Electrical Engineering

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Real-time interactive multiprogramming.
(1978) Heher, Anthony Douglas.; Nattrass, Henry Lee.
This thesis describes a new method of constructing a real-time interactive software system for a minicomputer to enable the interactive facilities to be extended and improved in a multitasking environment which supports structured programming concepts. A memory management technique called Software Virtual Memory Management, which is implemented entirely in software, is used to extend the concept of hardware virtual memory management. This extension unifies the concepts of memory space allocation and control and of file system management, resulting in a system which is simple and safe for the application oriented user. The memory management structures are also used to provide exceptional protection facilities. A number of users can work interactively, using a high-level structured language in a multi-tasking environ=ment, with very secure access to shared data bases. A system is described which illustrates these concepts. This system is implemented using an interpreter and significant improvements in the performance of interpretive systems are shown to be possible using the structures presented. The system has been implemented on a Varian minicomputer as well as on a microprogrammable micro= processor. The virtual memory technique has been shown to work with a variety of bulk storage devices and should be particularly suitable for use with recent bulk storage developments such as bubble memory and charge coupled devices. A detailed comparison of the performance of the system vis-a-vis that of a FORTRAN based system executing in-line code with swapping has been performed by means of a process control Case study. These measurements show that an interpretive system using this new memory management technique can have a performance which is comparable to or better than a compiler. oriented system.
The lightning ground flash : an engineering study.
(1979) Eriksson, Andrew John.
The thesis is concerned with a study of the electrical engineering parameters of the lightning ground flash - i.e. the statistical distributions of peak current amplitudes, discharge current waveform characteristics, and flash striking distances - in the event of flashes to practical engineering structures. In view of its predominating frequency of occurrence in practical situations, the discharge of primary concern is the downward progressing and negatively charged ground flash. A central feature of this work is the establishment of a lightning research station (incorporating a 60 m instrumented mast) in the Transvaal highveld region of South Africa. The design of this station and the related measurement techniques are fully described. Preliminary results accumulated over a 6-year period of observation are presented, and include recordings obtained during direct strikes to the mast, as well as data from associated measurements of additional thunderstorm and lightning parameters. The latter studies include the use of closed circuit television video recordings, together with electrostatic field mills and lightning flash counters. Analysis of the resultant data serves to provide a comprehensive characterisation of the thunderstorm and lightning climatology in the region - on the basis of electrical activity. With only few exceptions, it is concluded that the characteristics of lightning observed in the. Transvaal region are generally consistent with the trends of data from other regions of the world. A unique aspect of the project is a study of lightning striking distances. An attempt to estimate these distances using bi-directional photography of flashes to the research mast is described, and several preliminary results are also presented - in conjunction with the associated measurements of discharge current amplitude. These results are compared with previously used relationships between striking distance and peak current.
A software speech recognition system using a phonetic approach.
(1985) Everson, L Robert H.; Broadhurst, Anthony D.; Levy, David C.
Computer speech recognition techniques were investigated. This investigation included a study of the hearing and speech process. An algorithm was developed that used nine features to identify the phonemes in speech signals. Two of these features, the total energy and the number of zero crossings in a specific section of the speech signal, were obtained directly from the digitized speech signal. The other features, frequency energy bands and formant frequencies, were measured from a spectral analysis of the signal. A Hewlett Packard mini-computer was used for the development of the necessary software in FORTRAN. For the testing of the algorithm ten words, "zero" through to "nine" were used.
A graphic rasterizer IC.
(1987) Izzard, Martin John.; Poole, Kelvin F.; Peplow, Roger Charles Samuel.
