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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
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Wireless transmissions is going wider-band
Up to 160 MHz Up to 40 MHz ac (2013) n (2009) Up to 22 MHz a/b/g (1999)
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Selective fading in wideband channels
SNR 30+ dB! Subcarriers
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Selective fading in wideband channels
Heterogeneous BERs
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Can we predict the bit destiny before transmitting the packet?
Optimal bit rate selection Efficient partial packet recovery Unequal packet protection
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Can we predict the bit destiny before transmitting the packet?
Narrow band SNR-BER theory cannot apply Approximation with effective SNR inaccurate Work with commodity WiFi NICs
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Rehearsing before transmissions
(Output) Packet error rate (PER) Error-prone bit positions Rehearse transmissions CSI (Input)
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Complex but predictable
To this end, we need? Complex but predictable
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Channel CSI (as input)
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Interleaving Channel CSI (as input) Interleaving Deterministic
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Interleaving interleaver Block interleaver write in row read in column
π π π π π π π π π π π π π ππ π π π π π π π ππ π π π π π π π ππ π π π π π π π π π π β¦ π ππ π π π π π π write in row π π π π π π Generally, there are two kinds of interleaver widely used in communication systems: block interleaver and convolutional interleaver. system adopts block interleaver. π ππ π ππ π ππ read in column
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Interleaving interleaver Block interleaver write in row read in column
π= π© ππ Γ π πππ
ππ + π/ππ interleaver π π π π π π π π π π β¦ π ππ write in row Generally, there are two kinds of interleaver widely used in communication systems: block interleaver and convolutional interleaver. system adopts block interleaver. read in column π π π π π π π ππ π π β¦ π ππ
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So far Coded Bits Packet Bits
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Coding CSI (as input) Deterministic Predictable Channel Interleaving
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Convolutional coding + + Simple encoder Output Input Coding rate of Β½
3 registers (4 finite states) Convolutional Coding + Output Input +
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Convolutional coding Decoding Failure
Path with the minimal Hamming distance Failure Faulty path is chosen Error event
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Convolutional coding Error event probability (EVP) EVPi
Probability that a faulty path will be selected EVPi Any faulty path that diverges at state i
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Convolutional coding EVPi varies
EVPi varies Indicate the error-prone bit positions
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Convolutional coding
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Convolutional coding
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Convolutional coding lim πββ π β π π β π· π Viterbi theory
Andrew James Viterbi lim πββ π β π π β π· π Viterbi theory π· π is the probability that one path with Hamming distance π will be chosen as the decoding result π· π = π=(π+π)/π π π π π π π (πβ π π ) πβπ , π ππ ππ
π
π π π π/π π π π/π (πβ π π ) π/π + π= π π +π π π π π π π (πβ π π ) πβπ ,π ππ ππππ π π = 2k is the number of paths with Hamming distance π
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Convolutional coding lim πββ π β π π β π· π
Andrew James Viterbi lim πββ π β π π β π· π Viterbi theory Assumption Each coded bit has the same error probability π π Coded Bits
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However β¦ + + Output 1 Input Output 2 Coded Bits πΏ π πΏ πβπ πΏ πβπ πΏ πβπ
πΏ πβπ πΏ πβπ πΏ πβπ Input Output 2 +
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EVP calculation in 802.11 Diverging segments of faulty paths
Cannot be directly measured Use error burst length to approximate All possible error combinations
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EVP calculation in 802.11 (1) Short divergent segments
(2) Small number of error bits
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Reducing computations
Sort π πβπ π π Γ πβπ (πβ π π ) Γ π π,π π¬π½π·+=
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LDPC (Low Density Parity Check)
Block code Packet bits Coded bits
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LDPC (Low Density Parity Check)
Block code Packet bits Coded bits Decoding Error π¬π½π· π = π=π¬πͺπͺ π π πβ π π πβπ π π β πβπ (πβ π π )
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Finally CSI (as input)
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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)
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Benchmark tests Packet Error Rate (PER) prediction Convolutional Code
LDPC Code
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Benchmark tests Computational overhead Average delay < 0.2 ms
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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
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Application -- #2 Unequal protection
Error-prone positions Periodical pattern (in OFDM symbols) EVP accurately describes the BER
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Application -- #2 Unequal protection
Video streaming Rect-Video I-frames at more reliable positions P-, B- frames at other positions Stan-Video Average 6dB improvement on PSNR (peak SNR), a standard metric to measure video quality
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More evaluations System parameters Partial packet recovery
MIMO settings Mobility
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Takeaways 1. Frequency selective fading in wideband channels
2. Narrow band experience unsuitable 3. Complicated but predictable PHY operations
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