Tomorrow: Uplink Video Transmission

Today: Downlink Video Broadcast

Changing Landscape of Multimedia Applications

• Motion-Compensated Predictive Coding (MPEG/H.26)– High compression efficiency– Rigid complexity partition between encoder (heavy) & decoder (light) – High fragility to transmission losses

• Image Coding (Motion JPEG)– Low complexity– High robustness to transmission losses– Low compression efficiency

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Previous

frame

Current fr

ame

DFD (Displaced Frame Difference)

Motion se

arch

range

Mot

ion

Vec

tor

Contemporary Video Coding Standards

Challenges:• Low bandwidths high compression efficiency • Limited handheld battery power low end-device complexity• Lossy wireless medium robustness to transmission losses

Rethinking Video Over Wireless

• High compression efficiency

• Flexible partition of complexity between encoder & decoder

• Inbuilt robustness to channel loss

• Backward compatibility with existing video standards

Puri & Ramchandran, Allerton ’02

Light PRISMUplink Encoder

Light PRISMDownlink Decoder

Heavy PRISM Downlink Encoder

Heavy PRISM Uplink Decoder

Trans-coding Proxy

New Architecture: PRISM(Power-efficient, Robust, hIgh-compression Syndrome-based Multimedia coding)

DecoderEncoderX

Y

X̂ • X and Y are correlated sources

• Y is available only at decoder

Source Coding with side-information (Slepian–Wolf, Wyner-Ziv)

• Exploit side-information Y at the decoder while encoding X

• No MSE performance loss over case when Y is available at both encoder and decoder when innovations is Gaussian

• For the video coding case, X is the block to be coded and the side-information Y consists of the previously decoded blocks in the frame memory

Background: Distributed Source Coding

• The encoder does not have access to Y1’, Y2’, etc• Neither the encoder nor the decoder knows the correct side-

information• Can decoding work?

– Yes!

– A “modified” Wyner-Ziv paradigm is needed(Ishwar, Prabhakaran, & Ramchandran ICIP ’03.)

PredictiveDecoder

PredictiveEncoder … Quantized …

DFD

X

. . .Y1 YM

. . .Y1’ YM’

… Motion Vector … X

PRISMDecoder

PRISMEncoder

X

. . .Y1 YM

. . .Y1’ YM’

X?

Motion-Free Encoding?

X Wyner-ZivEncoder

bin index

Y1’

Wyner-ZivDecoder

X

YT’

Wyner-ZivDecoder

. . .

YM’

Wyner-ZivDecoder

. . .

Decoding failure

Decoding failure

PRISM

Robustness Comparisons:

• Predictive Coding: channel errors lead to prediction mismatch and drift

• PRISM: drift stopped if syndrome code is “strong enough”:

Targeted noise ≥ Correlation Noise + Induced Channel Noise + Quant. Noise

• Need concept of “motion compensation at decoder”!

• Need mechanism to detect decoding failure

• In theory: joint typicality (statistical consistency)

• In practice: use CRC

• Secondary description of video sent over auxiliary-channel.

• Need to find statistics of correlation noise Z = X – Xmain.– Can leverage algorithm of Zhang, Regunathan and Rose (Asilomar ’99) to

develop recursive correlation estimation algorithm.(Wang, Majumdar, Ramchandran, and Garudadri: PCS ’04.)

• Auxiliary channel allows drift correction without intra-refresh.

Standards-Compliant Auxiliary-Channel

MPEG/H.26x Encoder

X

Auxiliary-Channel Encoder

MPEG/H.26X Decoder

Auxiliary-Channel Decoder

Xmain

Final reconstruction

Coset Index Wireless

Channel

Auxiliary-Channel

X̂

MPEG/H.26x

bit-stream

Wireless Channel

Results• Channel simulator provided by Qualcomm Inc. conforming to a CDMA 2000 1x

standard.

• Performance comparison among 3 systems:

– H.263+ bitstream with 20% extra rate for FEC (RS codes)

– H.263+ bitstream with 20% extra rate for standard-compliant auxiliary channel

– PRISM

• Standard-compliant auxiliary channel version outperforms H.263+FEC by 2.5-4 dB between error rates of 2-10%.

• PRISM outperforms H.263+FEC by 6-8 dB between error rates of 2-10%.

H.263+ with FEC

Stefan, 352x240, 15fps, 2200 kbps, 8% error rate

H.263+ with Auxiliary Channel PRISM

PRISM for Wireless Video Broadcast

• Broadcast source coding studied in information theory literature.(Heegard & Berger, IT’85, Steinberg & Merhav IT’04)

• Lossy channel: need broadcast source-channel coding view.– Can use PRISM constructions. (Majumdar & Ramchandran, ICIP ’04)

• No need to deterministically track Yb and Yg at encoder.

• No need for multiple prediction loops complexity savings.• Multiple side-informations at each decoder motion search at each decoder.

– Standards-compliant implementations possibly using the auxiliary channel setup.

(Wang, Majumdar, & Ramchandran, ICASSP ’05)

Decoder Bad

Decoder GoodEncoderX

Yg (“good” side-information)

Yb (“bad” side-information)

Xg

Xb

Rate = ∆R

Rate = R