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

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

November 2003 System #1 OSTBC On the kth tone, Alamouti code on 2 transmit antennas and 2 OFDM symbols x1 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

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

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

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

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

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

Performance summary 1 receive antenna Alamouti gains 1.5-3 dB over CDD November 2003 Performance summary 1 receive antenna Alamouti gains 1.5-3 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

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

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 802.11n network with heterogeneous MIMO/SISO devices ? Sumeet Sandhu, Intel Corporation