published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

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This thesis has investigated radioclimatological study in a clear-air environment as applicable to terrestrial line of sight link design problems. Radioclimatological phenomena are adequately reviewed both for the precipitation effect and clear-air effect. The research focuses more on the clear-air effect of radioclimatological studies. Two Southern African countries chosen for case study in the report are Botswana and South Africa. To this end, radiosonde data gathered in Maun, Botswana and Durban, South Africa are used for model formulation and verification. The data used in the thesis ranges from three years to ten years in these two stations. Three to ten years of refractivity data gathered in Botswana and South Africa is used for the model formulation. On the other hand, eight months signal level measurement data recorded from the terrestrial line of sight link set up between Howard College and Westville Campuses of the University of KwaZulu-Natal, Durban South Africa is used for model verification. Though various radioclimatic parameters could affect radio signal propagation in the clear-air environment, this report focuses on two of these parameters. These two parameters are the geoclimatic factor and effective earth radius factor (k-factor). The first parameter is useful for multipath fading determination while the second parameter is very important for diffraction fading, modeling and characterization. The two countries chosen have different terrain and topographical structures; thus further underlying the choice for these two parameters. While Maun in Botswana is a gentle flat terrain, Durban in South Africa is characterized by hilly and mountainous terrain structure, which thus affects radioclimatological modeling in the two countries. Two analytical models have been proposed to solve clear-air radioclimatic problems in Southern Africa in the thesis. The first model is the fourth order polynomial analytical expression while the second model is the parabolic equation. The fourth order polynomial model was proposed after an extensive analysis of the eight month signal level measurement data gathered in Durban, South Africa. This model is able to predict the fade exceedance probabilities as a function of fade depth level. The result from the fourth order polynomial model is found to be comparable with other established multipath propagation model reviewed in the thesis. Availability of more measurement data in more location will be necessary in future to further refine this model. The second model proposed to solve clear-air propagation problem in the thesis is the modified parabolic equation. We chose this technique because of its strength and its simplistic adaptation to terrestrial line of sight link design problem. This adaptation is possible because, the parabolic equation can be modified to incorporate clear-air parameters. Hence this modification of the parabolic equation allows the possibility of a hybrid technique that incorporates both the statistical and mathematical procedures perfectly into one single process. As a result of this, most of the very important phenomena in clear-air propagation such as duct occurrence probabilities, diffraction fading and multipath fading is captured by this technique. The standard parabolic equation (SPE) is the unmodified parabolic equation which only accounts for free space propagation, while the modified parabolic equation (MPE) is the modified version of the parabolic equation. The MPE is classified into two in the thesis: the first modified parabolic equation (MPE1) and second modified parabolic equation (MPE2). The MPE1 is designed to incorporate the geoclimatic factor which is intended to study the multipath fading effect in the location of study. On the other hand, MPE2 is the modified parabolic equation designed to incorporate the effective earth radius factor (k-factor) intended to study the diffraction fading in the location of study. The results and analysis of the results after these modifications confirm our expectation. This result shows that signal loss is due primarily to diffraction fading in Durban while in Botswana, signal loss is due primarily to multipath. This confirms our expectation since a flatter terrain attracts signal loss due to multipath while hilly terrain attracts signal loss due to diffraction fading. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2010.

Providing guarantees in the Quality of Service (QoS) has become essential to the transmission of multimedia traffic over wireless links with fading channels. However this poses significant challenges due to the variable nature of such channels and the diverse QoS requirements of different applications including voice, video and data. The benefits of dynamic adaptation to system and channel conditions have been accepted, but the true potential of optimized adaptation is lost if the layers operate independently, ignoring possible interdependencies between them. Cross-layer design mechanisms exploit such interdependencies to provide QoS guarantees for the transmission of multimedia traffic over fading channels. Channel adaptive M-QAM schemes are examples of some of the earliest works in the area of cross-layer design. However, many of the original schemes use the assumption that thresholds designed for AWGN channels can be directly applied to slow-fading channels. The thresholds are calculated with a commonly used approximation bit error rate (BER) expression and the first objective of the thesis was to study the accuracy of this commonly used expression in fading channels. It is shown that that the inaccuracy of the expression makes it unsuitable for use in the calculation of the threshold points for an adaptive M-QAM system over fading channels. An alternative BER expression is then derived which is shown to be far more accurate than the previous one. The improved accuracy is verified through simulations of the system over Nakagami-m fading channels. Many of the cross-layer adaptation mechanisms that address the QoS provisioning problem only use the lower layers (physical and data link) and few explore the possibility of using higher layers. As a result, restrictions are placed on the system which introduces functional limitations such as the inability to insert more than one class of traffic in a physical layer frame. The second objective in this thesis was to design a physical and application layer cross-layer adaptation mechanism which overcomes this limitation. The performance results of the scheme in both AWGN and fading channels show that the cross-layer mechanism can be efficiently utilized for the purposes of providing error rate QoS guarantees for multimedia traffic transmissions over wireless links. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2009.

