Wireless Sensor Network, 2009, 1, 334-339
doi:10.4236/wsn.2009.14041 Published Online November 2009 (http://www.scirp.org/journal/wsn).
Distributed Video Coding Using LDPC Codes
for Wireless Video
P. APARNA, Sivaprakash REDDY, Sumam DAVID*
Department of Electronics and Communication Engineering,
National Institute of Technology Karnataka, Surathkal, India
Received May 19, 2009; revised June 16, 2009; accepted June 23, 2009
Abstract
Popular video coding standards like H.264 and MPEG working on the principle of motion-compensated pre-
dictive coding demand much of the computational resources at the encoder increasing its complexity. Such
bulky encoders are not suitable for applications like wireless low power surveillance, multimedia sensor
networks, wireless PC cameras, mobile camera phones etc. New video coding scheme based on the principle
of distributed source coding is looked upon in this paper. This scheme supports a low complexity encoder, at
the same time trying to achieve the rate distortion performance of conventional video codecs. Current im-
plementation uses LDPC codes for syndrome coding.
Keywords: Syndrome Coding, Cosets, Distributed Source Coding, Distributed Video Coding (DVC).
1. Introduction
With the proliferation of various complex video applica-
tions it is necessary to have advanced video and image
compression techniques. Popular video standards like ISO
MPEG and ITU-H.26x have been successful in accom-
plishing the requirements in terms of compression effi-
ciency and quality. However these standards are pertinent
to downlink friendly applications like video telephony,
video streaming, broadcasting etc. These conventional
video codecs work on the principle of motion co mpensated
prediction which increases the encoder complexity due to
the coexistence of the decoder with the encoder. Also mo-
tion-search algorithm makes the encoder computationally
intensive. The downlink friendly architectures belong to
the class of Broadcast model, where in high encoder com-
plexity is not an issue. The encoder of a Broadcast model
resides at the base-station where power consumption and
computational resources are not an issue. However this
Broadcast model of video is not suitable for uplink friendly
applications like mobile video cameras, wireless video
sensor networks, wireless surveillance etc which demands
a low power, low complexity encoder. These uplink
friendly applications which belong to wireless-video model
demands a simple encoder since the power and the co mpu-
tational resources are of primary concern in the wireless
scenario. Based on the information theoretic b ounds estab-
lished in 1970’s by Slepian-Wolf [1] for distributed lossless
coding and by Wyner-Ziv [2] for lossy coding with de-
coder side infor mat ion, it is s een that effi cien t co mpressio n
can also be achieved by exploiting source statistics partially
or wholly at the decoder. Video compression schemes that
build upon these theorems are referred as distributed video
coding which befits uplink friendly video applications.
Distributed video coding shifts the encoder complexity to
the decoder making it suitable for wireless video model.
Unlike conventional video codecs distributed coding ex-
ploits the source statistics at the decoder alone, thus inter-
changing the traditional balance of complex encoder and
simple decoder. Hence the encoder of such a video codec is
very simple, at the expense of a more complex decoder.
Such algorithms hold great promise for new generation
mobile video cameras and wireless sensor networks. In the
design of a new video coding paradigm, issues like com-
pression efficiency, robustness to packet losses, encoder
complexity are of prime importance in comparison with
conventional coding system. In this paper we present the
simulation results of distributed video coding with syn-
drome coding as in PRISM [3], using LDPC codes for
coset channel coding [4].
2. Background
2.1. Slepian-Wolf Theorem for Lossless Distrib-
uted Coding [1]
Consider two correlated information sequences X and Y.
*SMIEEE.

P. APARNA ET AL. 335
Encoder of each source is constrained to operate with-
out the knowledge of the other sou rce while the de coder
has access to both encoded binary message streams as
shown in Figure 1. The problem that Slepian-Wolf
theo rem a dd res ses is to de termi ne the mi ni mu m nu mber
of bits per source character required for encoding the
message stream in order to ensure accurate reconstruc-
tion at the decoder. Considering separate encoder and
the decoder for X and Y, the rate required is RX ≥ H(X)
and RY ≥ H(Y) where H(X) and H(Y) represents the en-
tropy of X and Y respectiv ely. Slepian -Wolf [1] show ed
that good compression can be achieved with joint de-
coding but separate encoding.
