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ParCast : Soft Video Delivery in MIMO-OFDM WLANs. Xiaolin Liu Wenjun Hu Qifan Pu Feng Wu Yongguang Zhang. Microsoft Research Asia University of Science and Technology of China. Wireless trends. Wireless trends. Channel capability. Trend: MIMO-OFDM becoming primitives.
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ParCast: Soft Video Delivery in MIMO-OFDM WLANs Xiaolin Liu Wenjun Hu Qifan Pu Feng Wu Yongguang Zhang Microsoft Research Asia University of Science and Technology of China
Wireless trends • Channel capability Trend: MIMO-OFDM becoming primitives
Wireless trends • Channel capability • Application demand Abcdef… Trend: MIMO-OFDM becoming primitives Trend: Video traffic dominates Video over wireless to rule!
Traditionally… Divide into blocks and Transform Quantize + entropy coding 10000100101 00111001000 11010011001 01001001110 …… Separate source and channel codes Packetize Modulate 100001001011000011 001110010000011100 10000100101 00111001000 + protection 110100110011101001 010010011100101110 11010011001 01001001110 ……
Challenge for source coding Digital rates do not fall back gracefully • Cliff effects at some SNR • Glitches due to bit errors Ideally, want graceful degradation with the channel Video quality Video quality Successfully received packet index Channel quality
Challenge for channel coding Efficiency vs complexity tradeoff • 100+ heterogeneous subchannels in practice • E.g., 52 OFDM subcarriers in 802.11n per 20MHz, 2-4 MIMO spatial paths per subcarrier • One code for all does not fully utilize capacity • Subchannel specific modulation impractical Channel quality Ideally, want one code to gracefully degrade on all subchannels Subchannel index Unicast over MIMO-OFDM resembles broadcast over narrowband SISO channels
Our solution • A single joint source-channel code to work gracefully with SNR on all subchannels • Take a leaf from SoftCast’s book • Apply similar principles to MIMO-OFDM unicast
Source characteristics First-frame 8x8 block DCT coefficient energy distribution Energy difference over several orders of magnitude
Source coding goals Essential to protect the most important bands well First-frame 8x8 block DCT coefficient energy distribution Discard least important bands for compression
Channel characteristics 3x3 MIMO-OFDM subchannel gains on 20MHz channel Gain difference over several orders of magnitude
Channel coding goals 3x3 MIMO-OFDM subchannel gains on 20MHz channel Want to utilize the good subchannels Want to mitigate effects from the bad subchannels
Source-channel similarities Source: First-frame 8x8 block DCT coefficient energy distribution Channel: 3x3 MIMO-OFDM subchannel gains on 20MHz channel
Source-channel synergy Source Decompose Allocate bits
Source-channel synergy Joint Source Channel Power Freq Decompose Allocate power
ParCast: Parallel video unicast Preprocessing Sorting and matching Power allocation Freq Power Analog modulation and transmission b d f a c e a + bi c + di e + fi Decoding per subchannel Separating components
ParCast: Preprocessing • Video source: • Channel: Whole-frame 3D-DCT Divide into chunks Decorrelate and separate source/channel components Real values with variances λ1 , λ2, … Precode s1 0 a+bi c+di s1 s2 f+gi h+ki 0 s2
ParCast: Sorting and matching • Video source: chunks sorted by importance • Channel: subchannels sorted by gain • Matching Fine-grained unequal error protection
ParCast: Power allocation • More power → higher rate/lower distortion • Can formulate/solve optimization problem • Fixed total budget • Minimize ∑ distortion over components • Source/channel component pair as unit • Power weight = f(λi/si2) Helpful when given similar energy spread for both source/channel
ParCast: Power allocation • More power → higher rate/lower distortion • Can formulate/solve optimization problem • Fixed total budget • Minimize ∑ distortion over components • Source/channel component pair as unit • Power weight = f(λi/si2) • Discard pair if λi/si2 too small • Per-chunk whitening for hardware consideration Compression and protection in a single code Further unequal error protection
ParCast: Analog modulation Chunk i: 1 2 3 4 5 6 7 8 9 I Complex subchannel Chunk j: 1 2 3 4 5 6 7 8 9 Q … Nspatial path x Nsubcarrier complex symbols … Preamble OFDM symbol OFDM symbol OFDM symbol Soft delivery per subchannel to leverage channel capacity 44 OFDM symbols per packet
ParCast: Decoding • No standard MIMO decoding • LLSE to decode DCT coefficients per chunk • Inverse 3D-DCT
ParCast: Parallel video unicast Preprocessing Sorting and matching Power allocation Freq Power Analog modulation and transmission b d f a c e a + bi c + di e + fi Decoding per subchannel Separating components
ParCast: Parallel video unicast Freq Power b d f a c e a + bi c + di e + fi Parallel, independent encoding/transmission and reception/decoding
ParCast: Parallel video unicast Freq Power b d f a c e a + bi c + di e + fi Joint source-channel coding Lossless compression over lossy communication
ParCast: Parallel video unicast Freq Power Linear codec per subchannel for unicast over MIMO-OFDM b d f a c e a + bi c + di e + fi
Implementation • Source codec in Matlab • Channel dependent modules implemented on Sora • Channel trace driven simulation [Halperin et al, SIGCOMM 2010]
Evaluation • Microbenchmarks • Software radio based experiments • Effects of individual modules • Channel precoding, matching, joint power allocation • Video quality (PSNR) Comparison • Variants of SoftCast adapted to MIMO • Omni-MPEG over 802.11n • Layered SVC over MIMO
Performance: ParCastvsSoftCast • Must decorrelate both! • For best unequal protection • SoftCast does not work well over MIMO • Mixing source chunks = mixing subchannels Separate source, mixed channel SoftCast (mixed source) ParCast (Separate source/channel) Mixed source, separate channel
Performance: Stationary links ~ 10 dB
Performance: Mobile scenario football ~ 5 dB Fairly stable with delayed CSI Channel feedback delay period
Related work • Unequal error protection over MIMO/OFDM • Layered video coding, one layer per subchannel • Channel codes for graceful degradation • HM over STBC (broadcast), Apex (unicast), etc. • Joint source-channel coding (single antenna) • Jointly optimizing separate codes • A single code for compression/protection • SoftCast (broadcast), FlexCast (unicast)
Conclusion ParCastis simple yet effective • One code to work gracefully with SNR on all MIMO-OFDM subchannels • Joint source-channel coding • Motivated by source-channel synergy • Non-uniform distribution at both provides inherent unequal error protection • Parallel delivery per source chunk/subchannel • Treating unicast as broadcast • Graceful degradation via linear codec
Thank you! Questions?