A single chip line-rasterizer that overcomes the major bottleneck in graphics display systems has been designed by the author on a 4408 element gate array marketed by Plessey Semiconductors limited. The rasterizer was fabricated by Plessey using their 2 micron, double-level metal ISO CMOS process, in the United Kingdom. Two identifiable bottlenecks in the redraw speed on a general graphics display system are video memory bandwidth and rasterization speed (in dots produced per second). The rasterizer described here is capable of working in parallel with other rasterizers to overcome the rasterization bottleneck. Systems incorporating it are flexible and expandable. The rasterizer requests a primitive from a host or master part of the system. Once it has a primitive to work on, it begins rasterization. The rasterizer queues requests to write dots to the video memory part of the system. The device accepts two ordered pairs of 16-bit numbers as start-of-line and end-of-line coordinates, on an 8-bit bus; the dot addresses are in the form of two 16-bit numbers on a 32-bit bus. Simulation with CLASSIC showed that the device could be clocked at up to 8 MHz and would then produce dots at between 2 MHz and 4 MHz (dependent on the type of line) after the initial analysis overheads. This means that any video memory bandwidth may be fully used with this device and any improvements in memory bandwidth may be taken advantage of in a system using the parallel rasterization scheme. The Plessey test engineers exercised the device to prove the success of the fabrication. Further tests were performed by the author. In these, the rasterizer was seen to gather data correctly. The rasterization of a range of different types of lines, manhattan and general, short and long and lines of different direction, was tested. The various algorithm terminations were verified and all branches exercised. The flow control on the pixel bus was checked. The device used for all the tests, performed correctly at 10 MHz (design specification 8 MHz) which corresponds to a maximum rasterization speed of 5 MHz for 0° and 90° lines and between 2.5 MHz and 3.3 MHz for general lines. The results show that the rasterizer performance will allow full use of the memory bandwidth of the system and hence overcome the major bottleneck in many graphics display systems.
Modelling and analysis of turbogenerators in single machine and multi- machine subsynchronous resonance studies.
(1987) Jennings, Glenn Douglas.; Harley, Ronald G.
Subsynchronous Resonance (SSR) is a condition which occurs when turbogenerators are connected to series capacitively compensated transmission systems and it can cause large scale damage to the turbogenerators. The accuracy of predictions of this phenomenon are limited by the accuracy of the mathematical models used for the various system elements. The modal method of modelling a turbogenerator shaft, in which parameters are associated with each natural torsional mode of the shaft, is investigated in detail and the sensitivity of SSR predictions (both small signal and transient) to uncertainties in the mode parameters is evaluated. The modal model is then used to obtain reduced order shaft models and the accuracy of these reduced order modal models in SSR predictions is ascertained. The determination of mode parameters from generator transient response waveforms is investigated. A continuing problem in this field is the separation of damping values obtained from measurements on a synchronized generator, into their mechanical and electrical components. A method is proposed in this thesis which uses eigenvalue scanning techniques together with FFT analysis to achieve this separation. The SSR stability of, and the torsional interaction between two adjacent generators at a power station is studied. The analysis covers identical generators, nominally identical generators with small differences between their mode parameters and different generators with a coincident torsional mode. In addition, the torsional interaction between generators at different power stations which are remote from each other is investigated. This entire analysis is greatly assisted by modelling the turbogenerator shafts in modal form. Finally the damping of SSR oscillations in two non-identical adjacent turbogenerators with a single controlled shunt reactor, which uses the sum of the generator speed signals as an input to the controller, is investigated.
Data base optimisation for an I.C. design layout package on the VAX.
(1987) Figg, Peter Gerald.; Peplow, Roger Charles Samuel.
The performance of an interactive graphics/computer aided design system, such as the IC layout and rule checking package implemented as part of the schematic drawing application on the Gerber Systems Technology IDS-80, is very closely related to the performance of the system's data base. This is due to the fact that most editing functions on an interactive graphics/CAD system are data base intensive functions and the data base and its management routines form one of the major building blocks of a CAD system. It can therefore be said that the performance of a CAD system is directly dependent on the data base access time and the efficiency of the managing routines. The primary objective of this project was to enhance the performance of the IC layout and rule checking package. This was done by improving the performance of the data base of the system. This was achieved by following two mutually supportive paths. The first was the transportation of the software to a new host machine which had a 32-bit processor and virtual memory capabilities. The second was to try and improve the performance of the transported data base by utilising sophisticated data base structures and memory management·techniques facilitated by the larger available memory of the new host to optimise the data base operations. The effectiveness of the two paths in achieving their respective goals was evaluated using evaluation programs which simulated characteristic data base activities.This thesis documents the above process, as well as expounds on some of the background related theory which was instrumental in the progress of the project and the drawing of the final conclusions.
Colour measurement and colour reproduction systems.
(1987) Chalmers, Andrew Neil.; Nattrass, Henry Lee.
