1. 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 Bit errors, frame losses 10 s 250 ms

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

5. Related Work

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

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

8. Design

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

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

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

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

14. SoftPHY Hints With Weak Signal

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

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

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

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

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

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

21. Implementation

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

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 Traces TCP Clients AP Wired LAN

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? ~80% of the time Almost always YES 2X - 4X

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