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Published byBertha May Modified over 9 years ago
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Sunghwa Son 2011.09.28
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Introduction
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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
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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
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Related Work
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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.
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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?
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Design
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SoftRate: Key Insight Channel Quality Per-bit Confidences Estimate BERDetect Interference Interference-free BER SoftPHY Hints SoftRate
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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
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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)
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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
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Computing Interference-free BER Compute Interference-free BER Interference
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SoftPHY Hints With Weak Signal
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The SoftRate Protocol Data BER Interference Detection Interference-free BER SoftPHY Hints Pick rate with max throughput Receiver Sender
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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
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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
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When to lower the rate? 10 -4 10 -3 10 -5 10 -6 10 -7 BER Throughput 18 Mbps 12 Mbps 24 Mbps
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When to increase the rate? 10 -4 10 -3 10 -5 10 -6 10 -7 BER Throughput 18 Mbps 12 Mbps 24 Mbps
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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
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Implementation
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Evaluation Method Rate Adaptation (SoftRate, SNR-based, Frame-based) TCP Experiments SoftPHY Traces ns-3 simulations Channel Simulator PHY: GNU Radio + USRP
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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?
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SoftPHY Evaluation
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SoftPHY Hints Predict BER
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SNR vs. BER: Varying Mobility
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Interference detection accuracy: various bit rate
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Softrate Evaluation
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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
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Is SoftRate close to optimal? (Walking Speed) Within 10% of the optimal
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SoftRate vs. Frame-based: Walking speed Up to 2X over best frame-based algorithm
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SoftRate vs. SNR-based: Varying Mobility
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Rate selection accuracy
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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
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Conclusion
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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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