Techniques of colour measurement and colour reproduction are important in a wide range of commercial and social activities in most modern economies. Their study thus constitutes one of the major areas of interest to the CIE. The project described in this thesis began as an outgrowth of studies of new types of light sources and of the colorimetry of colour-TV systems; plus a conviction that modern TV cameras can operate effectively with a wide range of different illuminating spectra. It was soon evident that two important prerequisites for this research were: an understanding of the processes of human colour vision; and a knowledge of the standard, international, colorimetric terminology of the CIE. These topics are discussed fully in the text. Also included is a review of modern gas-discharge lamps, the~y properties, and their applications. Both high-pressure (HID) types and low-pressure (fluorescent-tube) types are considered. Because of the need to measure the colours of surfaces and their TV reproductions as accurately as possible, various forms of colorimeter were examined, leading to the choice of a spectrophotometer system for this work. The design, construction, and evaluation of an original spetrophotometer system (the UND Spectrophotometer) are described fully in the text. Finally, attention is given to the operation of a television system under nonstandard lighting. Twelve different light sources were evaluated as TV ((taking" illuminants, using both subjective and colorimetric methods of assessment. The experimental results tend to confirm that colorimetric methods are unsuited to colour reproduction evaluation, and that subjective methods are more meaningful. A subjective scale of colour reproduction performance was established, and it was found to correlate closely with the CIE general colour rendering index (Ra) for the various test lamps. The work reported herein predates similar experiments with TV lighting by other workers, and it includes a wider range of light sources. In spite of differences in experimental technique, however, there is broad agreement with their general results.
The Analysis of computer systems for performance optimisation.
(1987) Meiring, Pierre Andre.; Nattrass, Henry Lee.
The project investigated the problem of performance optimisation of computer systems at the systems level. It was ascertained that no generally accepted technique for approaching this problem exist. A theoretical approach was thus developed which describes the system, the workload and the performance in terms of matrices which are deduced from measured data. An attempt is then made to verify this theory by applying it to a real system in a controlled environment. A dummy workload is used and measurements are made on the computer system for various configurations. The results thus obtained are compared with the expected trends in system performance and conclusions are drawn which appear to verify the validity of the theory proposed.
Quantitative feedback design and construction of a two by two system with large disturbances.
(1989) Boje, Edward Sidney.; Eitelberg, Eduard.
The quantitative feedback theory (QFT) of Horowitz is theoretically well developed for multivariable systems but there is not sufficient knowledge on its application to practical problems. A "flying machine" consisting of an airframe with two independently controlled sets of wings has been designed and constructed as a 2-input 2-output control problem. The airframe is constrained to move vertically on guide wires and to rotate about a pivot. Air flow over the wings is provided by two 7.SkW fans operated without any attempt at providing non-turbulent flow. The arrangement of the wings is such that in open loop, the dynamic behaviour of the airframe from the rear set of wings to the height is non-minimum phase. Additionally, the airframe is unstable for some flight conditions. This uncertain, non-linear and highly disturbed plant provides an ideal practical environment in which to test controller design theory. The construction, modelling, parameter estimation and simulation of the flying machine is described. Three different controller structures are disGussed, with actual controller designs arrived at from QFT understanding. The controller designs for the flying machine take into account parameter uncertainty and trade off disturbance attenuation against rate and amplitude saturation at the wing angle inputs.
Implementation of a proprietary CAD graphics subsystem using the GKS standard interface.
(1989) Davies, Trevor Rowland.; Peplow, Roger Charles Samuel.
This project involved porting a Graphical Software Package (GSP) from the proprietary IDS-BO Gerber CAD system onto a more modern computer that would allow student access for further study and development. Because of the popularity of Unix as an "open systems environment", the computer chosen was an HP9000 using the HP-UX operating system. In addition, it was decided to implement a standard Graphical Kernel System (GKS) interface to provide further portability and to cater for the expected growth of the GKS as an international standard. By way of introduction, a brief general overview of computer graphics, some of the essential considerations for the design of a graphics package and a description of the work undertaken are presented. Then follows a detailed presentation of the two systems central to this project i) the lDS-8O Gerber proprietary CAD system, with particular attention being paid to the Graphical Software Package (GSP) which it uses and ii) the Graphical Kernel System (GKS) which has become a widely accepted international graphics standard. The major differences between the lDS-8O Gerber GSP system and the GKS system are indicated. Following the theoretical presentation of the GSP and GKS systems, the practical work involved in first implementing a "skeleton" GKS interface on the HP9000 Unix System, incorporating the existing Advanced Graphics Package (AGP) is presented. The establishment of a GKS interface then allows an lDS-8O Gerber GSP interface to be developed and mapped onto this. Detailed description is given of the methods employed for this implementation and the reasons for the data structures chosen. The procedures and considerations for the testing and verification of the total .system implemented on the HP9000 then follow. Original lDS-8O Gerber 2-D .applications software was used for the purpose of testing. The implementation of the data base that this software uses is also presented. Conclusions on system performance are finally presented as well as suggested areas for possible further work.
