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Circular Delay Diversity

Advantages and Drawbacks of Circular Delay Diversity for MIMO-OFDM Hemanth Sampath Ravi Narasimhan hsampath@marvell.com ravin@marvell.com Marvell Semiconductor, Inc. Circular Delay Diversity. Signal on the k th Tx antenna is circularly delayed by t k samples.

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Circular Delay Diversity

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  1. Advantages and Drawbacks of Circular Delay Diversity for MIMO-OFDMHemanth Sampath Ravi Narasimhanhsampath@marvell.comravin@marvell.comMarvell Semiconductor, Inc. H. Sampath, R. Narasimhan, Marvell Semiconductor

  2. Circular Delay Diversity • Signal on the kth Tx antenna is circularly delayed by tk samples. • To obtain full Tx-diversity, we must have tk > effective channel delay spread[Gore & Sandhu -2002] H. Sampath, R. Narasimhan, Marvell Semiconductor

  3. Frequency Domain Representation • MR x MT channel H(f) collapses to a MR x 1 channel hr(f) at the receiver. • hr (f) = h1(f) + h2(f) exp(-j2p t2 f / N) + … + hMT(f) exp(-j2ptMT f/ N ) • where hi(f) is the MR x 1 channel from ith transmit antenna. H. Sampath, R. Narasimhan, Marvell Semiconductor

  4. Resultant Channel (E-LOS, 20 MHz, 64-pt FFT) Tone Index Circular Delay Diversity leads to increase in frequency selectivity (proportional to circular delays!) H. Sampath, R. Narasimhan, Marvell Semiconductor

  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. H. Sampath, R. Narasimhan, Marvell Semiconductor

  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. H. Sampath, R. Narasimhan, Marvell Semiconductor

  7. Simulations • Packet Error Rate (PER) vs. SNR results for 1x2 & 1x1 system. • 1x2 system employs cyclic delay diversity. • 2nd antenna has delay of t2samples w.r.t 1st antenna. • Notation: 1x2 - [0, t2] • 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. H. Sampath, R. Narasimhan, Marvell Semiconductor

  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 ! H. Sampath, R. Narasimhan, Marvell Semiconductor

  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 ! H. Sampath, R. Narasimhan, Marvell Semiconductor

  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 H. Sampath, R. Narasimhan, Marvell Semiconductor

  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 H. Sampath, R. Narasimhan, Marvell Semiconductor

  12. Optimum Choice of Circular Delay Parameters (tk) • High K-Factor  Low tk • Low K-Factor and low delay spread  Low tk • Low K-Factor and high delay spread  High tk • Weaker FEC  Lower tk • E.g: Rate 3/4 code cannot exploit high frequency selectivity. • Delay parameters needs to be optimized on a per-user basis, depending on coding rate, K-factor and delay-spread ! • Requires (coarse) estimation / feedback of K-factor and delay spread! H. Sampath, R. Narasimhan, Marvell Semiconductor

  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. H. Sampath, R. Narasimhan, Marvell Semiconductor

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