Comment on the Pre-coding of STC for 3&4 Transmit Antennas

Wen Tong, Peiying Zhu, Dongsheng Yu, Ming Jia, Jianglei Ma, Mo-Han Fong, Hang Zhang, Brian Johnson

Description

2004-08-24 IEEE C802.16e-04/403 Project IEEE 802.16 Broadband Wireless Access Working Group Title Comment on the Pre-coding of STC for 3&4 Transmit Antennas Date 2004-08-24 Submitted Wen Tong, Peiying Zhu, Dongsheng Yu, Ming Source: Voice: (613)-763-1315 Jia, Jianglei Ma ,Mo-Han Fong, Hang Zhang and Fax: [phone] Brian Johnson [e-mail] Nortel Networks 3500 Carling Avenue Ottawa, ON. K2H 8E9 CANADA 3 and 4 antennas pre-coding contributions Re: To highlight and compare the performance simulation results for 3 and 4 antenna pre-coding Abstract To provide the more simulation results on the rate-1 3 and 4 antenna pre-coding technique Purpose This document has been prepared to assist IEEE 802.16. It is offered as a basis for discussion Notice and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. The contributor grants a free, irrevocable license to the IEEE to incorporate material contained Release in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part ...

Informations

• Publié par: Vewyur
• Nombre de lectures: 21
• Langue: English

Extrait

2004-08-24

IEEE C802

.

16e-04/403

Project

IEEE 802.16 Broadband Wireless Access Working Group <

http://ieee802.org/16

>

Title

Comment on the Pre-coding of STC for 3&4 Transmit Antennas

Date

Submitted

2004-08-24

Source:

Wen Tong, Peiying Zhu, Dongsheng Yu, Ming

Jia,

Jianglei Ma

,Mo-Han Fong, Hang Zhang and

Brian Johnson

Nortel Networks

3500 Carling Avenue

Ottawa, ON. K2H 8E9

CANADA

Voice: (613)-763-1315

Fax:

(613)-765-7723

[e-mail]

Re:

3 and 4 antennas pre-coding contributions

Abstract

To highlight and compare the performance simulation results for 3 and 4 antenna pre-coding

Purpose

To provide the more simulation results on the rate-1 3 and 4 antenna pre-coding technique

Notice

This document has been prepared to assist IEEE 802.16. It is offered as a basis for discussion

and is not binding on the contributing individual(s) or organization(s). The material in this

document is subject to change in form and content after further study. The contributor(s)

reserve(s) the right to add, amend or withdraw material contained herein.

Release

The contributor grants a free, irrevocable license to the IEEE to incorporate material contained

in this contribution, and any modifications thereof, in the creation of an IEEE Standards

publication; to copyright in the IEEE’s name any IEEE Standards publication even though it

may include portions of this contribution; and at the IEEE’s sole discretion to permit others to

reproduce in whole or in part the resulting IEEE Standards publication. The contributor also

acknowledges and accepts that this contribution may be made public by IEEE 802.16.

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2004-08-25

IEEE C80216e-04/403

1

Comment on the Pre-coding of Space Time Code for 3&4

Transmit Antennas

1

Introduction

Several contributions proposed the pre-coding for the space-time-coding transmission for 4-Tx and 3-Tx rate-1 code.

These class of the non-linear pre-coding is based on the re-mapping of the real part and imaginary part of the modulation

symbols onto the space time coding constellations, namely the Matrix-A. Such a mapping or pre-coding is designed to

achieve the full diversity and full rate code at space time coding level only. However, when applying this technique into

the specific end-to-end coding modulation system of OFDMA PHY, the gain of the pre-coding is diminished.

1.1 Calibration at Raw Bit Error Rate

In order to perform cross comparison of different simulation results, we first verify the performance at raw BER. In Figure

1, the published data and the simulation data obtained are matched well. It shown that at the raw BER level the pre-coding

scheme does demonstrate the diversity order 4 and the provided additional gain over the matrix-A.

Comparison of Rate-1 Four Transmit Space Time Codes

PU S C , 2-subchannels, ITU -PB , QPSK

1 .0 0 E-0 5

1 .0 0 E-0 4

1 .0 0 E-0 3

1 .0 0 E-0 2

1 .0 0 E-0 1

1 .0 0 E+0 0

5

7

9

1 1

1 3

1 5

1 7

1 9

2 1

2 3

2 5

SNR (dB)

raw BER

Be c e e m da ta (P re -c ode ha rd-de c is ion)

Be c e e m da ta (M a trix A only ha rd-de c is ion)

IEEE8 0 2 .1 6 -2 0 0 4 3 k m /h

Be c e e m 3 k m /h

Figure 1 Raw BER Performance for the Pre-Coding

However, as we can show in the following that such diversity gain can be diminished if the performance is evaluated at

the FEC decoder output, full diversity gain achieved by the pre-coding is diminished. This is due to the fact that coding

and modulation chain built in the OFDMA PHY can extract the diversity via sub-channel permutation, bit-interleave and

FEC decoder.

