Reed Solomon codes, a subset of multi-level cyclic block codes with, powerful error-correcting capabilities, are computationally very efficient when algebraic decoding is applied. They may, however, give weaker performance compared to convolutional codes, at least at moderate bit error rates (around 10-5 to 10-6) on the AWGN channel. This disadvantage mainly results from the lack of a generally applicable method for soft decision decoding. Because of easy acceptance of soft decision information, trellis decoders incorporating soft decisions can produce quite useful improvements in coding gain over hard decisions with little increase in complexity. Although trellis methods are commonly used for convolutional codes they can be applied to block codes[23]. In this project, therefore, it is intended to present results of an investigation of soft decision trellis decoding as applied to Reed Solomon codes, and the development of effective decoding algorithms. As a means of maximum likelihood decoding, Viterbi decoding for Reed Solomon codes was attempted. However, the complexity of such a decoder is often prohibitive. In order to reduce the complexity of the Viterbi decoder, an existing reduced search algorithm, which was developed for convolutional codes, was applied to Reed Solomon codes. Moreover, new reduced search methods were developed to enhance its computational efficiency. In contrast to convolutional codes, low rate codes require higher decoder complexity than high rate codes for Viterbi decoding of Reed Solomon codes. For low rate Reed Solomon codes, therefore, the Fano sequential decoding algorithm was investigated, since the complexity of a sequential decoder is independent of the number of encoder memory elements. A simulation model incorporating an AWGN channel has been used to establish the performance of the various suggested decoding algorithms. Importance sampling simulation techniques have been used to reduce simulation time. Simulation results of Viterbi decoding showed that soft decision decoding for the Reed Solomon codes could give at least 2dB coding gain over hard decision decoding on the AWGN channel and that the performance of reduced search, and sequential decoding could approximate that of the Viterbi method with much better computational efficiency. Theoretical decoding performance has been established by using the weight distributions of the codes. Based upon the theoretical results, the simulation results have been analysed. Moreover, the performance and the computational efficiency of each decoding method have been compared, and the advantages and disadvantages of each method indicated. It has been shown that, by using trellis methods, the potential disadvantages of Reed Solomon codes can be removed, and thus the performance of trellis decoded Reed Solomon codes is comparable with that of convolutional codes on the AWGN channel at moderate bit error rate. Therefore, Reed Solomon codes may be used in a wider range of applications, possibly with the decoding method adjusted to the channel conditions.

Video transcoding is a generic name for a video gateway structure whereby the tandeming process does not involve any high complexity cascaded decoding and re-encoding operations in contrast to the existing conventional solutions. Diverse multimedia communication network characteristics, such as bandwidth limitations and varying congestion conditions, incur quality degradation in video transmissions. The matching of input and output network constraints and characteristics is possible with video transcoding at a centralised unit within the network. Moreover, video transcoding also provides a suitable translation mechanism for different video compression standards achieving a transparent interconnection between diverse network topologies. In addition, video transcoding offers a method for providing robustness against transmission error effects which occur during the transmission of compressed video streams over highly bandwidth- restricted communication media, such as popular mobile-wireless networks. Due to the severe bandwidth restrictions of such networks, the video signals require low bit rate coding which in turn renders video streams highly susceptible to radio channel errors. Therefore, error-resilient operations also need to be provided together with the syntax and transmission rate translation features of video transcoders. The unique features of video transcoding provide flexible and efficient ways to alleviate the previously addressed three major issues for various requirements. These requirements can be imposed by the diversity of networks on which numerous applications are running or by different standards themselves as well as by a wide range of users. This is strictly related to a universal interoperability issue of heterogeneous characteristics and requirements which demand effective end-to-end solutions. Thus, the aim of this research presented in this thesis is to develop algorithms for the provision of such remedial solutions to the interoperability problem. In the light of these facts, the research work focuses on the design of various video transcoding algorithms. The objectives of these algorithms are to ease network congestion and/or user bandwidth limitation conditions, support essential standard conversions between different compression schemes and provide necessary error robustness over highly error-prone transmission media, such as mobile radio networks. The ultimate target is to establish a common platform where all the above three aims are successfully satisfied. Extensive computer simulations demonstrate the effectiveness of the proposed and designed systems, throughout the course of the research work. These simulations are assessed with the use of objective and subjective performance measures.

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Double-sideband amplitude-modulation is widely used because of its simplicity in transmission and reception, but in a number of applications the inclusion of a data channel has become an additional requirement. An additional data signal, transmitted with a normal AM broadcast signal, can be used by a specially-equipped receiver to provide for display and control functions. The data signals should not impair the main broadcast signals, and should be imperceptible to listeners with ordinary radio receivers. The principles of a new modulation scheme, which permits an additional data channel to be superimposed on conventional DSB-AM signals without any bandwidth increase or envelope distortion, are described in this thesis. The proposed system uses combined amplitude/frequency modulation to provide for the additional data channel. It is shown that bandlimitation may be preserved by frequency modulating the carrier with frequency-modulating functions related to the complex zeros of the amplitude-modulating signal. The performance of the new modulation technique is assessed by computer simulation. The cross-talk between the two channels due to multipath propagation is considered to be a limiting factor. A novel complex filtering technique to detect complex zeros of realsignals is introduced and discussed. The distribution of the complex zeros of positive bandlimited signals is assessed experimentally, and the possible data rates of the proposed system are discussed.

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