For doing this an admissible rate region is defined [6]
as shown in Fi gure 2 given b y:
RX + RY ≥ H(X,Y) (1)
RX ≥ H(X/Y), RY ≥ H(Y) (2)
RX ≥ H(X), RY ≥ H(Y/X) (3)
Thus Slepian-Wolf [1] sh owed that Equation (1) is th e
necessary condition and Equation (2) or Equation (3) are
the sufficient conditions required to encode the data in
case of joint decoding. With the above result as the base,
we can consider the distributed coding with side infor-
mation at the decoder as shown in the Figure 3. Let X be
the source data that is statistically dependent to the side
information Y. Side information Y is separately encoded
at a rate RY ≥ H(Y) and is available only at the decoder.
Thus as seen from Figure 2 X can be encoded at a rate RX
≥ H(X/Y).
Figure 1. Compression of correlated sources by separate
encoder but decoded jointly.
Figure 2. Admissible rate region [5].
Source X/YLoss l ess
Encoder Lo ssless
Deco der
X
Side I nformationY
R
x
H(X/Y)
R
Y
H(Y)
Figure 3. Lossless decoder with side information.
2.1. Wyner-Ziv Rate Distortion Theory[2,6]
Aaron Wyner and Jacob Ziv [2,6] extended Slepian-
Wolf theorem and showed that conditional Rate-MSE
distortion function for X is same whether the side in-
formation is available only at the decoder or both at
encoder and decoder; where X and Y are statistically
dependent Gaussian random processes. Let X and Y be
the samples of two random sequences representing the
source data and side information respectively. Encoder
encodes X without access to side information Yas
shown in Figure 4.
Decoder reconstructs
X
using Y as side information. Let
D = E [d (
X
, X)] is the acceptab le distortion. Let RX/Y(D)
be the rate required for the case where side information is
available at the encoder also and represent the
Wyner-Ziv rate required when encoder doesn’t have access
to side information. Wyner-Ziv proved that Wyner-Ziv rate
distortion function is the achievable lower
bound for the bitrate for a distortion D
)(
/DRWZ
YX
)DR (
/
WZ
YX
Correlated
Sources Joint
De cod
Encoder
X
Encoder
Y
X
Y
Rx
Ry
er
X
ˆ
Y
ˆ
0 )()( // DRDR YX
WZ
YX (4)
They also showed that for Gaussian memoryless
sources
0)()( // DRDR YX
WZ
YX (5)
As a result source sequence X can be considered as the
sum of arbitrarily distributed side information Y and in-
dependent Gaussian No ise.
R
y
R
x
bit s
bits
H(x)
H(x/y)
H(y/x)
H(y)
Distributed video coding is based on these two funda-
mental theories, specifically works on the Wyner-Ziv
coding considering a distortion measure. In such a coding
system the encoder encodes each video frame separately
Figure 4. Lossy decoder with side information.
Vani s h i ng Errors
for long sequences
No Errors
H(x,y)=R
x
+ R
y
Source X/YLossy Enco derLossy D
X
Side Inf
(D)
wz
x
R
ecoder
ormation Y
X'
H(Y)
Y
R
H(X/Y)

336 P. APARNA ET AL.
The correlation between binary sources X = [X1,
X2.....,Xn] and Y = [Y1, Y2, ..., Yn] is modeled using a bi-
nary symmetric channel. We consider Xi and Yi to be
correlated according to Pr [Xi
 Yi] = p < 0.5. The rate
used for Y is its entropy RY = H(Y), therefore the theo-
retical limit for lossless compression of X is given by
with respect to th e correlation statistics betw een itself and
the side information. The decoder decodes the frames
jointly using the side information available only at the
decoder. This video paradigm is as opposed to the conven-
tional coding system where the side information is avail-
able both at the encoder and decoder as shown in Figure 5.
nRx ≥ nH(Xi/Yi) = nH(p) =n(−plog2p−(1−p)log2(1−p))
2.2. Syndrome Coding [5] (6)
The compressed version of X is the syndrome S which
is the input to the chann el. The source Y is assumed to b e
available at the decoder as side information. Using a lin-
ear (n,k) binary block code, it is possible to have 2n−k
distinct syndromes, each indexing a set of 2k binary
words of length n. This compression results in mapp ing a
sequence of n input symbols into (n−k) syndrome sym-
bols.
Let X be a source that is to be transmitted using least
average number of bits. Statistically dependent side in-
formation Y, such that X = Y + N is available only at th e
decoder. The encoder must therefore encode X in the
absence of Y, whereas the decoder jointly decodes X us-
ing Y. Distributed source encoder compresses X in to
syndromes S with respect to a Channel code C [7]. De-
coder on receiving the syndrome can identify the coset to
which X belongs and using side information Y can recon-
struct back X.
