1. doc.: IEEE 802.11-04/0075r1 Submission January 2004 H. Sampath, R. Narasimhan, Marvell SemiconductorSlide 1 Advantages and Drawbacks of Circular Delay Diversity for MIMO-OFDM Hemanth Sampath Ravi Narasimhan hsampath@marvell.com ravin@marvell.com Marvell Semiconductor, Inc. hsampath@marvell.com ravin@marvell.com

2. doc.: IEEE 802.11-04/0075r1 Submission January 2004 H. Sampath, R. Narasimhan, Marvell SemiconductorSlide 2 Circular Delay Diversity Signal on the k th Tx antenna is circularly delayed by t k samples. To obtain full Tx-diversity, we must have t k > effective channel delay spread [Gore & Sandhu -2002]

3. doc.: IEEE 802.11-04/0075r1 Submission January 2004 H. Sampath, R. Narasimhan, Marvell SemiconductorSlide 3 Frequency Domain Representation MR x MT channel H(f) collapses to a MR x 1 channel hr(f) at the receiver. hr (f) = h 1 (f) + h 2 (f) exp(-j2  t 2 f / N) + … + h MT (f) exp(-j2  t MT f/ N ) where h i (f) is the MR x 1 channel from i th transmit antenna.

4. doc.: IEEE 802.11-04/0075r1 Submission January 2004 H. Sampath, R. Narasimhan, Marvell SemiconductorSlide 4 Resultant Channel (E-LOS, 20 MHz, 64-pt FFT) Circular Delay Diversity leads to increase in frequency selectivity (proportional to circular delays!) Tone Index

5. doc.: IEEE 802.11-04/0075r1 Submission January 2004 H. Sampath, R. Narasimhan, Marvell SemiconductorSlide 5 Advantages of Delay Diversity Provides transmit diversity gain in NLOS fading channels if circular delay > channel delay spread. –Stronger FEC  Higher gain. –Diversity gain improves the slope of BER vs. SNR plots. –Note: Introducing high circular delay >> channel delay spread can lead to performance loss due to limited FEC correction capability. Scalable to number of transmit antennas –Orthogonal ST block codes (e.g. Alamouti) are not scalable with number of antennas. Backwards compatible with legacy 802.11 systems. –Does not require an increase in number of PHY preambles, unlike Orthogonal ST block codes.

6. doc.: IEEE 802.11-04/0075r1 Submission January 2004 H. Sampath, R. Narasimhan, Marvell SemiconductorSlide 6 Drawbacks of Delay Diversity Sensitivity to K-factor: For LOS channels with high K-factor, delay-diversity converts static channel to a channel with increased frequency-domain nulls. –Leads to performance loss w.r.t legacy systems (Example: 1x2 has worse performance compared to 1x1). –Performance loss : Greater for higher K-factor Greater for larger circular delay. Greater for weaker FEC.

7. doc.: IEEE 802.11-04/0075r1 Submission January 2004 H. Sampath, R. Narasimhan, Marvell SemiconductorSlide 7 Simulations Packet Error Rate (PER) vs. SNR results for 1x2 & 1x1 system. 1x2 system employs cyclic delay diversity. –2 nd antenna has delay of  2 samples w.r.t 1 st antenna. –Notation: 1x2 - [0,  2 ] –1 sample = 50 nsec. Assumptions: – Perfect channel estimation, perfect synchronization, no phase noise, no IQ imbalance, no nonlinearities in RF front-end. –1000 byte packets, 20 MHz channelization, 64 point FFT. –Channels generated using Laurent Schumacher v3.2 Matlab code. –Unit transmit power per OFDM data tone. –Channel realizations for each Tx-Rx antenna pair has average power (across all realizations) of unity.

8. doc.: IEEE 802.11-04/0075r1 Submission January 2004 H. Sampath, R. Narasimhan, Marvell SemiconductorSlide 8 12 Mbps in B-NLOS (15 nsec RMS delay spread & K= -100 dB) At 10% PER, gain of 1x2-[0,1] is 0.5 dB; gain of 1x2-[0,32] is 1 dB !

9. doc.: IEEE 802.11-04/0075r1 Submission January 2004 H. Sampath, R. Narasimhan, Marvell SemiconductorSlide 9 54 Mbps in B-NLOS channel (15 nsec RMS delay spread & K= -100 dB) At 10% PER, gain of 1x2-[0,1] is 0 dB; loss of 1x2-[0,32] is 2.5 dB !

10. doc.: IEEE 802.11-04/0075r1 Submission January 2004 H. Sampath, R. Narasimhan, Marvell SemiconductorSlide 10 54 Mbps in E-LOS channel (100 nsec RMS delay spread & K=6 dB) At 10% PER, loss of 1x2-[0,1] is 2 dB; and loss of 1x2- [0,32] is 4.5 dB

11. doc.: IEEE 802.11-04/0075r1 Submission January 2004 H. Sampath, R. Narasimhan, Marvell SemiconductorSlide 11 12 Mbps in E-LOS channel (100 nsec RMS delay spread & K=6 dB) At 10% PER, loss of 1x2 is 1.0 dB

12. doc.: IEEE 802.11-04/0075r1 Submission January 2004 H. Sampath, R. Narasimhan, Marvell SemiconductorSlide 12 Optimum Choice of Circular Delay Parameters (  k ) High K-Factor  Low  k Low K-Factor and low delay spread  Low  k Low K-Factor and high delay spread  High  k Weaker FEC  Lower  k –E.g: Rate 3/4 code cannot exploit high frequency selectivity. 1.Delay parameters needs to be optimized on a per-user basis, depending on coding rate, K-factor and delay-spread ! 2.Requires (coarse) estimation / feedback of K-factor and delay spread!

13. doc.: IEEE 802.11-04/0075r1 Submission January 2004 H. Sampath, R. Narasimhan, Marvell SemiconductorSlide 13 Conclusions Delay diversity provides transmit diversity gain for NLOS fading channels, if delays > effective channel delay spread. Delay diversity leads to performance loss in channels with non-zero K-factor. Implementation Issues: –Advantages: The scheme is backwards compatible with 802.11a/g receivers, and scalable with number of antennas. –Disadvantages: The delay parameter needs to be optimized using feedback of K-factor and delay spread.