Thesis (M.Eng.)(Electronic)--University of Pretoria, 2007. / Includes summary. Includes bibliographical references. Available on the Internet via the World Wide Web.

Thesis (Ph. D.)--Faculty of Science, University of Oulo, Finland, 2006. / Includes bibliographical references (p. 218-236).

Packet-switched wireless data networks built upon IP (Internet Protocol)-based infrastructure are being envisioned to provide ubiquitous Internet access to mobile users. Supporting packet-data services along with the cellular voice services in an integrated networking framework gives rise to new network infrastructure and protocol design issues that are to be resolved to facilitate the introduction of the next generation wireless IP networks. This thesis addresses several protocol design issues in the area of wireless packet data networking, namely, retransmission control design for multichannel protocols, radio link level protocol for dynamic rate and error control, inter-layer protocol dependency, radio link-layer and transport-layer protocol fairness and radio link-level dynamic bandwidth allocation. A retransmission control policy for a multichannel S-ALOHA (slotted ALOHA) protocol in a high speed wireless data network is proposed and analyzed. A sub-optimal dynamic rate adaptation procedure is proposed for uplink data transmission in WCDMA (Wideband Code Division Multiple Access)-based wireless IP networks. The performance of this scheme is analyzed using a novel ‘mean-sense’ approach for interference calculation in cellular WCDMA environment. The impact of macrodiversity packet combining on transport-protocol throughput performance is analyzed under different link-level retransmission control policies. A unified TDMA (Time Division Multiple Access)-based centralized bandwidth management mechanism is proposed as a link-level solution for providing service fairness among competing users for uplink data transmission in a wireless IP network. TCP (Transmission Control Protocol) performance is evaluated under different transport-level packet scheduling policies in a correlated fading environment and a time frame-based scheduling policy is proposed to provide service fairness among mobile users in the case of downlink transmission. A set of centralized burst-level bandwidth allocation policies are investigated as a means of service integration with QoS (Quality of Service) provisioning in the wireless IP air interface. / Graduate

M.Tech. / Mobile handheld devices are moving from being peripheral devices and are now fulfilling functionality provided by laptops and desktops. The capability and functionality of handheld devices have improved. This makes the devices more prominent within public and private environments, allowing information to be processed inside and outside of the organisation’s network. Of all mobile handheld devices, the personal digital assistant (PDA) is seen to be more robust and powerful, increasing its use and popularity among users. PDAs offer wireless connectivity like Bluetooth and operate with multiple operating systems, also allowing them to be considered as a private or organisational enterprise tool. Bluetooth connectivity allows workers to access information anywhere, including both personal and corporate information. Software and applications have been specifically developed for handheld devices such as PDAs, giving users a high level of usability and functionality. The purpose of this dissertation is to present an information security evaluation of a Bluetooth-enabled handheld device, such as a PDA. The use of Bluetooth wireless technology and functionality provides added benefits, but also brings new information security threats to an organisation’s information assets. The research attempts to understand the implications of using a Bluetooth-enabled handheld device in both public and private environments. Five high-level layers are defined for this discussion. Information security risks are evaluated based on current research into vulnerabilities, attacks and tools that exist to compromise a Bluetooth-enabled handheld device. A Bluetooth penetration testing methodology is suggested for the identified vulnerabilities, attacks and tools, where a practical assessment is performed for a critical analysis of the information security mechanisms implemented by the Bluetooth-enabled handheld device (PDA). Possible recommendations to mitigate identified information security risks are also made. This study motivates the necessity of understanding the risks presented by a mobile workforce using Bluetooth connectivity in mobile handheld devices which can be used in both private and public environments.

