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10thMCM-NoviSad,23-24March2006 Joaquim Bastos([email protected]) 1

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Equalization Schemes forExtended Alamouti Codes in

MC-CDMA Systems

J. Bastos and A. Gameiro

Instituto de Telecomunicaes

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IST 4MORE Project

The main objective of IST 4MORE project is to implement a costeffective low power System on Chip (SoC) solution for a 4G mobileterminal employing multiple antennas, based on MC-CDMAtechniques. 4MORE is a joint European project involving:

IETR

http://www.ist-4more.org
http://www.dlr.de/http://www.ee.surrey.ac.uk/CCSR/http://www.av.it.pt/http://www.gaps.ssr.upm.es/http://www.nokia.com/http://www.mitsubishi-electric-itce.fr/http://www.francetelecom.com/http://www.st.com/http://www-leti.cea.fr/

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Overview

Introduction

Double Alamouti

System model

Decoding and Equalization

Simulations setup

Results

Conclusion

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Introduction (I)

CDMA benefits from SS andreusable single frequency.

OFDM robustness againstfrequency selective fading.

MC-CDMA considered strongcandidate for B3G systems.

Space-Time coding enhanceshigh capacity of MIMO systems.

MIMO schemes are key solutionforcapacitylimitationimposedbyMAI on CDMA based systems.

The popular Alamouti schemebenefits both from space and

time diversity, at the expense ofmoderate complexity.

Promising combination for

emerging mobilecommunication systems.

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Space-Time coding is used to combat fading by exploiting diversity.

The popular Alamouti STBC (2 Tx, 1 Rx) can be extended throughreplication, allowing to reduce the constellation size of the modulationscheme used by the system while keeping the same data throughput:

Lower constellation sizes will allow better tolerance to phase errorsand automatic gain compensation;

The data throughput of a system using Double Alamouti (4 Tx, 2 Rx)and QPSK modulation is the same as for a system implementingstandard Alamouti, using 16-QAM modulation;

Spectral efficiency can be kept while reducing constellation size, atthe expense of a higher number of elements in both Tx and Rx

antenna arrays; On the other hand, keeping the same constellation size allows to

have twice the data throughput, without needing further bandwidth.

Introduction (II)

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Double Alamouti scheme (4x2)

STC 1Alamouti

s1 s2

STC 2Alamouti

s3 s4

s1 -s2*

s2 s1*

t0 t1

s3 -s4*

s4 s3*

ATx1

ATx2

ATx3

ATx4

ARx2

Double AlamoutiDecoding w/ ZF or

MMSE criteria

ARx1

s1 s2 s3 s4r1,t0 r1,t1

r2,t0 r2,t1

si

r1,t0=s1h1,1(t)+s2h2,1(t)+s3h3,1(t)+s4h4,1(t)+n1(t)

r1,t1 =-s2*h1,1(t+1)+s1*h2,1(t+1)-s4*h3,1(t+1)+s3*h4,1(t+1)+n1(t+1)

r2,t0=s1h1,2(t)+s2h2,2(t)+s3h3,2(t)+s4h4,2(t)+n2(t)

r2,t1 =-s2*h1,2(t+1)+s1*h2,2(t+1)-s4*h3,2(t+1)+s3*h4,2(t+1)+n2(t+1)

HTx,Rx

Received signals

after MIMO channel

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System model

This simulation chain represents the system model for the Downlink:

Convolutional encoding using UMTS code (Rc=);

Channel interleaving according to UMTS procedures;

MIMO channel implemented according 3GPP Spatial Channel Model;

STBC is achieved by MIMO Encoding, Decoding and Equalization.

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Double Alamouti decoding (I)

At the receiver, the two signals need to be decoded, and for that, theymust be combined taking into account the Double Alamouti scheme.

Considering the following expression, we implemented two ways toobtain the appropriate estimates of the transmitted symbols. Thesewere based on ZF and MMSE criteria, respectively.

ZF:

MMSE:

RCS .~ 1

RICCCS cHH ..~ 12

NSCR .

4

3

2

1

4

3

2

1

*2,3

*2,4

*2,1

*2,2

*1,3

*1,4

*1,1

*1,2

2,42,32,22,1

1,41,31,21,1

*

*.

1,2

1,1

0,2

0,1

n

n

n

n

s

s

s

s

hhhh

hhhh

hhhh

hhhh

r

r

r

r

t

t

t

t

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Double Alamouti decoding (II)

After decoding operation it is still necessary to perform symbol

equalization given that the chips, referring to each transmittedsymbol, were each exposed to dissimilar conditions as thepropagation channel is not flat-fading.

Filtering is necessary to equalize the received OFDM signal,demodulated and STBC decoded. There are several choices for non-

linear equalization. We propose in this work EGC and MRC: EGC: gi= wii

*/|wii| ; MRC: gi = wii

*/ceqTi .

W=T C (T is the transformation matrix used earlier in decoding:

ZF: T=C-1 ; MMSE: T=CH(C.CH+c2

I)-1 .)

Total variance per subcarrier:(assuming Kis high)

4

1

224

1

222

ijj

ija

j

ijcc wKteqTi

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Simulations setup

Carrier Frequency 5.2 GHz

Bandwidth 61.44 MHz

Spreading Factor 32 (Walsh-Hadamard codes)

Number of Subcarriers 1024 (672 used)

OFDM Symbol Duration 21.5 s (24 OFDM Symbols per frame)

Guard Period 4.2 sAntenna Spacing BS: 10 MT: /2

Number of Antenna Elements DAl.: 4 Tx, 2 Rx SAl.: 2 Tx, 2 Rx

Modulation (constellation size) DAl.: QPSK (2) SAl.: 16-QAM (4)

Coding Rate 0.5

Mobile Terminal Speed 60 km/h

Channel

Model

Spatial 3GPP Urban Macro

Time ETSI BRAN E

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Results

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Conclusion

An extension of the Alamouti scheme was proposed in this work, and

its performance was compared with the original scheme. In order tocompare them properly, similar spectral efficiencies were considered.

The attained results show that Double Alamouti scheme associatedwith QPSK, MMSE based decoding and EGC or MRC equalizationprovide quite similar performance as Single Alamouti with 16-QAM

and MMSE equalization. Using ZF based decoding associated with any proposed equalization

techniques provides better performance than Single Alamouti withMRC equalization, only when the existing SNR is higher than 5 dB.

The proposed scheme should be an interesting option when its extra

complexity isnt a setback for system accomplishment. It can allowthe system to use more tolerant modulation schemes, like QPSK,while preserving data throughput, bandwidth and performance.

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