. 21114321ii

kii

kii

kii

kii

kii

k cacacacacaca

N

,...0,0,0,,0,0,0, 1ik

ik aa

C(z)

N-fold upsampling



18/5/00p. 15


Direct Sequence CDMA (DS-CDMA) • Reception : if received signal = transmitted chip sequence,

multiply chips with (synchronized) code sequence + sum.

• Example :

transmitted symbols +1……… -1……… -1……… +1………

code sequence +1,+1,-1

transmitted chips +1,+1,-1, -1,-1,+1, -1,-1,+1, +1,+1,-1

received chips +1,+1,-1, -1,-1,+1, -1,-1,+1, +1,+1,-1

+1,+1,-1

received symbols +1……… -1……… -1……… +1………



18/5/00p. 16


Direct Sequence CDMA (DS-CDMA)• Reception block scheme: basic operation is `matched filter’

+ downsampling (=symbol-rate sampling) (see Lecture-3)

,....,.,.,.,.,. 21114321ii

kii

kii

kii

kii

kii

k cacacacacaca

,..., 1ik

ik aa

N

,*,*,*,....,*,*,*,. 1ik

ik aa

C(1/z)

N-fold downsampling

24

23

22

21

31

22

134

3 )()()()( )....()1

( iiiiiiii cccczczczcczz

C



18/5/00p. 17


Direct Sequence CDMA (DS-CDMA)• Multiple access based on code orthogonality

,....,.,.,.,.,. 21114321jj

kjj

kjj

kjj

kjj

kjj

k cacacacacaca

,...0,0

N

,*,*,*,....0,*,*,*,.0 1jk

jk aa

C(1/z)

N-fold downsampling

)....()1

( 31

22

134

3 zczczcczz

C iiii

)( 0...... 2211 jicccccc iN

jN

ijij

chip sequence user-j

code sequence user-i



18/5/00p. 18


Direct Sequence CDMA (DS-CDMA)• Need for code synchronization (acquisition+tracking)

procedure : not addressed here.• Reception for -Asynchronous CDMA -Dispersive channel (hence inter-symbol and/or inter- chip-interference (ISI/ICI), multi-user-interference (MUI))

needs more refined techniques…….see below• PS : here we only consider `short codes’ 1 code period for each symbol in practice also `long codes’ (`scrambling codes’) code period >> symbol period



18/5/00p. 19


CDMA• PS : Direct sequence spreading and frequency hopping are `Spread

Spectrum’ (SS) techniques, where transmission bandwidth for every signal is much larger than information bandwidth. SS Techniques originally developed for military radar and communication systems because of robustness against (narrow band) jamming.

• PS: CDMA application examples :

- IS-95 cellular telephony (Qualcomm)

- IMT2000-UMTS (`wideband CDMA’)

- IEEE 802.11 wireless LANs

- GPS

- cable modems, power line comms, ….



18/5/00p. 20


Space Division Multiple Access (SDMA)- Different users are separated based on spatial properties

- Hence different users can use the same time slots and/or

frequencies, or the same codes (= SDMA on top of

TDMA/FDMA or CDMA)



18/5/00p. 21


Space Division Multiple Access (SDMA)

- Signal separation based on

-Beamforming techniques (`1G’)

-Advanced (multi-path) channel modeling & signal

processing techniques

- May be viewed as `dynamic sectorization’

(currently only `static sectorization’ used (e.g. GSM), based on directional antennas)

- See Lecture-10



18/5/00p. 22

DS-CDMA

General CDMA setup, with several users + noise :

• Code orthogonality is

• Receiver structure a la page 13 (with code orthogonality) sufficient/optimal ?

