In meteor-burst communications systems, the channel is bursty with a continuously fluctuating signal-to-noise ratio. Adaptive data rate systems attempt to use the channel more optimally by varying the bit rate. Current adaptive rate systems use a method of closed-loop decision-feedback to control the transmitted data rate. It is proposed that an open-loop adaptive data rate system without a decision feedback path may be possible using implicit channel information carried in the first few milliseconds of the link establishment probe signal. The system would have primary application in low-cost half-duplex telemetry systems. It is shown that the key elements in such a system would be channel predictors. The development of these predictors is the focus of this research. Two novel methods of predicting channel parameters are developed. The first utilises early fast Doppler information that precedes many long duration, large signal-to-noise-ratio overdense trails. The presence of early fast Doppler at the trail commencement is used as a toggle to operate at a higher data rate. Factors influencing the use of early fast Doppler for this purpose are also presented. The second method uses artificial neural networks. Data measured during trail formation is processed and presented to the neural networks for prediction of trail parameters. Several successful neural networks are presented which predict trail type, underdense or overdense, and peak trail amplitude from the first 50ms of the trail's lifetime. This method allows better estimation of the developing trail. This fact can be used to implement a multi-rate open-loop adaptive data rate system. / Thesis (Ph.D.)-University of Natal, Durban, 1994.

The process of designing high power RF amplifiers has in the past relied heavily on measurements, in conjunction with simple linear theory. With the advent of the harmonic balance method and increasingly faster computers, CAD techniques can be of great value in designing these nonlinear circuits. Relatively little work has been done in modelling RF power MOSFETs. The methods described in numerous papers for the nonlinear modelling of microwave GaAsFETs cannot be applied easily to these high power devices. This thesis describes a modelling procedure applicable to RF MOSFETs rated at over 100 W. This is achieved by the use of cold S parameters and pulsed drain current measurements taken at controlled temperatures. A method of determining the required device thermal impedance is given. A complete nonlinear equivalent circuit model is extracted for an MRF136 MOSFET, a 28 V, 15 W device. This includes two nonlinear capacitors. An equation is developed to describe accurately the drain current as a function of the internal gate and drain voltages. The model parameters are found by computer optimisation with measured data. Techniques for modelling the passive components in RF power amplifiers are given. These include resistors, inductors, capacitors, and ferrite transformers. Although linear ferrite transformer models are used, nonlinear forms are also investigated. The accuracy of the MOSFET model is verified by comparison to large signal measurements in a 50 0 system. A complete power amplifier using the MRF136, operating from 118 MHz to 175 MHz is built and analysed. The accuracy of predictions is generally within 10 % for output power and DC supply current, and around 30 % for input impedance. An amplifier is designed using the CAD package, and then built, requiring only a small final adjustment of the input matching circuit. The computer based methods described lead quickly to a near-optimal design and reduce the need for extensive high power measurements. The use of nonlinear analysis programs is thus established as a valuable design tool for engineers working with RF power amplifiers. / Thesis (Ph.D.)-University of Natal, Durban, 1992.

Wireless cellular communications is witnessing a rapid growth in, and demand for, improved technology and range of information types and services. Future 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. This thesis examines the topic of multiple access protocols and models suitable for modem third-generation wireless networks. The major part of this thesis is based on a proposed Medium Access Control (MAC) protocol for a Code Division Multiple Access (CDMA) data packet radio network, as CDMA technology is proving to be a promising and attractive approach for spectrally efficient, economical and high quality digital communications wireless networks. The proposed MAC policy considers a novel dual CDMA threshold model based on the Multiple Access Interference (MAl) capacity of the system. This protocol is then extended to accommodate a mixed voice/data traffic network in which variable length data messages share a common CDMA channel with voice users, and where the voice activity factor of human speech is exploited to improve the data network performance. For the protocol evaluation, the expected voice call blocking probability, expected data throughput and expected data message delay are considered, for both a perfect channel and a correlated Rayleigh fading channel. In particular, it is shown that a significant performance enhancement can be made over existing admission policies through the implementation of a novel, dynamic, load-dependent blocking threshold in conjunction with a fixed CDMA multiple access threshold that is based on the maximum acceptable level of MAl. / Thesis (Ph.D.)-University of Natal, Durban, 1999.

