1. Authors: Joachim Hagenauer, Thomas Stochhammer
Paper Presentation Channel Coding and Transmission Aspects for Wireless Multimedia
Authors: Joachim Hagenauer, Thomas Stochhammer
Source: Proceedings of the IEEE , Volume: 87 Issue: 10 , Oct 1999, pp
Originally Presented by Hong Hong Chang, Feb 17, 2003

2. Overview Introduction System Architecture
The Links between Source and Channel Coding
RCPC, UEP
PCM Transmission example
Transmission
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3. Wireless Channel Multipath fading Doppler spreading Effect of distance
Quite noisy
High BER
average error rates up to 10%
Channel coding is necessary
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4. Source Coding & Channel Coding (I)
Shannon’s separation theorem
source coding - long blocks of source symbols
channel coding -a sequence of random block codes with infinite length
Infinite delay
data
Source Coding
Channel Coding
Modulation
transmission
(C) 2005 by Yu Hen Hu

5. Source Coding & Channel Coding (II)
Shannon’s separation theorem is no longer applicable
short blocks, small delays
Combined and joint source and channel coding
MPEG II audio layer
Source-controlled channel decoding
Uses the residual redundancy of the uncompressed or partly compressed source data to improve channel decoding
(C) 2005 by Yu Hen Hu

6. Transmissions - Two Kinds of Data Channels
Mode 1
Error free delivery
Using ARQ
Delay and bit throughput rate (BTR) vary according to the channel conditions
Mode 2
Guarantees constant bit rate and delay
Errors occur
(C) 2005 by Yu Hen Hu

7. System for Transmission of Multimedia Applications over Mobile Channels
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8. Application Properties
Delay-sensitive applications
Speech, video telephony
Use frequent resynchronization, reduced predictive coding
No ARQ, deep interleaving or long block codes
BTR-sensitive applications
Audio, video
Use bidirectional predictive coding, long term rate control algorithms
Might use error protection interleaving, serial or parallel concatenated coding or ARQ to exploit the provided bandwidth as optimally as possible
(C) 2005 by Yu Hen Hu

9. Application Properties (Cont)
BER-sensitive applications
Data
Error-free delivery
Use ARQ, FEC
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10. Multimedia Transmission
Each application may request different QoS
All application are combined into one single transmission stream
New layer necessary for multimedia transmission
Adaptation Layer
Multiplex Layer
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11. Adaptation Layer and Multiplex Layer
Adapt the requesting upper application to transmission condition according to the required QoS
Have tools for error detection, error correction, bit reordering, retransmission protocols
Multiplex layer
Multiplex the adaptation layer bit streams or packets into one single bit steam
Optimizing the throughput, minimize misdeliveries
(C) 2005 by Yu Hen Hu

12. Transmission Scheme over a Mobile Channel
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13. Links between Source Coding and Channel Coding
Channel State Information (CSI)
Connected by soft decision of demodulator/detector
Soft decision gains 2-3dB
Source Significant Information (SSI)
For unequal error protection (UEP)
Rate-compatible punctured convolutional code (RCPC)
Decision Reliability Information (DRI)
Soft output from channel decoder
Source a priori/a posteriori information (SAI)
probability of next bit, correlation
Reduce channel decoder error rate
(C) 2005 by Yu Hen Hu

14. Rate-Compatible Punctured Convolutional Code for Unequal Error Protection
Start with a rate 1/n0 linear convolutional code
Encode k input bits to produce n0k output bits
Delete n0k−n bits from the output bits
The code rate is
The corresponding n0k perforation matrix has n ones and n0k−n zeros
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15. Punctured Convolutional Code Example
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16. Puncture Pattern and Perforation Matrix
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17. Rate Compatible Convolutional Code
2/3
2/3
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18. Rate Compatible Punctured Convolutional Code
A family of punctured codes are rate compatible if the codeword bits from the higher-rate code are embedded in the lower rate codes.
The zeros in perforation matrices of the lower rate codes are also the zeros in the perforation matrices of the higher rate
The ones in in perforation matrices of the higher rate codes are also ones in in perforation matrices of the lower rate codes.
(C) 2005 by Yu Hen Hu

