Comment on the Pre-coding of STC for 3&4 Transmit Antennas
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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: (613)-765-7723 Brian Johnson wentong@nortelnetworks.com 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 ...
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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
wentong@nortelnetworks.com
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.
