1. Transmit diversity for MIMO-OFDM
November 2003
Transmit diversity for MIMO-OFDM
Sumeet Sandhu, Minnie Ho
Communications Technology Lab
Intel Corporation
November 2003
Sumeet Sandhu, Intel Corporation

2. Overview Diversity # 1 : OSTBC
November 2003
Overview
Diversity # 1 : OSTBC
Orthogonal space-time block codes (e.g Alamouti)
Diversity # 2 : CDD
Cyclic delay diversity
Sumeet Sandhu, Intel Corporation

3. November 2003
System #1 OSTBC
On the kth tone, Alamouti code on 2 transmit antennas and 2 OFDM symbols
x x2
x2* -x1*
Channel vector at nth receive antenna
hn,k (2 x 1)
Equivalent SISO channel after receiver processing
|hn,k|
Full diversity !
Sumeet Sandhu, Intel Corporation

4. November 2003
System #2 CDD
Signal on 2 transmit antennas with N=12-point FFT, cyclic prefix=4, cyclic delay d=2
( x9 x10 x11 x12 ) ( x1 x2 x3 x4 x5 x6 x7 x8 x9 x10 x11 x12 )
( x7 x8 x9 x10 ) ( x11 x12 x1 x2 x3 x4 x5 x6 x7 x8 x9 x10 )
Channel vector at nth receive antenna and kth tone
hn,k (2 x 1)
Equivalent SISO channel
w * hn,k , where w = [1 exp(j*2*pi*d*k/N)]
Sumeet Sandhu, Intel Corporation

5. OSTBC vs CDD : 2 transmit antennas
November 2003
OSTBC vs CDD : 2 transmit antennas
0.9 dB for 2x3
2.2 dB for 2x1/2
Should channel model be standard (50 ns rms) ?? Gains look too high and show Alamouti in good light
Sumeet Sandhu, Intel Corporation

6. OSTBC vs CDD : 6 Mbps vs 54 Mbps
November 2003
OSTBC vs CDD : 6 Mbps vs 54 Mbps
1.6 dB for 6 Mbps
3 dB for 54 Mbps
May need to do 40 MHz simulations (to check gain of Alamouti), may need to include channel estimation errors
Sumeet Sandhu, Intel Corporation

7. OSTBC : 4 transmit antennas
November 2003
OSTBC : 4 transmit antennas
Rate 3/4 orthogonal space-time block code
Provides full order 4 diversity
Sumeet Sandhu, Intel Corporation

8. OSTBC vs CDD : 4 transmit antennas
November 2003
OSTBC vs CDD : 4 transmit antennas
-0.5 dB
Sumeet Sandhu, Intel Corporation

9. Performance summary 1 receive antenna Alamouti gains 1.5-3 dB over CDD
November 2003
Performance summary
1 receive antenna
Alamouti gains dB over CDD
3 receive antennas
Alamouti gains <1 dB
40 MHz instead of 20 MHz
Gain likely to be smaller
Channel estimation errors
Alamouti likely to be more sensitive
Change numbers
Sumeet Sandhu, Intel Corporation

10. Practical issues Scalability Alamouti : not scalable with Tx antennas
November 2003
Practical issues
Scalability
Alamouti : not scalable with Tx antennas
CDD : easily scalable for 1-4 Tx antennas
Backward compatibility
Alamouti : requires HT receivers
CDD : transparent to legacy devices
Latency
Alamouti : 1 OFDM symbol delay, even # padding
CDD : no additional delay
Make bigger case of decoding delay and SIFS
Sumeet Sandhu, Intel Corporation

11. Conclusions Alamouti Pros : optimal performance
November 2003
Conclusions
Alamouti
Pros : optimal performance
Cons : incompatible with 11a, decoding delay (SIFS)
CDD
Pros : 11a compatible, transparent to Rx, scalable
Cons : sub optimal performance
Question for TGn
How important is range in a mixed n network with heterogeneous MIMO/SISO devices ?
Sumeet Sandhu, Intel Corporation