,..., 1ik

ik aa

N )(zCi )(zH i

channel noise

N )(zC j )(zH j

,..., 1jk

jk aa received signal

)( 0...... 2211 jicccccc iN

jN

ijij



18/5/00p. 23

DS-CDMA : Single-User Receiver

`Single-user’ (`matched filter’) receiver :

• Aim is to suppress inter-symbol-interference (ISI for user-of-interest i) and multi-user interference (from other users)

noise

,..., 1ik

ik aa

N )(zCi

N )(zC j

,..., 1jk

jk aa

)(zH i

channel

)(zH j

N)1

(z

Ci



18/5/00p. 24


`Single-user’ (`matched filter’) receiver : case-1

• if Hi(z)=Hj(z)=1 (no channel dispersion & synchronous)

- MUI=0 because of code-orthogonality

- ISI=0 because no channel dispersion

Hence optimal receiver structure (in the sense of Lecture-4)

noise

,..., 1ik

ik aa

N )(zCi

N )(zC j

,..., 1jk

jk aa

1

channel

1

N)1

(z

Ci



18/5/00p. 25



• if Hi(z)=1, Hj(z)=pure delay (e.g asynchronous CDMA)

- MUI=0 only if code cross-correlations are zero :

Hence need for proper code design (or need for other receiver)

noise

,..., 1ik

ik aa

N )(zCi

N )(zC j

,..., 1jk

jk aa

1

channel

z

N)1

(z

Ci

)( 0..... 2211 jicccc ijij



18/5/00p. 26


`Single-user’ (`matched filter’) receiver : case-2 (ctnd)

• if but , then MUI may still ruin performance (=`near-far problem’).

• Solutions: - Power control

- `Near-far’ resistant receiver.

noise,..., 1

ik

ik aa

N )(zCi

N )(zC j

,..., 1jk

jk aa

channel

z.

N)1

(z

Ci

)( 0..... 2211 jicccc ijij



18/5/00p. 27



• if Hj(z)=0 (hence no MUI), but Hi(z)=multipath propagation

- ISI=0 only if code auto-correlations are zero :

Hence need for proper code design (or need for other receiver)

noise

,..., 1ik

ik aa

N )(zCi

N )(zC j

,..., 1jk

jk aa

)(zH i

channel

0

N)1

(z

Ci

0..... 2211 iiii cccc



18/5/00p. 28


`Single-user’ (`matched filter’) receiver : case-3• alternative structure is a `true’ matched filter receiver:

• This is the `RAKE receiver’ (mostly used in practice)

noise

,..., 1ik

ik aa

N )(zCi

N )(zC j

,..., 1jk

jk aa

)(zH i

channel

N)1

().1

(z

Hz

C ii



18/5/00p. 29


`Single-user’ (`matched filter’) receiver : case-3• RAKE receiver example :

• One `finger’ for each channel tap. Each `finger’ draws energy from one channel component (reflection).

noise

,..., 1ik

ik aa

N )(zCi

N )(zC j

,..., 1jk

jk aa

110 . zhh

channel

)1

(z

Ci N

1z

)1

(z

Ci N

1h

0h



18/5/00p. 30


Conclusion :• Single-user receivers o.k. for synchronous DS-

CDMA with mild multi-path effects• Requires proper code design, with suitably small

auto-correlations & cross-correlations (see below)• Near-far problem• More general receivers : Multi-user detection

(MUD) based receivers (see below)



18/5/00p. 31

DS-CDMA : Code Design

• `Pseudo-random’ codes : deterministic sequences with noise-like (statistical) properties

• Example : Maximum length codes

-linear shift register : binary & length n

-EXOR operation on selected bits

-code repetition period is 12 n

1 52 3 output=code4

example: 5-stage, [2,3]



18/5/00p. 32


• Example : Maximum length codes, 4-stage, [3,4]

1 2 3 4

state 1000 output 0 0100 0 0010 0 1001 1 1100 0 0110 0 1011 1 0101 1 1010 0 1101 1 1110 0 1111 1 0111 1 0011 1 0001 1



18/5/00p. 33


• Example : Maximum length codes, 4-stage, [3,4]

properties:

