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Cross-Layer Wireless Bit Rate Adaptation

Cross-Layer Wireless Bit Rate Adaptation. Sunghwa Son 2011.09.28. Introduction. Time-varying wireless channel. Large-scale attenuation Due to changing distance Small-scale fading Due to multipath Interference Unpredictable. 10 s. 250 ms. Bit errors, frame losses.

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Cross-Layer Wireless Bit Rate Adaptation

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  1. Cross-Layer Wireless Bit Rate Adaptation Sunghwa Son 2011.09.28

  2. Introduction

  3. Time-varying wireless channel • Large-scale attenuation • Due to changing distance • Small-scale fading • Due to multipath • Interference • Unpredictable 10 s 250 ms Bit errors, frame losses

  4. Online Bit Rate Adaptation • Varying modulation & coding (redundancy) • Better channel  Higher rate • Huge gains possible (802.11g has 1-54 Mbps) Requirements Responsive Do not react to interference Estimate channel accurately

  5. Related Work

  6. Existing Algorithms Frame-based SNR-based • RRAA, Wong et. al., 2006. • SampleRate, Bicket, 2005. • RBAR, Holland et. al., 2001. • CHARM, Judd et. al., 2008. Data Data ACK SNR using preamble Estimate frame loss rate at each bit rate Lookup table SNR  best rate

  7. Problems With Existing Algorithms Indoor Channel Quality Outdoor SNR-based “SNR  bit rate” table specific to environment Which table to use when? X Frame-based Too slow Reacts to interference

  8. Design

  9. SoftRate: Key Insight Channel Quality SoftPHY Hints Per-bit Confidences Estimate BER Detect Interference SoftRate Interference-free BER

  10. SoftRate: Contributions • Adapts to channel accurately and quickly • Robust to collision losses • Feedback: interference-free BER from SoftPHY hints • 2X gain over frame-based and SNR-based

  11. Computing SoftPHY Hints TX Modulator Encoder Symbols Bits Coded bits Soft Output Decoder RX Demodulator Decoder Symbols Coded Bits Bits SoftPHY Hints Error vectors (rcvd symbol – closest valid symbol)

  12. 1 1-p log p 1 + es Pr(correctly decoded) log Pr(incorrectly decoded) BER from SoftPHY Hints decoded bits Soft Output Viterbi (or) BCJR decoder For linear block or convolutional code Log Likelihood Ratio s SoftPHY hint of a bit = p = Probability of bit error BER = Average p over all bits in the packet

  13. Computing Interference-free BER Compute Interference-free BER Interference

  14. SoftPHY Hints With Weak Signal

  15. The SoftRate Protocol Data Receiver Interference-free BER BER Interference Detection Sender Pick rate with max throughput SoftPHY Hints

  16. 10-7 10-6 10-5 10-4 10-3 Rate Selection at the Sender BER Throughput 24 Mbps 18 Mbps 12 Mbps Adjacent rates have an order of magnitude difference in BER

  17. 10-7 10-6 10-5 10-4 10-3 When is the current rate optimal? BER Throughput 24 Mbps Optimality range for 18 Mbps 18 Mbps 12 Mbps

  18. 10-7 10-6 10-5 10-4 10-3 When to lower the rate? BER Throughput 24 Mbps 18 Mbps 12 Mbps

  19. 10-7 10-6 10-5 10-4 10-3 When to increase the rate? BER Throughput 24 Mbps 18 Mbps 12 Mbps

  20. The SoftRate Protocol Data Receiver Interference-free BER BER Interference Detection Sender • Precompute optimality ranges • If BER below optimality range, increase rate. • If above range, decrease rate. • Otherwise, continue at current rate. SoftPHY Hints

  21. Implementation

  22. Evaluation Method TCP ns-3 simulations Rate Adaptation (SoftRate, SNR-based, Frame-based) SoftPHY Traces PHY: GNU Radio + USRP Experiments Channel Simulator

  23. Evaluation Questions SoftPHY • Can SoftPHY hints estimate channel BER? • Can SoftPHY hints identify interference? SoftRate • Gains of SoftRate in mobile channels? • SoftRate robust to interference?

  24. SoftPHY Evaluation

  25. SoftPHY Hints Predict BER

  26. SNR vs. BER: Varying Mobility

  27. Interference detection accuracy: various bit rate

  28. Softrate Evaluation

  29. SoftRate Evaluation in Mobile Channels • Compare with • StaticBest: omniscient—best for each pkt • SNR-based: RBAR and CHARM • Frame-based: RRAA and SampleRate TCP AP Clients Wired LAN Traces

  30. Is SoftRate close to optimal? (Walking Speed) Within 10% of the optimal

  31. SoftRate vs. Frame-based: Walking speed Up to 2X over best frame-based algorithm

  32. SoftRate vs. SNR-based: Varying Mobility

  33. Rate selection accuracy

  34. Evaluation Answers SoftPHY • Can SoftPHY hints estimate channel BER? • Can SoftPHY hints identify interference? SoftRate • Gains of SoftRate in mobile channels? • SoftRate robust to interference? YES ~80% of the time 2X - 4X Almost always

  35. Conclusion

  36. SoftRate: Summary • Accurate, responsive, robust to collision losses • Feedback: interference-free BER from SoftPHY hints • 2X over frame-based, 4X over untrained SNR-based Looking ahead • BER computation from SoftPHY hints useful for other cross-layer protocols

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