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Many of todays' sophisticated communication systems, particularly those operating in the microwave frequency range, have a growing demand for highly selective and frequency agile filters with fast tuning rates. At microwave frequencies above 2GHz, filters that combine both tuning speed and high selectivity are difficult to realise. Unloaded Q values in the range of 1000 are required for reasonable selectivity. At present, these requirements with the exception of tuning speed are mainly fulfilled by electronically tuned filters employing yttrium iron garnet (YIG) resonators. An alternative solution to this problem is to utilise high-Q dielectric resonators (DRs) in-conjunction with an appropriate tuning mechanism to realise tunable filters. DRs can provide unloaded Q-factors comparable to waveguide resonators (typically 10,000 @ 10GHz), but at a substantially lower weight and smaller volume. DRs are also compatible with a microstrip environment which eliminates the need for microstrip to waveguide adaptors. This thesis is devoted to the study of the design and realisation of electronically tuned microwave filters employing dielectric resonators. Knowledge of the values of the coupling coefficient between a pair of DRs is essential for the design of both the bandpass and bandstop filters using these resonators. This necessitates the use of an accurate expression for the coupling coefficient which was derived by performing an electromagnetic analysis on the DR structure. This analysis also took into account the effects of the actual circuit environment such as the resonator support spacer and the dimensions of the enclosure. The experimental data presented shows very good agreement with the computed data. Novel techniques for the tuning of the DR operating in its fundamental mode are presented. These tuning configurations incorporating GaAs varactor diodes have been investigated to determine a circuit which greatly perturbs the DR fundamental mode. The amount of tuning that can be obtained by these methods exceeds more than 60MHz of the unperturbed value of resonant frequency with acceptable amount of Q-factor degradation. These tuning characteristics are superior to similar devices reported to date. A simple analytical expression is also derived for determining the tuning range of such a structure. Techniques for the design of novel electronically tunable DR bandpass and bandstop filters having either a Chebyshev or Butterworth response are presented. Design for prescribed response, starting from a low-pass lumped-element prototype filter is outlined. The bandpass filter structure consists of cascaded resonators, with the end resonators coupled to microstrip arcs on a substrate, whereas, the bandstop filter consists of a microstrip line with resonators mounted at intervals of one-quarter-wavelength. Tuning is achieved by varying a bias voltage to the varactor. The measured performances of several practical devices operating in the S-band are presented, these agree closely with theory. The centre frequency of these filters is within 2.46% and the bandwidth is within 3.75% of the desired values.

This thesis describes the in-orbit measurements, design and simulation of error-control strategies and channel modelling for low-Earth orbiting satellites, specifically in a restrictive small satellite platform. The motivation to pursue such a direction is the quest to optimise the low-Earth orbit (LEO) satellite communication link. Until a few years ago, advanced designs for satellite communications systems had focused on conventional geostationary Earth orbit (GEO) communication technologies which are not necessarily tailored to the LEO environment. Such sub-optimal designs were accommodated by the typical application of LEO satellites - remote sensing satellites - normally being large satellites equipped with huge parabolic dishes. With the recent rapid development in numerous LEO satellite constellations and trend in the satellite industry towards smaller and affordable satelhtes, the prominence of satellites in LEO cannot be ignored and it has become crucial to efficiently utilise the communications link. Until now, a suitable channel model for low-Earth orbit satellites has not existed. In order to provide a viable satellite network, a proper knowledge of the dynamic characteristics of the link is essential. A UHF measurement campaign from the UoSAT -3 microsatellite in LEO was therefore undertaken and analysis of the error statistics provides unique information on the channel behaviour for a broad range of elevation angles. The thesis has investigated generative Markov models as a means of representing the observed error statistics and established that the LEO satellite channel can be accurately described by a multiple-good-state, one-error state Fritchman model, or a four-state Markov model under special circumstances which are outlined. Various error control strategies have been evaluated based on the in-orbit measured data and proposed channel models, therefore verifying the precision of the Fritchman and four-state Markov models. During the course of this work, two schemes comprising type-II hybrid-ARQ based on punctured Reed Solomon codes and byte interleaving were investigated and the tradeoffs were identified. A novel type-II + delay hybrid protocol has been proposed and has been demonstrated to provide further increases in throughput performance. The principal conclusion from this thesis is that, using the results of the measurement campaign with an in-orbit satellite, it has been possible to generate a realistic fading model for use in future planning for LEO satellite systems. Furthermore, the type-II, type-II+delay hybrid and byte interleaving techniques are shown to provide an improvement in throughput performance of the existing store-andforward communications protocol with varying tradeoffs between the techniques.

The widespread use of radio frequencies of wavelengths small compared with the major terrain irregularities has led to the development of theoretical deterministic models for the prediction of field strengths over paths of given profile. The examination of these models is the main objective of the present thesis. Although present radio links are mainly based on empirical developments, theoretical approaches may offer considerable alternative for the design of future wireless communications systems. It is well known that the methods applied are based on multidimensional integral equations, which only in certain and idealised cases reduce to a practical form suitable for realistic utilisation. The present work attempts to reveal the physical processes that characterise the radio channel and how these are approached by certain models for common engineering applications. Since the major mechanism of propagation in radio environments is diffraction, extensive analysis is performed for this physical process. In particular, a new fast implementation of the Vogler multiple knife-edge diffraction algorithm is described with the additional benefit of improved accuracy at path profile configurations where the original solution fails considerably. An entirely new approach to slope-Uniform Theory of Diffraction is introduced and shown to produce essentially identical results to Vogler within much shorter computation times. This is applied to 3D urban propagation and to terrestrial fixed links and is shown to produce accurate results compared with measurements. Finally, new physical-statistical models are introduced in order to overcome the excessive cost of high resolution building databases. Application to both mobilesatellite and to broadband fixed access systems revealed a high degree of statistical accuracy