As a design and analysis aid for the development of an experimental TWT, a computer program is written which allows the small-signal gain to be computed for various operating conditions, such as various conditions of tube bias (beam voltage and current) and frequency. In order to arrive at a value for the gain, a number of parameters need first to be defined or calculated. Using the method (Approach II) of Jain and Basu [17] which is applicable to a helix with a free-space gap between it and circular dielectric support rods surrounded by a metal shell, the dielectric loading factor (DLF) for the structure is found and the dispersion relation then solved to obtain the radial propagation constant y and axial propagation constant B. The method is tested for a helix with measured data and found to be acceptably accurate. Helix losses are calculated for the low-loss input and output sections of the helix, using the procedures developed by Gilmour et al [14,18], from which values are found for the helix loss parameter d. Another value for d, obviously much larger, is also found for the lossy attenuator section of the helix. Here measured data for the attenuator is used as a basis for a polynomial which models the attenuator loss as a function of frequency. The Pierce gain parameter C is found using the well-known equations of Pierce [21,22,26], and then the space-charge parameter Q. Here knowledge of the space-charge reduction factor F is required to find Q, and a simple non-iterative method is presented for its calculation, with some results. From the other parameters already calculated the velocity parameter, b, is then found. since sufficient information is now available, the electronic equations are solved. These equations are in a modified form, better accounting for the effects of space-charge than the well-known standard forms. Results are compared and slight differences found to exist in the computed gain. Now that the x's and y's (respectively the real and imaginary parts of the complex propagation constants for the slow and fast space-charge waves) are known the launching loss can be calculated. Launching losses are found for the three space-charge waves, not just for the gaining wave. The gain of the TWT is not found from the asymptotic gain equation but from a model which includes the effects of internal feedback due to reflections at the ports and attenuator. Values of reflection coefficients are modelled on the results of time-domain measurements (attenuator) and found by calculation (ports). This model permits the unstable behaviour of the tube to be predicted for various conditions of beam current and voltage and anticipates the frequencies at which instability would be likely. Results from simulations are compared with experimental observations. The need to pulse the experimental tube under controlled conditions led to the development of a high-voltage solid state pulse modulator providing regulated output pulses of up to 5000V and 200mA directly, without the use of transformers. The pulse modulator design embodies two unusual features a) its operation is bipolar, delivering positive or negative output pulses, depending only on the polarity of the rectifier input, and b) the use of multiple regulating loops and stacked pass elements to achieve high-voltage operation. Some results are presented. / Thesis (Ph.D.)-University of Natal, Durban,1994.

Ad hoc networks are commonly known to implement IEEE 802.11 standard as their medium access control (MAC) protocol. It is well known that token passing MAC schemes outperform carrier-sense-multiple-access (CSMA) schemes, therefore, token passing MAC protocols have gained popularity in recent years. In recent years, the research extends the concept of token passing ' scheme to wireless settings since they have the potential of achieving higher channel utilization than CSMA type schemes. In this thesis, a hybrid Token-CDMA MAC protocol that is based on a token passing scheme with the incorporation of code division multiple access (CDMA) is introduced. Using a dynamic code distribution algorithm and a modified leaky-bucket policing system, the hybrid protocol is able to provide both Quality of Service (QoS) and high network resource utilization, while ensuring the stability of a network. This thesis begins with the introduction of a new MAC protocol based on a token-passing strategy. The input traffic model used in the simulation is a two-state Markov Modulated Poisson Process (MMPP). The data rate QoS is enforced by implementing a modified leaky bucket mechanism in the proposed MAC scheme. The simulation also takes into account channel link errors caused by the wireless link by implementing a multi-layered Gilbert-Elliot model. The performance of the proposed MAC scheme is examined by simulation, and compared to the performance of other MAC protocols published in the literature. Simulation results demonstrate that the proposed hybrid MAC scheme is effective in decreasing packet delay and significantly shortens the length of the queue. The thesis continues with the discussion of the analytical model for the hybrid Token CDMA protocol. The proposed MAC scheme is analytically modelled as a multiserver multiqueue (MSMQ) system with a gated service discipline. The analytical model is categorized into three sections viz. the vacation model, the input model and the buffer model. The throughput and delay performance are then computed and shown to closely match the simulation results. Lastly, cross-layer optimization between the physical (PHY) and MAC layers for the hybrid token-CDMA scheme is discussed. The proposed joint PHY -MAC approach is based on the interaction between the two layers in order to enable the stations to dynamically adjust the transmission parameters resulting in reduced mutual interference and optimum system performance. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2009.