19. RCPC Example
Note that the rates are progressive. So that the amount of FEP can be adapted to different importance of the content.
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20. Recursive Systematic Encoder Structure
Memory M=4 , Mother code rate = ½, Puncturing rate = 8/12
Nonsystematic vs Systematic
G(D) = (1+D3+D4, 1+D+D2+D4, 1+D2+D3+D4)
Gs(D) =
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21. Error Probability Upper Bound
df – free distance, the minimum distance of any path from the correct path
cd – the sum of all information weights on all wrong path of distance d starting inside one puncturing period
Pd – the pairwise error probability of two code sequences at distance d
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22. Puncturing Table Rate Table df d df +1 df +2 8/10 3 cd ad 14 5 138 41 3 cd ad 14 5
138 41
1114
276
8/12 4 10 81 22
307 74
8/14 1 82
126 29
8/16 7 64 16 96 24
128 32
(C) 2005 by Yu Hen Hu

23. Comparison of systematic recursive convolutional code with nonsystematic codes
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24. Encoder & Decoder Encoder Decoder
Puncture
Repeat – replacing “1” by “2” or any higher integer in the puncturing tables
Decoder
Punctured bits are stuffed with zeros
Repeated bits are combined by adding soft values
Header of frame contains the coding rate information of payload
Easily adapted to multimedia and channel requirements via puncturing control
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25. BER Performance of Systematic Recursive PCPC code
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26. Soft-In/Soft-Out Decoding
Decoding algorithm
Viterbi (VA)
Maximum-a-posteriori-probability-symbol-by-symbol (MAP)
VA and MAP can accept soft values
Source a priori information
Channel state information
VA and MAP can deliver soft outputs
(C) 2005 by Yu Hen Hu

27. PCM Transmission example - EEP
Analog source
Source coding: m-bit linear quantization (m=20)
Quantized sample
smaller k -> more important.
Transmission distortion
equal Pb for all k=1,2,…,m
(C) 2005 by Yu Hen Hu

28. PCM Transmission Example – Applying Soft Bits
CSI is transformed to a DRI and directly passed to the source decoder. Thus, λ(x) (soft value) is obtained
Reconstructed PCM value
Gain of about 1.6dB in SNRPCM
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29. PCM Transmission Example – Apply Channel Coding
m is smaller, quantization noise increases
Channel coding rate = ½
RCPSRC 8/16
Improves total performance
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30. PCM Transmission Example – UEP
Let all bits contribute the same transmission distortion. Then,
Small k, small Pb
Use this information for unequal error-protection design
Require that transmission distortion of each bit is smaller than quantization distortion. We have
(C) 2005 by Yu Hen Hu

31. PCM Transmission Example: RCPSRC code for UEP
Employ
the upper bound for the bit error probability
Distance spectra of puncture table
Obtain a certain rate R(k) for each bit class at different channel SNR
Rate distribution for PCM Transmission
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32. PCM Transmission Example - Simulation Results
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33. Approaches to Improve the Transmission of Multimedia I
Approaches to Improve the Transmission of Multimedia I. Error Resilient Source Coding
Fixed length coding
more stable against channel error
MPEG-4 error resilient mode
Space the Resync markers evenly throughout the bit stream
All predictively encoded information is confined within one video packet to prevent the propagation of errors
(C) 2005 by Yu Hen Hu

34. II. Improved Receiver Algorithms
European Digital Satellite TV-Broadcasting standard
MPEG-2 based source coding
Concatenated coding scheme
Error-concealment techniques based on temporal, spatial, frequency
Joint-source channel coding
Instead of remove residual redundancy by using VLC, keep it and use it at the receiver side to achieve more reliable decoding
Soft source decoding
(C) 2005 by Yu Hen Hu

35. III. Source Adapted UEP RCPC Application to GSM speech
Turbo Code
Channel coding is applied according to the bit sensitivity
Application to hierarchical video broadcast
Base layer and enhancement layer
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36. IV. Channel Adapted Combined Source-Channel Coding Methods
Goal
Allocate bit rates in an optimal way between source and channel encoders as the source and channel vary
Minimize end-to-end distortion
Feed back the CSI from the decoder to the encoder on a reverse channel
(C) 2005 by Yu Hen Hu