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One-Size-Fits-All Wireless Video
Szymon Jakubczak
with Hariharan Rahul and Dina Katabi
• Mobile TV• Live streaming– sports,
concerts, conferences, lectures, …
• Broadcast TV
Wireless Video Has Important Applications
All involve multicast, and some involve mobilityCurrent design struggles with multicast and mobility
Multicast Challenges Current Wireless Design
High bitrate Starves the far receiver
6Mb/s
1Mb/s
• Currently, the sender has to pick a bitrate• But different receivers support different bitrates
Multicast Challenges Current Wireless Design
High bitrate Starves the far receiverLow bitrate Reduces everyone to the worst receiver
• Currently, the sender has to pick a bitrate• But different receivers support different bitrates
6Mb/s
1Mb/s
Mobility Makes Things Worse
High rate Video stalls when SNR dipsLow rate Overall video quality is low
Successive frames may experience a different channel
200ms
Time [ms]Rece
ived
Sig
nal L
evel
[dBm
]Mobility causes fast unpredictable SNR variations
Common Problem
Hard to pick a single rate that matches the channel
Wrong bitrate video degrades drastically
But …
In principle, video quality should degrade smoothly with channel quality
Sender should be able to simply transmit:Noisy channel decoded pixels approximate
original pixelsGood channel decoded pixels match originals
Why Cannot Current Design Provide Smooth Degradation?
• Compression and error protection convert real-valued pixels to bits
• Bits destroy the numerical properties of original pixels11110 and 11111 could refer to pixels as different as 5 and 149
• If all bit errors can be corrected all pixels are correct• Even one residual bit error arbitrary errors in pixels
Analog TV Degraded Smoothly
Real-Valued Pixels2, 153, …
Transmitted Values2α, 153α, …
Transmitted values are linearly related to pixel luminance
But Analog TV was not efficient:• No compression• No error protection
α
Small perturbation on channel
Small perturbation in pixel values
It did not convert pixels to bits
SoftCast Combines the Best of Both Worlds
Like Digital TV,It codes for compression and error protection
Like Analog TV,It provides smooth degradation
Goal: transmitted signal is linearly related to the pixels smooth degradation
SoftCast uses a new coding technique that:
– converts pixels to real-valued codewords, not bits
– provides compression and error protection while preserving linearity between pixels and codewords
– passes the codewords to the PHY, which transmits them directly on the channel
SoftCast
Pixels in an image change gradually In frequency domain, most high frequencies are zero
STEP1: Convert a frame to frequency domain using DCT
STEP2: Send only non-zero frequencies in the frame Compressing the frame
How Does SoftCast Compress?
Zeros
DCT ofwhole frame
Encoder needs to tell the decoder the location of zeros– Easy because zeros are clustered
Divide into chunks and drop zero chunks– Use a bit map to tell receiver locations of zero chunks
Drop Zero Chunks
• DCT is a linear operator• Dropping zero chunks does not break linearity SoftCast’s compression preserves linearity
How Does SoftCast Provide Error Protection?
2.5
SoftCast protects real-valued codewords using magnitude-scaling
Codeword Transmitted Received Decoded
24.9
25.1±0.1
2.492.51
±0.01
Channel Noise±0.1
25
x10
Before Tx Scale up
/10
After Rx Scale down
How Does SoftCast Provide Error Protection?
2.5
SoftCast protects real-valued codewords using magnitude-scaling
Codeword Transmitted Received Decoded
24.9
25.1±0.1
2.492.51
±0.01
Channel Noise±0.1
25
x10
Before Tx Scale up
/10
After Rx Scale down
Scaling the codeword up, scales down the effective noise on the channel by the same factor
But Can’t Scale All Codewords UpScaled-up values are larger take more power to transmitBut hardware has limited powerWe find the optimal scaling factors that minimize video errors given hardware power
Theorem • Let λi be the variance of chunk i• The linear encoder that minimizes video errors scales
the values xi in chunk i as follows:
yi = gi xi where gi ~ λi-1/4
Scaling is linear SoftCast’s error protection preserves linearity
How Does the PHY Transmit?