Modern telecommunications companies are moving away from conventional circuit-switched architectures to more versatile packet-switched infrastructures. Traditional First-In-FirstOut (FIFO) queues that are currently multiplexing IP traffic are not able to meet the strict Quality-of-Service (QoS) requirements of delay sensitive real-time traffic. Two main solution families exist that separate heterogeneous traffic into appropriate classes. The first is known as Generalized Processor Sharing (GPS), which divides the available bandwidth among the contending classes, proportionally to the throughput guarantee negotiated with each class. GPS and its myriad of packetised variants are relatively easy to analyse, as the service rate of individual classes is directly related to its throughput guarantee. As GPS splits the arriving traffic into separate queues, it is useful for best-effort traffic, supplying each class of traffic with either a maximum or minimum amount of bandwidth that it deserves. The second solution is the Earliest Deadline First (EDF) scheduler, also known as Earliest Due Date (EDD). Each traffic class has a delay deadline, by which the individual packets need to be served in order to meet their heterogeneous QoS requirements. EDF selects packets that are closest to their deadline. It is therefore primarily useful for delay sensitive real-time traffic. Although this is a simple algorithm, it turns out to be surprisingly difficult to analyse. Several papers attempted to analyse EDF. Most of them found either discrete bounds, which lie far away from the mean, or stochastic bounds which tend to capture the delay behaviour of the traffic more accurately. After the introductory first chapter, this thesis simulates a realistic cellular environment, where packets of various classes of service are transmitted across an HSDPA air interface. The aim is to understand the behaviour of EDF and its channel aware Opportunistic EDF scheduler compared to other scheduling families commonly used in HSDPA environments. In particular, Round Robin is simulated as the most simplistic scheduler. Max ell chooses packets solely based on the best channel conditions. Finally, PF -T is a scheme that tries to maximise the overall transmission rate that packets experience, but this metric gets divided by the throughput that each class already achieved. This introduces a form of long-term fairness that prevents the starvation of individual classes. The third chapter contains the main analysis, which uses Large Deviation principles and the Effective Bandwidth theory to approximate the deadline violation probability and the delay density function of EDF in a wired network. A definition for the fairness of EDF is proposed. The analysis is extended to approximate the stochastic fairness distribution. In the fourth chapter of the thesis an opportunistic EDF scheduler is proposed for mobile legs of a network that takes advantage of temporary improvements in the channel conditions. An analytical model is developed that predicts the delay density function of the opportunistic EDF scheduler. The channel propagation gain is assumed to be log-normally distributed, which requires graphical curve fitting, as no closed-form solution exists / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2009.

Modem technology has revived interest in the once popular area of meteor scatter communications. Meteor scatter systems offer reliable communications in the 500 to 2000 km range all day, every day. Recent advances in microprocessor technology have made meteor scatter communications a viable and cost effective method of providing modest data rate communications. A return to the basic fundamentals has revealed characteristics of meteor scatter propagation that can be used to optimize the protocols for a meteor scatter link. The duration of an underdense trail is bounded when its initial amplitude is known. The upper bound of the duration is determined by maximizing the classical underdense model. The lower bound is determined by considering the volume of sky utilized. The duration distribution between these bounds is computed and compared to measured values. The duration distribution is then used to specify a fixed data rate, frame adaptive protocol which more efficaciously utilizes underdense trails, in the half duplex environment, than a non-adaptive protocol. The performance of these protocols is verified by modeling. / Thesis (Ph.D.)-University of Natal, Durban, 1987.