- # 1-outputs (8) = # 0-outputs (7) + 1

- 4 runs of length 1

2 runs of length 2

1 run of length 3

1 run of length 4

properties always hold for max.length codes



18/5/00p. 34


• Example: Maximum length codes

auto-correlation : (cyclic)

cross-correlation :

worst-case cross-correlation in a `family’ of

codes (same n), does not go to zero for large n

ps: for this, replace {0,1} by {-1,+1} (BPSK)

0)1(2 mod for 12

10)1(2 mod for 1

)( nn

n

,

iiR



18/5/00p. 35


• Other (improved) codes :

Gold Codes

Kasami Codes

Barker Codes

Walsh-Hadamard Codes

….



18/5/00p. 36


• Example : IS-95 (CDMA/TDMA)

- Carrier spacing (bandwidth) 1.25 MHz

- Chip rate 1.2288 Mchips/sec

- QPSK/O-QPSK modulation

- Walsh codes (length 64) to separate physical channels

- Long code of length (2^42-1) for baseband

data-scrambling

- Long code of length (2^15-1) for quadrature spreading

(one for I, one for Q)

- Soft handover

- Power control



18/5/00p. 37

DS-CDMA : Multi-user detection

• Problem Statement : optimal receiver structure for the general case (asynchronous and/or multipath and/or near-far…) ??

• Basic principles are those of Lecture-4 :

minimum-distance receiver, matched filter front-

ends and/or Nyquist-rate sampling front-end, etc..

,..., 1ik

ik aa

N )(zCi )(zH i

channel noise

N )(zC j )(zH j

,..., 1jk

jk aa received signal



18/5/00p. 38


• Nyquist-rate sampling is chip-rate sampling (if excess bandwidth sufficiently small)

• Matched filter front-end consists of a bank of matched filters, one for each active user.

-> Need to know all the user-codes : only @ base station

,..., 1ik

ik aa

N )(zCi

N )(zC j

,..., 1jk

jk aa

)(zH i

channel

N)1

(z

Ci

)(zH j

noise

N)1

(z

C j

MU

-det

ecti

on



18/5/00p. 39


• Minimum-distance receiver : possible, but complexity is major impediment

(exponential in number of users !)

• Other :

Zero-forcing (ISI/MUI)

MMSE

SIC (`serial interference cancellation’)

PIC (`parallel interference cancellation’)

= Active research area, vast recent literature & considered for W-CDMA...



18/5/00p. 40

CDMA/DMT Combined Schemes

Several schemes have been presented that combine CDMA features with DMT features:

• Example-1 : multicarrier-CDMA (MC-CDMA)

first spreading, then different chips on different carriers

• Example-2 : MC-DS-CDMA

frequences for different carriers spread by 1 and the same

spreading sequence • Example-3 : MT-CDMA

first DMT, then time domain spreading

Pros & Cons : see literature...



18/5/00p. 41


Example-1 : multicarrier-CDMA (MC-CDMA)

[Yee, Linnartz en Fettweis] [Fazel en Papke]



18/5/00p. 42


• Example-2 : MC-DS-CDMA [DaSilva en Sousa]



18/5/00p. 43


• Example-3 : MT-CDMA [Vandendorpe]



18/5/00p. 44

Other...

• Block Spreading (`chip interleaving’) : [Cirpan en Tsatsanis]

for a code sequence

transmit a block of symbols multiplied by ,

then the same block multiplied by , etc…

Leads to simpler (block) channel models.

• DMT-CDMA with block spreading

• etc.

Conclusion : Active research area, more to come...

,...,, 321iii ccc

ic2

ic1



18/5/00p. 45

Conclusions

• Multiple Access :

- TDMA/FDMA/CDMA/SDMA

• DS-CDMA:

- Single-user receiver structures (RAKE) :

mostly used in practice

- Code design

- Multi-user detection receivers

- CDMA/DMT

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18/5/00 p. 1 Postacademic Course on Telecommunications Module-3 Transmission Marc Moonen Lecture-9...

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