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Recitation: Rehearsing Wireless Packet Reception in Software Zhenjiang Li, Yaxiong Xie, Mo Li, Nanyang Technological University Kyle Jamieson University College London Slide 2 Up to 160 MHz Up to 40 MHz Up to 22 MHz 802.11 a/b/g (1999) 802.11 n (2009) 802.11 ac (2013) Wireless transmissions is going wider-band Slide 3 Channel 30 + dB! Subcarriers SNR Selective fading in wideband channels Slide 4 Channel Selective fading in wideband channels Heterogeneous BERs Slide 5 Can we predict the bit destiny before transmitting the packet? Slide 6 Narrow band SNR-BER theory cannot apply Approximation with effective SNR inaccurate Work with commodity WiFi NICs Slide 7 CSI Rehearse transmissions (Input) Rehearsing before transmissions (Output) Packet error rate (PER) Error-prone bit positions Slide 8 To this end, we need? Complex but predictable Slide 9 CSI (as input) Channel Slide 10 Interleaving Deterministic CSI (as input) Channel Slide 11 Interleaving interleaver write in row read in column Block interleaver Slide 12 Interleaving Block interleaver interleaver write in row read in column Slide 13 So far Coded Bits Packet Bits Slide 14 CSI (as input) Deterministic Predictable Channel Interleaving Coding Slide 15 Convolutional coding + + Convolutional Coding Simple encoder Coding rate of 3 registers (4 finite states) Input Output Slide 16 Convolutional coding Decoding Path with the minimal Hamming distance Failure Faulty path is chosen Error event Slide 17 Convolutional coding Error event probability (EVP) Probability that a faulty path will be selected EVP i Any faulty path that diverges at state i Slide 18 Convolutional coding EVP i varies Indicate the error-prone bit positions Slide 19 Convolutional coding Slide 20 Convolutional coding Slide 21 Andrew James Viterbi Viterbi theory Slide 22 Convolutional coding Andrew James Viterbi Viterbi theory Coded Bits Slide 23 However Coded Bits + Input + Output 1 Output 2 Slide 24 EVP calculation in 802.11 Diverging segments of faulty paths Cannot be directly measured Use error burst length to approximate All possible error combinations Slide 25 EVP calculation in 802.11 (1) Short divergent segments (2) Small number of error bits Slide 26 Sort Reducing computations Slide 27 LDPC (Low Density Parity Check) Block code Packet bits Coded bits Slide 28 LDPC (Low Density Parity Check) Block code Packet bits Coded bits Decoding Error Slide 29 Finally CSI (as input) Slide 30 Experiment evaluation TP-Link TL-WDR7500 Wi-Fi router Atheros 9580 NIC (802.11n) SoC QCA9558 Data collected 50 locations (U-shape route) 93,000+ UDP packets with 1000 byte random payloads Iterating all 8 data rates (6.5, 13, 19.5, 26, 39, 52, 58.5, 65 Mbps) CSIs and transmitted packets (correct and corrupted) Slide 31 Benchmark tests Packet Error Rate (PER) prediction Convolutional Code LDPC Code Slide 32 Benchmark tests Computational overhead Average delay < 0.2 ms Slide 33 Application -- #1 Rate selection Throughput improvement Average: 25.6% Rect-Rate: PER by Recitation < 0.1 ESNR: ESNR rate selection approach OPT: Oracle selection based on actual transmissions Slide 34 Application -- #2 Unequal protection Error-prone positions EVP accurately describes the BER Periodical pattern (in OFDM symbols) Slide 35 Application -- #2 Unequal protection Average 6dB improvement on PSNR (peak SNR), a standard metric to measure video quality Rect-Video I-frames at more reliable positions P-, B- frames at other positions Stan-Video Video streaming Slide 36 More evaluations System parameters Partial packet recovery MIMO settings Mobility Slide 37 Takeaways 1. Frequency selective fading in wideband channels 3. Complicated but predictable 802.11 PHY operations 2. Narrow band experience unsuitable