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TitleCompression and Robust Transmission of Images Over Wireless Channels
TagsTelecommunication Code Division Multiple Access Channel Access Method Forward Error Correction Error Detection And Correction
File Size7.8 MB
Total Pages220
Document Text Contents
Page 1

Abstract

In this dissertation we study compression and robust transmission of images over wireless

channels. Due to the problems associated with wireless channels, image communication

over these channels requires error-resilient coding schemes that must offer good compression

and low complexity. We propose an analysis-by-synthesis coding technique called Variable
Block-size two-dimensional Code Excited Linear Predictive (VB 2D-CELP) coding that

implements block-adaptive prediction and variable block-size coding. The method can be

used for still picture coding or for periodic intra-frame coding required in time-varying

image coding. The scheme demonstrates its merits over the DCT-based JPEG standard
by reducing the block effects of the DCT method while having low decoder complexity and

offering provisions for error-resilience.

Another important problem studied in this dissertation is the transmission of images

over wireless channels, in particular over CDMA Rayleigh fading channels. We develop a

robust coding scheme and propose error-resilient tools that are implemented in the source

coding scheme to mitigate the effect of uncorrected channel errors and to limit error p rop
agation. Source error detection and concealment techniques are implemented under the

source coding constraining conditions or under separation of the responses into zero-input

response and zero-state response of each image block.

Based on an investigation of the error sensitivity of the bit-stream of coded images, we
propose and investigate strategies that combine error-resilient source coding and channel

error control for the purpose of providing robust transmission. The a priori knowledge

of the bit-sensitivity of the different types of information of the compressed image data

enables us to perform an efficient unequal error protection for robust transmission. For

the channel error control, we investigate a type-I hybrid ARQ protocol using concatenated

Reed-Solomon/Convolutional coding. We study the system performance for two extreme

channel conditions: the perfectly interleaved channel and a quasi-static highly correlated

channel. For applications with different quality of service requirements, we study the system

performance in terms of reliability and transmission delay and examine the effect of outer

interleaving and maximum number of retransmissions on the system performance using a

quasi-analytical method for the case of the channel with non-independent errors.

Finally, a coding control technique that dynamically adapts the source coder rate and

channel error control to the channel condition is proposed. Based on an estimate of the

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channel condition, the rate control adapts the source coder rate so as the compression ratio
is changed to provide higher channel protection when the channel is severe, a d improve
the source rate and provide better performance when the conditions are favorable.

Page 110

5 Protocol Performance Analysis i n C D M A Rayleigh Fading Channels 86

ical bound on throughput. The computation of the bound is based on bounds provided in

Section 5.42. It is clear that the throughput of the protocol approaches zero a s the channel
error rate increases, that is at wry low values of Eb/No.

The effect of RS interleaving depth variation on the performance of the truncated pro-
tocol is shown in Figure 5.7 for the throughput and in Figure 5.5 for the protocol error
probability. The effect of finite interleaving on the probability of total error of the concate-

nated scheme was also shown in Figure 5.3. A value of I = 1 corresponds to no interleaving.
In regard to the effect of outer interleaving we have the following comments:

There is a certain degradation in performance resulting from finite interleaving. The

cuwes cross-over at a certain Eb/No value. This shows that when the channel is in
poor conditions, interleaving makes the problem worse because symbols in errors out

of the Viterbi decoder, spread over more frames which means that more frames need

to be retransmitted.

For Eb/No values greater than the cross-over point it is impossible for the truncated
protocol to achieve the same reliability as the untruncated protocol even if inter-
leaving is used. However, a reasonable amount of interleaving along with one or two

retransmissions can achieve reasonable reliability. This is because the extra transmis-

sions in the untruncated protocol are simply wasted if the goal of the system design

is to provide reasonable reliability (say lo-') instead of error-free performance (say
< 10-lo).

The normalized delay versus Eb/No is shown in Figure 5.8 where comparison with
the analytical bound is performed. In Figure 5.9, results are provided for different outer
interleaving depth. It is shown that interleaving does not significantly decrease the number

of retransmissions needed for a frame to be accepted a t the receiver. The normalized delay

of the truncated protocol is also shown in Figure 5.10.
The average transmission delay in seconds versus Eb/No is shown in Figure 5.11. The

results clearly show that the transmission delay of the truncatcd protocol is bounded by the

maximum delay due to the limited number of retransmissions. By contrast, the transmission

delay for the untruncated protocol (Figure 5.12) can be very long when the channel error
rate is high, which occasionally occurs in time-varying fading channels.

Finally, in Figure 5.13 we show the average transmission delay of the truncated pro-
tocol as function of the reliability. In order to achieve high communication reliability, the

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5 Protocol Performance Analysis in CDMA Rayleigh Fading Channels 87

Fig. 5.8 Average normalized de-
lay of the untruncated protocol:
comparison between simulation re-
sults and upper theoretical bound.

Fig. 5.10 Average normalized de-
lay of the truncated protocol as
function of maximum number of re-
transmissions L and outer interleav-
ing depth I.

Fig. 5.9 Average normalized de-
lay of the untruncated protocol as
function of RS interleaving depth I.

I
t m 2m 301 1UI ! - [dB1

Fig. 5.11 Average transmission
delay of the truncated protocol a s
function of maximum number of re-
transmissions L and RS interleaving
depth I.

Page 219

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