ParCast : Soft Video Delivery in MIMO-OFDM WLANs
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Transcript of ParCast : Soft Video Delivery in MIMO-OFDM WLANs
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ParCast: Soft Video Delivery in MIMO-OFDM WLANs
Xiaolin Liu Wenjun HuQifan Pu Feng Wu Yongguang Zhang
Microsoft Research AsiaUniversity of Science and Technology of China
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Wireless trends
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Wireless trends
• Channel capability
Trend: MIMO-OFDM becoming primitives
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Wireless trends
• Channel capability • Application demand
Abcdef…
Trend: MIMO-OFDM becoming primitives
Trend: Video traffic dominatesVideo over wireless to rule!
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Traditionally…Divide into blocks and Transform
10000100101001110010001101001100101001001110
……
Quantize + entropy coding
1000010010100111001000
1101001100101001001110
……
Packetize
100001001011000011001110010000011100
110100110011101001010010011100101110
+ protection
Separate source and channel codesModulate
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Challenge for source coding
Digital rates do not fall back gracefully– Cliff effects at some SNR– Glitches due to bit errors
Vide
o qu
ality
Channel quality
Vide
o qu
ality
Successfully received packet index
Ideally, want graceful degradation with the channel
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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
Chan
nel
qua
lity
Subchannel indexUnicast over MIMO-OFDM resembles broadcast over narrowband SISO channels
Ideally, want one code to gracefully degrade on all subchannels
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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
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Source characteristics
First-frame 8x8 block DCT coefficient energy distribution
Energy difference over several orders of magnitude
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Source coding goals
First-frame 8x8 block DCT coefficient energy distribution
Essential to protect the most important bands well
Discard least important bands for compression
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Channel characteristics
3x3 MIMO-OFDM subchannel gains on 20MHz channel
Gain difference over several orders of magnitude
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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
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Source-channel similarities
Source: First-frame 8x8 block DCT coefficient energy distribution
Channel:3x3 MIMO-OFDM subchannel gains on 20MHz channel
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Source-channel synergy
Source
Decompose
Allocate bits
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Source-channel synergy
Source Channel Joint
Decompose
FreqPo
wer
Allocate power
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ParCast: Parallel Video Unicast
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ParCast: Parallel video unicast
Freq
Preprocessing
a c e
b d f
a + bi c + di e + fi
Analog m
odulation and transm
ission
Sorting and matching
Power allocation
Separating components
Decoding per subchannel
Pow
er
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ParCast: Preprocessing
• Video source:
• Channel:
Whole-frame 3D-DCT
a+bi c+di
f+gi h+ki
s1 0
s2
Divide into chunks
Precode
s1s20
Real values with variances λ1 , λ2 , …
Decorrelate and separatesource/channel components
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ParCast: Sorting and matching
• Video source: chunks sorted by importance
• Channel: subchannels sorted by gain
• Matching Fine-grained unequal error protection
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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/si
2)
Helpful when given similar energy spread for both source/channel
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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/si
2)
– Discard pair if λi/si2 too small
– Per-chunk whitening for hardware considerationCompression and protection in a single code
Further unequal error protection
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ParCast: Analog modulation
OFDM symbol OFDM symbol OFDM symbol…Preamble
44 OFDM symbols per packet
…
Chunk i: 1 2 3 4 5 6 7 8 9 Chunk j: 1 2 3 4 5 6 7 8 9
IQ
Complex subchannel
Soft delivery per subchannel to leverage channel capacity
Nspatial path x Nsubcarrier complex symbols
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ParCast: Decoding
• No standard MIMO decoding• LLSE to decode DCT coefficients per chunk• Inverse 3D-DCT
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ParCast: Parallel video unicast
Freq
Preprocessing
a c e
b d f
a + bi c + di e + fi
Analog m
odulation and transm
ission
Sorting and matching
Power allocation
Separating components
Decoding per subchannel
Pow
er
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ParCast: Parallel video unicast
Freq
a c e
b d f
a + bi c + di e + fiParallel, independent
encoding/transmission and reception/decoding
Pow
er
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ParCast: Parallel video unicast
Freq
a c e
b d f
a + bi c + di e + fi
Joint source-channel codingLossless compression over
lossy communication
Pow
er
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ParCast: Parallel video unicast
Freq
a c e
b d f
a + bi c + di e + fi
Linear codec per subchannel for unicast over MIMO-OFDM
Pow
er
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Performance
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Implementation
• Source codec in Matlab• Channel dependent modules implemented
on Sora • Channel trace
driven simulation [Halperin et al, SIGCOMM 2010]
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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
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Performance: ParCast vs SoftCast
• Must decorrelate both! – For best unequal protection
• SoftCast does not work well over MIMO– Mixing source chunks =
mixing subchannels
Separate source, mixed channelSoftCast (mixed source) ParCast (Separate source/channel)Mixed source, separate channel
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Performance: Stationary links
~ 10 dB
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Performance: Mobile scenariofootball
~ 5 dBFairly stable with
delayed CSI
Channel feedback delay period
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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)
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Conclusion
ParCast is 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
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Thank you!
Questions?