Numerical alogrithms for PWM modulators.
(1989) Green, Walter Battman.; Harley, Ronald G.
The development of a simple efficient Pulse Width Modulation (PWM) modulator has been a goal for many research workers. In general three techniques have been used, namely; the analogue triangular wave technique; the use of look-up tables, and the use of Analogue to Digital converters together with analogue circuitry. The modulator described in this thesis is based on an iterative numerical algorithm, and is thus fundamentally different from all previous techniques. The algorithm is limited only by the speed and precision of the associated digital circuitry and can achieve higher modulating frequencies with greater accuracy than can be realised using any of the methods that have previously been investigated. The use of high switching frequencies simplifies the design of filters to reduce both unwanted harmonics and acoustic noise. In this thesis, an equation of a multiphase digital oscillator is derived which is simple to implement and will operate over a wide range of frequencies. The conditions for stable oscillation are derived, and two classes of oscillator are developed. It is shown how the frequency and amplitude of oscillations can be independently and continuously varied. The errors in computing the amplitude and frequency are analysed, and are shown to be cyclic. Upper bounds for the amplitude errors are derived. Single and three phase PWM modulators are described and the implementation procedures for their practical realisation are developed. Two specific implementations of the algorithm are investigated and experimental results confirm theoretical analyses. The algorithm can be incorporated in the Space Vector Modulation (SVM) method of PWM, to improve the resolution at low speeds and to enable the SVM technique to be applied at high gear ratios. A 3-phase 16-bit PWM modulator was built and operated satisfactorily with a pulse switching frequency of 20 kHz and an output frequency range of 1000:1.
A gridless, variable perveance Pierce electron gun.
(1994) Foulis, Bruce David.; Nattrass, Henry Lee.
This thesis covers the design and development of a modulated Pierce electron gun used in the construction of experimental travelling wave tube (TWT) amplifiers. The gun incorporated an open aperture switching electrode, positioned mid-way between anode and cathode, to pulse the beam. This method of modulation did not have the same adverse effects on electron trajectories as in the case of a conventional mesh grid, but rather the electrode could be used to alter the focus conditions within the gun and subsequently improve certain beam characteristics. Ion focusing effects could also be eliminated with the electrode, allowing dual mode operation of the guns without the complications normally associated with such a practice. The switching electrode was simulated to ascertain its effect on electron trajectories within the gun, using finite element analysis as well as an electron optics design program. A test gun was constructed in a glass envelope in order to investigate the performance of the new design. The glass gun allowed a beam analysis to be performed, as well as thermal measurements to be made. Results from this gun compared favourably with earlier simulations. The results of two metal/ceramic construction TWTs are presented, showing the beneficial effects of the switching electrode on the performance of the tubes as a whole, and the electrode's potential to compensate for constructional anomalies. The joining of metals to ceramic using active brazing techniques is also an important aspect tackled by the thesis, with several innovative ideas being implemented in the construction of the devices. A simple yet reliable electrical feed-through was developed for those guns having a ceramic envelope. Extensive work was also performed on the manufacture of impregnated tungsten cathodes for use in the electron guns. Several test diodes, including a water-cooled demountable test vehicle, were constructed to test the performance of the cathodes. An analysis was performed on the patchy behaviour of some of the initial cathodes to improve the preparation methods used in the laboratory. The emission results obtained from the cathodes are documented, as is the successful incorporation of several of them into the new modulated gun design.
Performance, models and topologies of common channel signalling system number seven.
(1998) Ramlakan, R. S.; Mneney, Stanley Henry.
