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Mythili Vutukuru MIT CSAIL Joint work with Hari Balakrishnan and Kyle Jamieson

Cross-layer Wireless Bit Rate Adaptation. Mythili Vutukuru MIT CSAIL Joint work with Hari Balakrishnan and Kyle Jamieson. Time-varying wireless channel. Large-scale attenuation Due to changing distance Small-scale fading Due to multipath Interference Unpredictable. 10 s. 250 ms.

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Mythili Vutukuru MIT CSAIL Joint work with Hari Balakrishnan and Kyle Jamieson

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  1. Cross-layer Wireless Bit Rate Adaptation Mythili Vutukuru MIT CSAIL Joint work with Hari Balakrishnan and Kyle Jamieson

  2. 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

  3. 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

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

  5. 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

  6. SoftRate: Key Insight Channel Quality SoftPHY Hints Per-bit Confidences Estimate BER Detect Interference SoftRate Interference-free BER SoftPHY design more general compared to [JB’07, KKB’08]

  7. 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

  8. Rest of the talk… • Computing SoftPHY hints • Interference-free BER from SoftPHY hints • SoftRate algorithm • Evaluation

  9. 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)

  10. 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

  11. Computing Interference-free BER Compute Interference-free BER Interference

  12. SoftPHY Hints With Weak Signal

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

  14. 10-7 10-6 10-5 10-4 10-3 Rate Selection at the Sender R . Frame Delivery Rate = f(BER) BER Throughput 24 Mbps 18 Mbps 12 Mbps Adjacent rates have an order of magnitude difference in BER

  15. 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

  16. 10-7 10-6 10-5 10-4 10-3 When to lower the rate? BER Throughput 24 Mbps 18 Mbps 12 Mbps

  17. 10-7 10-6 10-5 10-4 10-3 When to increase the rate? BER Throughput 24 Mbps 18 Mbps 12 Mbps

  18. 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

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

  20. 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? ~80% of the time Almost always

  21. SoftPHY Hints Predict BER

  22. 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

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

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

  25. SoftRate vs. SNR-based: Varying Mobility (Walking speed) 1 10 100 1000 (Walking speed) Approx speed (mph) (Train speed) 4X over untrained SNR-based algorithm

  26. SNR vs. BER: Varying Mobility SNRBER and SNRbest rate specific to operating environment

  27. 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

  28. 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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