Electrical and Electronic Engineering - Theses

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    Interleaving techniques for high speed data transmission
    Hui, Wing Hong ( 1993)
    Interleaving is a technique used to convert a transmission channel with memory into one that is memoryless. The performance of Forward Error Correction (FEC) systems operating in the presence of burst errors is improved by passing the coded signal through an interleaving process. Commercial FEC sub-systems such as Viterbi and Reed-Solomon decoders are now commonplace, however interleavers, while indispensable, are still quite rare. This dissertation provides a comprehensive review of the two main interleaver types: block and convolutional interleavers. Following this review, the optimum convolutional interleaver is chosen for further analysis. To gain some "real-time" experience and to investigate the commercial potential of a convolutional interleaver, a variable rate interleaver has been successfully implemented on a TMS320C51 Digital Signal Processor (DSP). Many factors were considered in this implementation: throughput, synchronisation, interleaving depth and full-duplex interleaving and de-interleaving. To test the implementation, the proposed convolutional interleaver was finally interfaced to a commercial 1024 QAM 2Mbit/s modem. The investigation of the implementation of interleavers with DSP indicates that there is a need for more compact and flexible interleaver structures which can be readily integrated (in VLSI or DSP). The final part of the dissertation focused on cascaded and adaptive interleavers. Cascaded interleavers allow more sophisticated interleavers to be constructed from simple interleaving blocks. Adaptive interleavers provide the ability to adjust the interleaving depth (and thus the burst error protection) dynamically. A comprehensive computer simulation was developed and used for these investigations. The previously mentioned DSP based interleaver was also interfaced to the host personal computer (PC). This system facilitates rapid simulation results with the interleaving part of the simulation being run in real-time. In summary, this thesis provides new designs and associated implementation results for various interleaving systems including high speed single chip, variable rate, byte oriented convolutional interleavers. Based on a novel dynamic interleaver concept, a new adaptive interleaving system is proposed and this is supported with successful simulation results for advanced high speed data transmission system.
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    The design of an interface between a hardware ATM cell-stream splitter and the system bus of an experimental B-ISDN terminal
    Liew, Selbyn ( 1992)
    As worldwide standards on global networking and B-ISDN (Broadband Integrated Services Digital Network) are developed and increasing in detail and clarity, the stage is set for an advancement towards standardised high-speed global networking. With the enormous transmission bandwidth available through the use of optical communications technology, it is equally important that new applications are developed to make this worthwhile and to meet the demand and supply of communication needs today, which are calling for the increase of both volume and sophistication. In response to the imminent deployment of B-ISDN in the future, an experimental B-ISDN terminal is being developed in this department with a view to eventually provide an integration of video, voice and data communication services. This has been made possible with the CCITT adoption of Asynchronous Transfer Mode (ATM) as the basis for B-ISDN, which provides the flexibility to accommodate a variety of services with varying bit-rates. The work treated in this thesis is a continuation of this B-ISDN terminal project and is specifically targeted at completing the design of a hardware ATM cell-stream splitter to sort out ATM cells so that different cell types may be directed appropriately to where they are to be processed. The work here aims at completing the interface of the hardware splitter to the system bus of the computer workstation being used as the experimental terminal. A fair amount of attention has also been given to the use of the XC4000 Field Programmable Gate Array (FPGA) technology provided by Xilinx Inc.. The use of FPGAs has been a chief feature of the work on the hardware splitter.
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    Linearization of analogue optical transmitter by feedforward compensation
    Kwan, Anthony Chiu-Chi ( 1993)
    In recent years, analogue optical systems have received a lot of attention. Although conventional optical systems mostly employ digital format because of the low power budget requirement and good immunity to noise and distortion, analogue systems prove to have significant advantage over digital systems in applications like video distribution and satellite communication systems. However, analogue transmission systems require transmitters of low noise and low distortion. A number of linearization schemes have been proposed to reduce the distortion introduced by the analogue transmitter. One of the most widely used linearization scheme for analogue transmitter is feedforward compensation. Previous research has shown that feedforward compensation can reduce laser intensity noise as well as distortion. The work described in this thesis is to investigate design optimization of feedforward compensation and to develop a prototype of this optimized system. Experimental testing shows that the feedforward prototype is capable of reducing distortion products by 15 dB over 2.7 GHz. This work also involves modelling the feedforward system to investigate the factors limiting the operation bandwidth of the system. The model developed is a useful design tool for feedforward systems because the distortion reduction performance of the feedforward system can be predicted by characterising the individual components that made up the linearization scheme. Theoretical analysis of an alternative implementation of the feedforward system is also performed. Unlike conventional feedforward systems this new implementation uses only one laser source. Although the distortion reduction performance is degraded, the advantage is that problems associated with the use of two optical sources of different wavelengths is eliminated.
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    Feedforward compensation of directly modulated semiconductor lasers for analogue fibre communications systems
    Fock, Lap-Shun ( 1991)
    Optical fibre communication has become very attractive in the field of telecommunication. Recently, subcarrier multiplexing has emerged as a method for providing a way for each customer to directly access voice, data, and video transmissions. Although digital transmission is becoming very popular in telecommunications, analogue transmission still has its attractive features due to its low cost and compatibility to recent customer electronics. Directly modulated semiconductor lasers are important optical sources in optical communication systems. In analogue fibre communications, nonlinear distortion and intensity noise in these directly modulated semiconductor laser are main obstacles. The thesis first presents experimental characterization of semiconductor lasers, including light-current characteristics, static longitudinal mode spectrum, and small-signal frequency response. Since the linearity of semiconductor lasers plays an important role in analogue fibre communications, distortion characterization of semiconductor lasers has been investigated in considerable detail. This thesis gives details of distortion characteristics, particularly in experimental work. The main work presented in this thesis is an optical feedforward compensation scheme. This feedforward system was successfully built for experimental investigation. Since nonlinear distortion in semiconductor lasers is a severe problem in analogue fibre-communications, the feedforward compensation has been employed to compensate the distortion. Reduction of harmonic distortion and intermodulation distortion was achieved by feedforward compensation. The thesis describes the optical feedforward system in considerable detail. A simultaneous reduction of intensity noise and distortion was achieved using the feedforward technique. This provided a great improvement in signal-to-noise ratio in analogue subcarrier multiplexed systems. The experimental results are presented in this thesis. Also, the theoretical calculation of noise limitations in the feedforward system is presented in this thesis.
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    Bearings only tracking
    Logothetis, Andrew ( 1994)
    This thesis addresses the problem of tracking a single target when only bearing information is available from the sensor. The aim is to determine target information, such as heading, speed and position. A number of existing techniques are described. A new HMM based algorithm is proposed as a solution to the bearings only tracking problem. Track initiation, maintenance and termination are automatically provided in the HMM framework. The key issue of the new algorithm is the representation of a continuous state process, such as target position and velocity, using a discrete time finite state process. Rules and assumptions for constructing a finite state HMM are presented. Bearing measurement imperfections are modeled by additive white noise with state dependent standard deviation reflecting the target-observer geometry. Observer maneuvering strategies, that assist in the quality of track, are not addressed. A large number of simulations have been performed and the results show the excellent tracking capability of the new algorithm.