An Intelligent Network (IN) can be visualised as a central computer interconnected to the telecommunication network that allows the creation and deployment of services as consumer demand arises. Messages or information are transferred between the central processor and exchanges by a standardised form of signalling, known as Common Channel Signalling System Number Seven (CCS7). The thesis focuses on CCS7 performance, models and topologies. A requirement of the research entails the study of the CCS7 performance parameters: signalling delays and signalling network dependability. Signalling delays may be comprised of signalling message transfer delays within the message transfer part and queueing delays. Signalling network dependability usually include availability, reliability, maintainability and network robustness. For the purpose of modelling, the decomposition of the CCS7 message transfer part into subsystems is essential. A generic model for the message transfer part was used to implement certain functionalities of CCS7 in OPNET. OPNET (Optimised Network Engineering Tools) is a comprehensive engineering system capable of simulating large communication networks with detailed protocol modelling and performance analysis. The simulation software (OPNET) and performance parameters were used to analyse CCS7 networks resulting in the selection of a particular topology for a given region. The network structures analysed include the mesh topology, standard quad topology, hierarchical topology and Telstra's new topology. It was found that the mesh structure had the best simulated and availability/reliability results but was impractical for large networks due to the cost implications. This cost factor led to the selection of a hierarchical signalling system for Kwa-Zulu Natal.
Multiple ring networks in clustered traffic environments.
(1998) Green, Breton.
Ring networks are appropriate for the full range of network levels, including multiprocessor systems, local area computer networks and high speed backbones. The most well known and widely implemented examples are the IBM token ring and FDDI networks. Ring networks have the advantages of high channel utilisation and bounded delay if an n-limited service policy is used. The packet transfer delay, defined as the average time a packet spends in the network from the time it is generated until the time it is received at its destination node, improves with the number of rings on which a node is connected. However, many ring connections are not economically feasible since the cost of the ring interface increases with the number of rings. There has been an abundance of previous work on single token ring networks. A number of papers on slotted rings, register insertion rings and more complex ring architectures have also been published. However, there is very little existing literature on multiple ring networks as well as ring networks in clustered traffic environments, i.e. where nodes from the same cluster tend to communicate more with each other than with other nodes in the network. This thesis focuses on two network topologies that make use of multiple rings and are well suited to clustered traffic environments: the two-connected multiple ring (2-MR) and the destination removal double ring (DRDR). For the 2-MR network, three different practical token-based protocols are investigated in an attempt to optimise performance. It is further shown that significant performance improvements can be achieved by employing a slotted ring protocol rather than the token ring protocol. The DRDR network is also examined and its performance compared to the aforementioned architectures. For each of the six cases, both random and clustered traffic patterns are considered and compared. Analytical results are derived which are verified by results obtained from computer simulations. Furthermore, we look at exact methods of analysing ring networks. A mean value analysis of a single token ring network with a I-limited service discipline is performed, which clearly shows the complexity exact methods introduce. Finally, although it has been stated in the literature that an exact analysis of a multiple symmetrical token ring network is intractable, we present a novel Markov chain approach that gives exact results for near zero loads.
The Coriolis effect and travelling waves in porous media convection subject to rotation.
(2000) Patrascoiu, Mihail Radu.; Vadasz, Peter.
This study intends to recover and expand the analytical work of Vadasz (1998) for linear and weak non-linear stability of a rotating porous media heated form below and subject to gravity and Conolis forces. It is shown that the viscosity has a destabilising effect at high rotation rate. It has been established that the critical wave number in a plane containing the streamlines is dependent on rotation. Finite amplitude calculations provide a set of differential equations for the amplitude and phase, corresponding to the stationary and over-stable convection, identifying the post-transient conditions that a fluid is subject to, i.e. a pitchfork bifurcation for the stationary case, or a Hopf bifurcation in the case of over-stable convection. The previous model (Vadasz [1998]) was extended with an additional time scale in order to represent amplitude fluctuations and a short space scale to include horizontal modes of oscillations. When the complete solution for the stream function or temperature is analysed, where left and right travelling waves are considered, we obtain a set of differential equations for the amplitude and phase. The solutions are discussed in this context.
Performance analysis of cellular networks.
(2000) Rajaratnam, Myuran.; Takawira, Fambirai.