3. Implementation
2.3. Correlation Channel and the Channel Codes [4] 3.1. Encoder
The performance of the channel codes is the key factor
of the distributed video coding system in both error cor-
recting and data compression. Turbo and LDPC codes
are two advanced channel codes which have astonishing
performance near the Shannon Capacity limit. The use of
LDPC codes for syndrome coding was first suggested by
Liveris in [4], where the message passing algorithm was
modified to take syndrome information in to account.
The encoder block diagram is shown in the Figure 6. The
video frames are divided into blocks of 8x8 and each
block is processed one by one. Block DCT (Discrete
Cosine Transform) is applied to each 8x8 block (or
16x16) and the DCT coefficients are zig-zag scann ed so
that they are arranged as an array of coefficients in or-
der of their importance. Then the transformed coeffi-
cients are uniform quantized with reference to target
distortion measure and desired reconstruction quality.
After quantization a bitplane is formed for each block
as shown in Figure 7 [3]. Main idea behind distributed
video coding is to code source X assuming that the side
information Y is available at the decoder such that X =
Y + N, where N is Gaussian random noise. This is done
in the classification step where bitplane for each coeffi-
cient is divided into different levels of importance.
Classification step strongly rely on the correlation noise
Figure 5. Lossless decoder with side information.
Figure 6. Video encoder.
Encod er
X
ˆ
De co de r
X
Side Information
Y
DCT &
Zigzag ScanUniform Quantization Classification
Syndrome Codi ng
using
LDPC co de s
En t ropy Coding us in g
Adapt ive Huff man
Codin g
Bit Plan e
Formation
Input
Block
Correlat ion Noise
Estimation
Encode
Bit S t r e
da m
One Frame
De lay
MSB
LS B
Class Info

P. APARNA ET AL. 337
Figure 7. Bit planes for each coefficient blocks.
structure N between the source block X and the side
information block Y. Less is the correlation noise be-
tween X and Y, more is the similarity and hence less
number of bits of X can be transmitted to the decoder.
In order to classify the bitplanes offline training is done
for different types of video files without any motion
search. On the basis of offline process 16 types of
classes are formed, where each class considers different
number of bitplanes for entropy coding and syndrome
coding for each coefficient in the block. In the classifi-
cation process, MSE (mean square error) for each block
is computed with respect to the zero motion blocks in
the previous frame. Based on the MSE and the offline
process appropriate class for that particular block is
chosen. As a result some of the least significant bit
planes are syndrome coded and some of the bitplanes
that can be reconstructed from side information are to-
tally ignored. The syndrome coding bitplanes shown in
black and gray in Figure 7 and skip planes shown in
white in Figure 7. Skip planes can be reconstructed
back using side information at the decoder and hence
need not be sent to the decoder. The important bits of
each coefficient that cannot be determined by side in-
formation has to be syndrome coded [3]. In our imple-
mentation we code two bitplanes using coset channel
coding and the remaining syndrome bitplanes using
Adaptive Huffman coding. Among the syndrome cod-
ing bitplanes we code the most significant bit planes
using Adaptive Huffman coding. The number of bit-
planes to be syndrome coded is directly used from class
information that is hard coded. Hence we need not send
four-tuple data (run, depth, path, last) as in PRISM [3].
Rest of the least significant bitplanes is coded using
coset channel coding. This is done by using a parity
check matrix H of a (n,k) linear channel code. Com-
pression is achieved by generating syndrome bits of
length (n-k) for each n bits of data. These syndrome bits
are obtained by multiplying the source bits with the
parity check matrix H such that
x
0
x
1
x
2
x
3
x
63
b
0
b
2
b
m
b
1
Coe fficients
Bitplanes
S = HbX
where S represents the syndrome bits. H represents the
parity check matrix of linear channel code. bX represents
the source bits.
These syndromes identify the coset to which the
source data belongs to. In this implementation we have
considered two biplanes for coset coding marked gray in
the Figure 7. We have implemented this using irregular
3/4 rate LDPC coder [4].
3.2. Decoder
The Decoder block diagram is shown in the Figure 8.
The entropy coded bits are decoded by an entropy de-
coder and the coset coded bits are passed to the LDPC
decoder. In this implementation, previous frame is con-
sidered as the side information required for syndrome
decoding. Once the syndrome coded bits are recovered
they identify the coset to which Xi belongs and hence
using the side information Yi we can correctly decode the
entire bits of Xi. The quantized codeword sequence is
then dequantized and inverse transformed to get the
original coefficients.