Traditional PHY maps bits to reals (I and Q) using modulation
SoftCast PHY directly transmits the real-valued codewords as I and Q
Recall: Channel transmits pairs of real values (I and Q)
QAM modulation IQ
…0011001
…y[5]y[4]y[3]y[2]y[1]
I
Q
SoftCast achieves its goal of ensuring that the transmitted signal is linearly related to the pixels
…y[5]y[4]y[3]
y[1]
y[2]
…y[5]
y[3]y[1]
y[4]y[2]
Performance
Compared Schemes
• SoftCast• MPEG-4 (H.264) over 802.11– Implemented in libx264 via ffmpeg
• 2-Layer Video– A base layer and an enhancement layer– Implemented in libx264 via ffmpeg
Test Setup
WARP
Locations of trace collection
• Collected channel traces with WARP between node in testbed
Test Setup• Collected channel traces with WARP between node in testbed
• Extracted noise patterns as differences between transmitted and received soft values
Trace-Driven Channel
(802.11 OFDM)
MPEG4
2-Layer Video
SoftCast
MPEG4
2-Layer Video
SoftCast
Encoders Decoders
• Compare schemes for the same trace-driven channels
Video Quality vs. Channel Quality
0 5 10 15 20 2520
25
30
35
40
45
Channel Quality – SNR [dB]
Vide
o Q
ualit
y –
PSN
R [d
B]
Video Quality vs. Channel Quality
0 5 10 15 20 2520
25
30
35
40
45
MPEG 6Mbps
Channel Quality – SNR [dB]
Vide
o Q
ualit
y –
PSN
R [d
B]
Video Quality vs. Channel Quality
0 5 10 15 20 2520
25
30
35
40
45
MPEG 6MbpsMPEG 12Mbps
Channel Quality – SNR [dB]
Vide
o Q
ualit
y –
PSN
R [d
B]
Video Quality vs. Channel Quality
0 5 10 15 20 2520
25
30
35
40
45
MPEG 6MbpsMPEG 12MbpsMPEG 18MbpsMPEG 24MbpsMPEG 36MbpsMPEG 48MbpsMPEG 54Mbps
Channel Quality – SNR [dB]
Vide
o Q
ualit
y –
PSN
R [d
B]
MPEG degrades drastically when the bitrate does not match channel SNR
0 5 10 15 20 2520
25
30
35
40
45
SoftCastMPEG 6MbpsMPEG 12MbpsMPEG 18MbpsMPEG 24MbpsMPEG 36MbpsMPEG 48MbpsMPEG 54Mbps
Channel Quality – SNR [dB]
Vide
o Q
ualit
y –
PSN
R [d
B]
SoftCast combines efficiency with smooth video degradation
Video Quality vs. Channel Quality
Multicast
• Receiver 1 has SNR = 5dB – best bitrate 6Mb/s• Receiver 2 has SNR = 21dB – best bitrate 48Mb/s
Multicast
MPEG SoftCast20
25
30
35
40
Vide
o PS
NR
[dB]
• Receiver 1 has SNR = 5dB – best bitrate 6Mb/s• Receiver 2 has SNR = 21dB – best bitrate 48Mb/s
Multicast
MPEG SoftCast20
25
30
35
40
Vide
o PS
NR
[dB]
Layered video:• Base layer at 6Mb/s, enhancement layer at 48 Mb/s• Have to divide medium time between the layers
Multicast
MPEG SoftCast Layered 4:1 Layered 3:2 Layered 2:320
25
30
35
40
Vide
o PS
NR
[dB]
Layered video:• Base layer at 6Mb/s, enhancement layer at 48 Mb/s• Have to divide medium time between the layers
Multicast
MPEG SoftCast Layered 4:1 Layered 3:2 Layered 2:320
25
30
35
40
Vide
o PS
NR
[dB]
In 2-layer video, enhancement reduces transmission time of base Weak receiver becomes worse off
Layered video:• Base layer at 6Mb/s, enhancement layer at 48 Mb/s• Have to divide medium time between the layers
Preliminary Mobility Results
1517192123252729
SNR [dB]
PSN
R [d
B]
7 6.5 6
Preliminary Mobility Results
1517192123252729
MPEG
SNR [dB]
PSN
R [d
B]
7 6.5 6SNR variations cause major glitches in MPEG
1517192123252729
SoftCastMPEG
SNR [dB]
PSN
R [d
B]
7 6.5 6
Preliminary Mobility Results
SoftCast reacts smoothly to changes in SNR
Conclusion
• Digital video can achieve smooth degradation• Key Idea: – Continue to compress and protect against errors– But make codewords linearly related to pixels
• Experimental results show this approach is highly promising for multicast and mobile scenarios