Performance analysis in cellular networks is the determination of customer orientated grade-of-service parameters, such as call blocking and dropping probabilities, using the methods of stochastic theory. This stochastic theory analysis is built on certain assumptions regarding the arrival and service processes of user-offered calls in a network. In the past, cellular networks were analysed using the classical assumptions, Poisson call arrivals and negative exponential channel holding times, borrowed from earlier fixed network analysis. However, cellular networks are markedly different from fixed networks, in that, they afford the user a unique opportunity: the ability to communicate while on the move. User mobility and various other cellular network characteristics, such as customer-billing, cell· layout and hand·off mechanisms, generally invalidate the use of Poisson arrivals and negative exponential holding times. Recent measurements on live networks substantiate this view. Consequently, over the past few years, there has been a noticeable shift towards using more generalised arrival and service distributions in the performance analysis of cellular networks. However, two shortcomings with the resulting models are that they suffer from state space explosion and / or they represent hand off traffic as a state dependent mean arrival rate (thus ignoring the higher moments of the hand-off arrival process). This thesis's contribution to cellular network analysis is a moment-based approach that avoids full state space description but ensures that the hand-off arrival process is modelled beyond the first moment. The thesis considers a performance analysis model that is based on Poisson new call arrivals, generalised hand-off call arrivals and a variety of channel holding times. The thesis shows that the performance analysis of a cellular network may be loosely decomposed into three parts, a generic cell traffic characterising model, a generic cell traffic blocking model and a quality of service evaluation model. The cell traffic characterising model is employed to determine the mean and variance of hand-off traffic offered by a cell to its neighbour. The cell traffic-blocking model is used to detennine the blocking experienced by the various traffic streams offered to each cell. The quality of service evaluation part is essentially afued-point iteration of the cell traffic characterising and cell traffic blocking parts to determine customer orientated grade-of-service parameters such as blocking and dropping probabilities. The thesis also presents detailed mathematical models for user mobility modelling. Finally, the thesis provides extensive results to validate the proposed analysis and to illustrate the accuracy of the proposed analysis when compared to existing methods.
Feasibility study of a neural network current controller for a boost rectifier.
(2000) Worthmann, Cedric Alwyn.; Diana, Gregory.
During the past two decades, Quality of Supply has become a serious problem for Variable Speed Drives in the industrial and commercial sectors. Quality of Supply problems can trip Variable Speed Drives, which results in loss of production, which is a significant problem in the paper and pulp industry. Researchers have proposed that Quality of Supply problems can be minimised in-house, using controlled front end rectifiers (boost rectifier), to maintain a regulated DC link voltage in the Variable Speed Drive configuration, as most faults are created by a varying supply voltage. This thesis extends the work performed on boost rectifiers by investigating the feasibility of replacing the classical controllers with a Continual Online Trained Artificial Neural Network current controller. The approach adopted in this thesis was to evaluate and extend the work previously performed on conventional boost rectifier current controllers and Continual Online Trained Artificial Neural Network current controlled inverter, at the University of Natal. During this evaluation, the respective controller shortcomings were identified and addressed. Thereafter the Continual Online Trained Artificial Neural Network current controller was modified, according to the control requirements of the boost rectifier, and used as a replacement for the conventional current controller in the boost rectifier system. Finally, the Continual Online Trained Artificial Neural Network current controller was evaluated to assess its viability as a current controller for a boost rectifier. The concept of implementing the real-time Continual Online Trained Artificial Neural Network current controller using a DSP (Digital Signal Processor) was described, along with the main features and practical limitations of existing commercial DSP's. It is shown that at the time of writing of this thesis, the commercially available DSP' s are not powerful enough to implement the Continual Online Trained Artificial Neural Network current controller. However this thesis also shows that it is feasible to implement the real-time controller on the newly released TMS320C67 DSP card.
The design of an HF band direct sequence point-to-point link for rural telecommunications.
(2000) Selmer, Roland Marc.