An experimental, field replaceable travelling wave tube (TWT) with low profile, TEM transitions has been developed with the aim of combining the cost advantages of field replaceable TWTs with the bandwidth advantages of TEM transitions. Field replaceable TWTs are cheaper because their periodic permanent magnet (PPM) stacks (which are expensive) are easier to produce and can be reused. Standard TEM transitions have a high profile and prevent TWTs from being plugged into PPM stacks. The stacks of packaged TWTs are therefore built onto the tubes, using split ring magnets, before the tubes are potted. Upon failure, such tubes are discarded together with their focusing, coupling and cooling structures. Two types of low profile, TEM transitions were investigated: a ring and a plug transition. In the former, the helix was joined to a metal ring that was sandwiched between two ceramic rings in the vacuum envelope. Due to its high shunt capacitance, this transition was narrowband. In the plug transition, the helix was joined to a small metal plug in the wall of a ceramic tube. Plug transitions with a return loss of better than 10 dB over an octave were made but some technological difficulties were encountered. Metal/ceramic joining with active solder is therefore discussed. The development of graphite attenuators was attempted but graphite of high enough resistivity to achieve the attenuation and matching required was not available. Thin film attenuators were therefore used instead. Some good attenuators were made but the molybdenum films were sensitive to processing conditions. An empirical procedure was developed to magnetize PPM stacks to a wanted profile. The stacks that were designed and built resulted in good beam focusing. TWT tests were made using an automated test station that was built. Maximum RF gains and output powers of 20 dB and 43 dBm respectively were measured. The TWT design, construction and testing was a combination of materials science and microwave work. Some simulations were used to solve electromagnetic problems but much of the work was practical, involving trial and error and many measurements. / Thesis (Ph.D.)-University of Natal, Durban, 1994

Broadcasting authorities presently measure intermodulation distortion by applying a three tone simulation of a composite video and sound signal to the transposer and then measuring the relative amplitude of the major in-band intermodulation product (nominally at vision carrier frequency plus 1,57 MHz in the 625 line I/PAL System) on a spectrum analyser. This method is slow and requires a skilful operator to achieve repeatable results. Furthermore it tests the common RF amplification equipment at one luminance level and one chrominance level and therefore does not subject the transposer equipment to operation over its full range. A new sampling measurement technique has been proposed which overcomes all these problems by selectively mixing, while transmitting a colour bar test pattern, the demodulated output video signal of the frequency transposer with a pulse train coinciding with a particular colour. This thesis describes the design of a very stable proportional timing generator and its application to the measurement of intermodulation distortion on frequency transposers. The timing generator, which locks automatically onto the video signal and produces narrow sampling pulses which coincide accurately with a particular section of each line over a 50°C temperature range, is applicable to all PAL and NTSC TV Systems. / Thesis (Ph.D.)-University of Natal, Durban, 1989.

Radio frequencies at millimeter wavelengths suffer greatly from rain attenuation. It is therefore essential to study rainfall characteristics for efficient and reliable design of radio networks at frequencies above 10GHz. These characteristics of rain are geographically based, which need to be studied for estimation of rain induced attenuation. The ITU-R, through recommendations P.837 and P.838, have presented global approaches to rain-rate variation and rain-induced attenuation in line-of-sight radio links. Therefore, in this dissertation characteristics of rainfall rate and its applications for South Africa are evaluated. The cumulative distributions of rain intensity for 12 locations in seven regions in South Africa are presented in this dissertation based on five-year rainfall data. The rain rate with an integration time of 60 minutes is converted into an integration time of 1 minute in accordance with ITU-R recommendations. The resulting cumulative rain intensities and relations between them are compared with the global figures presented in ITU-R Recommendation P.837, as well as with the work in other African countries, notably by Moupfuma and Martin. Based on this work, additional rain-climatic zones are proposed alongside the five identified by ITU-R for South Africa. Finally, the study compares the semi-empirical raindrop-size distribution models such as Laws and Parsons, Marshall and Palmer, Joss, Thams and Waldvogel, and Gamma distribution with the estimated South Africa models. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, 2006.