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
Wireless trends
• Channel capability
• Application demand
Abcdef…
Trend: MIMO-OFDM
Trend: Video traffic
Video over wireless to rule!
becoming primitives
dominates
Traditionally…
Divide into
blocks and
Transform
Quantize + 10000100101
entropy coding 00111001000
11010011001
01001001110
……
Packetize
Separate source and channel
codes
Modulate
100001001011000011
001110010000011100
110100110011101001
010010011100101110
+ protection
10000100101
00111001000
11010011001
01001001110
……
Challenge for source coding
Digital rates do not fall back gracefully
– Cliff effects at some SNR
– Glitches due to bit errors
Video quality
Video quality
Ideally, want graceful degradation with the channel
Channel quality
Successfully received packet index
Challenge for channel coding
Channel
quality
Efficiency vs complexity tradeoff
Ideally, want one code
to gracefully degrade on all subchannels
Subchannel index
Unicast over MIMO-OFDM resembles
–broadcast
100+ heterogeneous
subchannels
in practice
over narrowband
SISO
channels
• 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
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
Energy difference over
several orders of magnitude
First-frame 8x8 block DCT coefficient energy distribution
Source coding goals
Essential to protect the
most important bands well
Discard least important
bands for compression
First-frame 8x8 block DCT coefficient energy distribution
Channel characteristics
Gain difference over several
orders of magnitude
3x3 MIMO-OFDM subchannel gains on 20MHz channel
Channel coding goals
Want to utilize the
good subchannels
Want to mitigate effects
from the bad subchannels
3x3 MIMO-OFDM subchannel gains on 20MHz channel
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
Power
Source-channel synergy
Channel
Joint
Freq
Decompose
Allocate power
ParCast: Parallel Video Unicast
ParCast: Parallel video unicast
Sorting and
matching
Power
allocation
Freq
a
c
e
Decoding per
subchannel
b
d
f
Separating
components
a + bi
c + di
e + fi
Analog modulation
and transmission
Power
Preprocessing
ParCast: Preprocessing
• Video source:
Whole-frame
3D-DCT
Divide into
chunks
Decorrelate and separate
source/channel components
Real values with
• Channel:
Precode
a+bi
c+di
f+gi
h+ki
s1
0
s1
0
s2
s2
variances λ1 , λ2 , …
ParCast: Sorting and matching
• Video source: chunks sorted by importance
• Channel: subchannels sorted by gain
Fine-grained unequal error protection
• Matching
ParCast: Power allocation
• More power → higher rate/lower distortion
• Can formulate/solve optimization problem
– Fixed total budget
– Minimize
distortion
over
components
Helpful∑when
given
similar
energy
spread for component
both source/channel
• Source/channel
pair as unit
– Power weight = f(λi/si2)
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)
2 too small in a single code
Compression
and
– Discard pair if
λi/sprotection
i
Furtherwhitening
unequalfor
error
protection
– Per-chunk
hardware
consideration
ParCast: Analog modulation
Chunk i: 1 2 3 4 5 6 7 8 9
Chunk j: 1 2 3 4 5 6 7 8 9
I
Q
Complex
subchannel
…
Nspatial path x Nsubcarrier complex symbols
Preamble
OFDM symbol
OFDM symbol
…
Soft44delivery
per subchannel
OFDM symbols
per packet
to leverage channel capacity
OFDM symbol
ParCast: Decoding
• No standard MIMO decoding
• LLSE to decode DCT coefficients per chunk
• Inverse 3D-DCT
ParCast: Parallel video unicast
Sorting and
matching
Power
allocation
Freq
a
c
e
Decoding per
subchannel
b
d
f
Separating
components
a + bi
c + di
e + fi
Analog modulation
and transmission
Power
Preprocessing
Power
ParCast: Parallel video unicast
Freq
Parallel, independent
encoding/transmission
and reception/decoding
a
c
e
b
d
f
a + bi
c + di
e + fi
Power
ParCast: Parallel video unicast
Freq
a
c
e
b
d
f
Joint source-channel coding
Lossless compression over
lossy communication
a + bi
c + di
e + fi
Power
ParCast: Parallel video unicast
Freq
a b
a + bi
Linear codec per subchannel
d
c
c + di
e f
e + fi
for unicast over MIMO-OFDM
Performance
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: ParCast vs SoftCast
Separate source, mixed channel
SoftCast (mixed source)
ParCast (Separate source/channel)
Mixed source, separate channel
• Must decorrelate both!
– For best unequal protection
• SoftCast does not work
well over MIMO
– Mixing source chunks =
mixing subchannels
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
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
Thank you!
Questions?