The following work documents the design of an HF band direct sequence point-to-point link as used in a rural environment. The dissertation begins with a description of the overall document layout. An introduction into the problems associated with providing rural access is then given, with special emphasis on wireless technologies. It is argued that the attributes of HF band radio make it a good candidate for providing wireless communications links for under serviced rural areas in South Africa. The pitfalls and disadvantages of using an ionospheric-based medium in which to propagate an electromagnetic wave are discussed and several solutions are put forward. One of these solutions is Direct Sequence Code Division Multiple Access (DS-CDMA). A thorough analysis of the principles of DS-CDMA is given with emphasis on its abilities to combat the negating effects of the HF channel. A CDMA HF system is then proposed, outlining the various practical and theoretical aspects. Next, an HF channel model is designed and simulated, first with no spreading or coding, then with just spreading and finally with spreading and coding. It is found that although the extra diversity of the spreading and coding aid in reducing the bit error probability, more detailed local measurements and refinement in the design of the channel model and simulated system are needed to increase performance. Various aspects of a practical system that was built are then discussed, highlighting issues such as hardware interfacing and the software design of a man-machine-interface. Field measurements are also given with estimates on an upper limit on expected performance of a practical system. Finally conclusions are given, detailing the achievements and shortcomings of the research.
Power control in multimedia CDMA cellular networks.
(2000) Sharma, Neeru.; Takawira, Fambirai.
Wireless mobile communication is witnessing a rapid growth in, and demand for, improved technology and range of information types and services. Further, third generation cellular networks are expected to provide mobile users with ubiquitous wireless access to a global backbone architecture that carries a wide variety of electronic services. We examine the topic of power control and models that arc suitable for modem third generation wireless networks. CDMA technology is proving to be a promising and attractive approach for spectrally efficient, economical and high quality digital communications wireless networks. This thesis addresses the challenge of integrating heterogeneous transmitting sources with a broad range of Quality of Service characteristics in the cellular COMA networks. Provided the right power control can be devised, COMA offers the potential of extracting gain from the statistical multiplexing of such sources. A distributed power control algorithm is proposed which is required to update the transmitted power of the mobiles in each of the service classes locally. and enhance the performance of the system significantly. Algorithms for pragmatic issues like power level quantization and truncation of power are derived and incorporated into the proposed distributed power control algorithm.
The application of controllable inverter-based series compensation to power oscillation damping.
(2000) Chonco, Nkosinathi Stanford.; Rigby, Bruce S.; Harley, Ronald G.
Poorly damped oscillations that occur between the generators in large interconnected power systems often limit the amount of power that can be transmitted through a transmission corridor and are a threat to secure system operation. Coordinated insertion and removal of capacitors in series with a transmission line is one of the approaches that has been known for many years to be capable of enhancing the damping of power system oscillations. Unfortunately however, this approach historically relied on the operation of mechanical circuit breakers which were too slow and unreliable for the high-speed and repetitive operation that such an application demands. Recently-emerged, high-speed power-electronic-based switching devices are finding increasing use in modem power systems in the so-called Flexible AC Transmission Systems (F ACTS) concept. One particular FACTS impedance controller, namely the inverter-based series compensator, can rapidly alter the magnitude of capacitive compensating reactance in series with the line to make it practically feasible to enhance the damping of power system oscillations via dynamically-controlled series compensation. This thesis identifies, in the literature, an insightful approach to the design of an idealised controllable series compensator (CSC) damping scheme; such an approach has been considered in the analyses of the thesis. Three mathematical models of a single-machine infinite bus (SMIB) system are developed and are subsequently used in the initial design and analysis of a CSC damping controller carried out in the thesis. The simple SMIB system case study is used to identify and investigate the factors that have a significant impact on the performance of a CSC damping controller before studying the more complex issue of inter-area mode damping using a CSC. This thesis successfully confirms the results of a previous analytical study in which an idealised representation of the CSC was used, and extends the scope of that previous study by also considering a detailed representation of one particular type of CSC: the inverter-based series compensator. The two key findings of this extended investigation are that the inverter-based form of controllable series compensator can successfully be used to damp power oscillations and that, where the damping of oscillations is the particular focus of study, an idealised representation of the inverterbased CSC is suitable for the analyses. In the case of the inter-area mode damping problem, the selection of an appropriate input signal to the CSC damping controller is a key issue, since the oscillations that are to be damped involve a number of participating generators. This thesis examines the suitability of a few candidate input signals that have been proposed in the literature using the conceptually simpler SMIB system analytical models that have been developed. Finally, the thesis applies the understanding of CSC damping controller design gained from the SMIB study to the problem of inter-area mode damping on a four-generator study system. Time-domain simulation results are presented to demonstrate the impact of the controlled inverter-based series compensator on the damping of the inter-area mode of this system.

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