one-size-fits-all wireless video
DESCRIPTION
One-Size-Fits-All Wireless Video. Szymon Jakubczak with Hariharan Rahul and Dina Katabi. Wireless Video Has Important Applications. Mobile TV Live streaming sports, concerts, conferences, lectures, … Broadcast TV. All involve multicast, and some involve mobility - PowerPoint PPT PresentationTRANSCRIPT
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One-Size-Fits-All Wireless Video
Szymon Jakubczak
with Hariharan Rahul and Dina Katabi
![Page 2: One-Size-Fits-All Wireless Video](https://reader035.vdocuments.net/reader035/viewer/2022062520/56816388550346895dd47791/html5/thumbnails/2.jpg)
• 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
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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
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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
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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
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Common Problem
Hard to pick a single rate that matches the channel
Wrong bitrate video degrades drastically
But …
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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
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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
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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
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SoftCast Combines the Best of Both Worlds
Like Digital TV,It codes for compression and error protection
Like Analog TV,It provides smooth degradation
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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
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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
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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
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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
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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
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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
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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]
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Performance
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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
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Test Setup
WARP
Locations of trace collection
• Collected channel traces with WARP between node in testbed
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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
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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]
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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]
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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]
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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
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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
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Multicast
• Receiver 1 has SNR = 5dB – best bitrate 6Mb/s• Receiver 2 has SNR = 21dB – best bitrate 48Mb/s
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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
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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
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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
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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
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Preliminary Mobility Results
1517192123252729
SNR [dB]
PSN
R [d
B]
7 6.5 6
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Preliminary Mobility Results
1517192123252729
MPEG
SNR [dB]
PSN
R [d
B]
7 6.5 6SNR variations cause major glitches in MPEG
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1517192123252729
SoftCastMPEG
SNR [dB]
PSN
R [d
B]
7 6.5 6
Preliminary Mobility Results
SoftCast reacts smoothly to changes in SNR
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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