1.2 Channel Weighted Soft De-mapping

In addition to the channel bit-interleave, at the FEC decoder side, the soft de-mapping of the bit should be

weighted by MIMO channel information, this has a significant impact over the diversity performance of the

FEC output. (see e.g. Proakis: ”Digital Communications”)

.

2004-08-25

IEEE C80216e-04/403

2

Comparison of Rate-1 Four T ransmit Space T ime Codes

PUSC, 2-subchannels, ITU-PB, QPSK, CTC, R=3/4

1.00E-04

1.00E-03

1.00E-02

1.00E-01

1.00E+00

0.00

2.00

4.00

6.00

8.00

10.00

12.00

SNR (dB)

BLER

Demap w ith H w eight

Demap w ithout H w eight

Beceem data (Pre-code hard-decision)

Beceem data (Matrix A only hard-decision)

Figure 2 Channel W eighted Soft Demapping Impact on Diversity Performance

As we can see the hard decision FEC suffers the diversity loss and the simple channel weighted soft demapping should be

implemented when comparing the different FEC coded STC methods.

1.3 Bit Interleave

The bit-interleave as a basic building block should also incorporated into the performance evaluation. As we can see the

bit-interleave process can provide and extract the temporal diversity gain. As can be seen from Figure 3, @BLER=1%, we

have the following conclusions for the 4 transmit rate-1 space time code:

1.

There is about 1dB bit interleave gain for pre-coded matrix-A and

2.

There is more than 1.5dB bit interleave gain for the basic matrix-A

3.

The performance of pre-coded matrix-A has only 0.3dB gain over the basic matrix-A

4.

For code rate lower and ¾ the gain of pre-coding is diminished

1.4 Complexity of Decoding of Pre-coded Matrix-A

In all the simulations for the pre-coding scheme in this contribution, the true MLD decoding is employed. Due

to the constellation rotation at transmitter, the MLD decoder can not enjoy the simple slicing to perform the soft

bit QAM demapping, the received constellation de-rotation prior to the QAM demapping is sub-optimal and

suffers performance loss, therefore, the MLD decoder for the pre-coding can be only achieved by computing the

Euclidean distance. This drastically increases the de-mapping complexity as compared the non-pre-coded

matrix-A where the simplest bit slicing can be used for optimal QAM demapping.

2004-08-25

IEEE C80216e-04/403

3

Comparison of Rate-1 Four Transmit Space Time Codes

PUSC, 2-subchannels, ITU-PB, QPSK, CTC, R=3/4

1.00E-03

1.00E-02

1.00E-01

1.00E+00

2

3

4

5

6

7

8

9

10

11

12

SNR (dB)

BLER

IEEE802.16-2004 (w ith bit-interleave)

Samsung

Beceem data (pre-code)

Beceem data (Matrix-A only)

IEEE802.16-2004 (no bit interleave)

Beceem

Figure 3

Performance comparison of the pre-coded and non-pre-coded matrix-A

1.5 Comparison of 2/3/4 Transmit Antenna Rate-1 STC Performance

For the rate-1 space time code matrix-A, @BLER=1%, 3-antenna matrix-A has only 0.2 dB gain over 2-

transmit matrix-A, while 4-antenna matrix-A has about 1.5dB gain over 2-antenna matrix-A. See Figure 4, as

we can see, there is not much room for the full-rate full diversity space time code, such as pre-coding to achieve

significant performance enhancement for 3 antenna and 4 antenna case.

Comparison of Rate-1 2/3/4-T ransmit Space T ime Codes

PUSC , 2-subchannels, ITU -PB , QPSK , C TC , R =1/2

1.00E-04

1.00E-03

1.00E-02

1.00E-01

1.00E+00

0

2

4

6

8

10

12

SNR (dB)

BLER

4x1 Matrix-A

3x1 Matrix-A

2x1 Matrix-A

B eceem 3x1 STFTD

Figure 4 comparison of the rate-1 STC for 4/3/2 transmit antenna cases

2004-08-25

IEEE C80216e-04/403

4

2

Conclusions

The diversity gain of full-rate full-diversity rate-1 space time pre-coding diminishes when the performance is

evaluated at CTC decoder output. The added complexity for the pre-coding at both encoder and decoder (in

particular for the QAM demapping) is significant and does not have the merit to tradeoff with the performance

benefit.