Small mobile devices which have an ability to access the world wide web (WWW) wirelessly are in demand of late. This demand is attributed to the fact that video and audio streaming are cost effectively accessible via the WWW through wireless fidelity (Wi-Fi). This high demand for cheap real-time multimedia access via Wi-Fi makes it imperative for researchers to develop a wireless local area network (WLAN) standard, such as IEEE (802.11n), that has high data throughput and/or link reliability. The current drawback with the IEEE (802.11n) standard is that it is not power efficient for battery powered small mobile devices because of the high complexity multiple-input-multiple-output (MIMO) scheme implemented within the standard. Spatial modulation (SM) is a recently proposed low complexity MIMO scheme that can achieve high data throughput with good link reliability whilst being power efficient for small mobile devices. This study is aimed at further improving data throughputs of SM and also determining the bit error rate (BER) performance of SM in a city centre environment. Conventional spatial modulation has been investigated in literature with most research efforts geared towards improving the BER performance and minimizing receiver complexity of the scheme over non-cascaded fading channels. We propose adaptive M-ary quadrature amplitude spatial modulation (A-QASM) as a scheme that will improve the average throughput in comparison to conventional spatial modulation given a target BER constraint. The analytical BER lower bound is derived for this proposed scheme and validated by the Monte Carlo simulation results. The simulation results also prove that the average throughput of the proposed scheme (A-QASM) outperforms that of conventional spatial modulation. The definition for the received SNR of the A-QASM scheme is also proposed. In research literature, conventional spatial modulation has been discussed in depth in non-cascaded wireless fading channels. The performance analysis derived in literature in non-cascaded wireless fading channels; does not apply in predicting the BER performance of a mobile device, using conventional spatial modulation, in an environment where there is signal diffraction (i.e city centre or a forest) which makes the signal susceptible to independent cascaded fading. This study contributes by developing an analytical framework for the BER lower bound of conventional spatial modulation over cascaded fading channels. Simulation results closely agree with the derived theoretical framework. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2012.

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Thesis (Ph. D.)--University of Hawaii at Manoa, 2005. / Includes bibliographical references (leaves 84-89). / Also available by subscription via World Wide Web / xi, 89 leaves, bound ill. 29 cm

The analysis of the distortion which results when frequency-modulated waves are passed through linear networks is investigated by the Fourier method and the Quasi-steady-state method. The major enphasis is placed on the Fourier method, and extensive digital computer programs are developed to allow this method to be implemented on the modern, high-speed digital computer. In the Fourier method, the frequency-modulated wave which is applied to the input of a linear network is broken up into its Fourier spectrum. Each of the resulting ‘'sideband'' frequencies is then passed through the network and is subjected to alterations in amplitude and phase. The output wave is then synthesized by taking the vector sum of the "weighted" sideband components. In contrast to the single pair of sideband frequencies generated by amplitude modulation, the spectrum of a frequency-modulated wave contains an infinite number of sideband components. Fortunately, only a relatively small number of these sidebands have significant influence on the total makeup of the waveform. The number of significant sidebands is proportional to the value of modulation index. When the modulation index is high, the number of significant sidebands is very large and the number of computations required by the Fourier method becomes enormous. Previously considered to be completely impractical, the Fourier method was usually abandoned in favor of the Quasi-steady-state approach. However, the digital computer techniques developed in the course of this investigation allow for a fast, economical, and convenient analysis based on the Fourier method even when the modulation index is relatively high. Analyses were performed for values of modulation index up to 45 and techniques are discussed for increasing this range. The Quasi-steady-state method is based on the assumption that the frequency of the input wave is changing slowly enough that the frequency of the output wave at any instant is equal to the "instantaneous fregquency' of the input wave. This method is inherently in error since it neglects the transient terms generated by the changing frequency. To compensate for this error, it is the general practice to incorporate correction terms, usually in the form of an infinite series. The Quasi-steady-state method is more effective at low modulating frequencies (high modulation index). While the analysis contained in this paper considers in detail only a first-order correction, the application of higher-order correction terms is discussed. The results obtained from applying both analyses to a complex, multi-section filter indicate that the computer solution of the Fourier method is preferable for intermediate values of modulation index. Experimental verification of the Fourier method is obtained by simulating the system on an analog computer. The advantages of this rather novel approach are discussed in some detail. The agreement between the results predicted by the digital computer and those obtained experimentally leaves no doubt to the validity and accuracy of the analysis. Digital computer programs for analyzing the distortion using each of the above methods are given. Subprograms are also included, some of which can be used independently. Among these are a program that computes Bessel functions of the first kind for positive and negative orders and a program that computes the minimum phase shift of a network from its atténuation. All programs are written in the FORTRAN IV computer language and were executed on the IBM 7040/1401 system. / Ph. D.