4. Simulation Results
Video Codec is designed for a single camera scenario
which is an application to wireless network of video
camera equipped with cell phones. The video codec is
simulated and tested with a object oriented approach
Encoded
Bit Stream
Entropy
D ec oding
Syndro me
D ecoding using
LDPC dec o der
Side I nf ormation
Generation
Bit Plane
Formation
LS B
MSB
Uniform
DeQuantization IDCT &
Zigzag Scan
Reco nstructe
d
Block
Figure 8. Video decoder.

338 P. APARNA ET AL.
Table 1. Filename: foreman. QCIF, frame rate=30fps. Table 2. Filename: football. QCIF, frame rate=30fps.
Figure 9. a) Error resilience characteristics of DVC, 4th, 10th, 20th frames are lost for football; b) Error resilience character-
istics of DVC, 4th, 10th, 20th frames are lost for foreman.
using C++ in gcc. The program processes frames one by
one and within each frame, block wise processing is
done. The input to the encoder is a QCIF video file
(Quarter Common Intermediate Format). Encoder allows
the storage of one previous frame. Objective perform-
ance evaluation of the system is done by measuring the
Compression Ratio (CR), MSE and the Peak Signal to
Noise Ratio (PSNR) between the original and the recon-
structed video. The PSNR and CR for various video se-
quences is computed. These are compared with that of
H.263+ Intra and H.263+ Predictive video codec [8]. The
encoder and decoder block as shown in Figure 6 and
Figure 8 respectively are implemented and some pre-
liminary simulation results are presented in this paper
for two video files Football and Foreman in QCIF
resolution with a frame rate of 30 fps. The rate distor-
tion performance and the error resilience characteris-
tics of the distributed video coder is presented in this
paper. As seen from the Table 1, for the same bitrate
distributed video coder has better PSNR than DCT
based intraframe coder and but is slightly inferior to
H.263+ predictive coder [8] for Foreman file. As seen
from Table 2 distributed video coder has better PSNR
than DCT based intraframe coder and H.263+ predic-
tive coder for Football file. With some enhancements
to the current coding scheme such as accurate model-
ing of correlation statistics between the source data and
the side information, proper motion search module for
side information generation etc, better rate-distortion
performance can be achieved with a low complexity
encoder model.
Error Resilience characteristics of Distributed
video scheme is as shown in Figure 9a for Football
and Figure 9b for Foreman. Effect on the quality of
the reconstructed video sequence is seen by dropping
4th, 10th, 20th frames at the decoder in our imple-
mentation. It is seen that distributed video coder re-
covers quickly. In Distributed video scheme, decod-
ing is dependent on the side information Y that is
universal for all source data X as long as correlation
structure is satisfied.
5. Conclusion
In this paper we have tried PRISM [3] like implementa-
tion using LDPC coset channel coding. By proper mod-
eling of correlation structure of source and the side in-
formation for video we can achieve better compression
performance with better quality of reconstructed video
sequence. However the main aim of distributed video
coding scheme is to reduce encoder complexity to con-
form with wireless-video model, which seems to be sat-
isfied. Distributed codec is more robust to packet /frame
Luma PSNR (dB) for different Methods
BitRate
(Mbps) DVC Implementation H.263+ Predictive Coder IntraCoder (Motion JPEG)
3.52 30.724 25.62 30.07
3.67 31.834 25.76 30.92
4.87 34.005 26.59 33.80
Luma PSNR (dB) for different Methods
BitRate
(Mbps) DVC Implementation H.263+Predic- tive Coder IntraCoder (Motion JPEG)
2.57 31.357 34.72 30.092
2.67 33.554 35.03 32.863
3.55 35.534 35.86 34.92

P. APARNA ET AL. 339
loss due to the absence of pred iction loop in the encod er.
In a Predictive coder accuracy of decoding is strongly
dependent on a sing le predictor from the encoder, lo ss of
which results in erroneous decoding and error propaga-
tion. Hence Predictive coder can recover from packet or
frame loss by only some extent. The quality of the re-
constructed signal for the same CR can be improved by
performing more complex motion search. However it is
seen that the current implementation operates well in
high quality (PSNR of order of 30dB) regime. The ex-
tension to lower bit rates withou t any compromise in the
quality so that it is comparable with the conventional
codecs will be the next part of the work.
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