Thesis (Ph.D.)--University of Delaware, 2007. / Principal faculty advisor: Stephan K. Bohacek, Dept. of Electrical and Computer Engineering. Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1988. / Vita. Abstract. Includes bibliographical references (leaves 48-49). Also available via the Internet.

Thesis (Ph. D.) -- University of Texas at Dallas, 2007. / Includes vita. Includes bibliographical references (leaves 78-80)

Thesis (Ph. D.)--Ohio State University, 2007. / Title from first page of PDF file. Includes bibliographical references (p. 142-147).

Bibliography : p. 275-283. Investigates the characteristics of the handover algorithm; how the handover request response time affects the call quality or call drop; and, studies a linear highway model to examine handover performance at the system level in which heavy new call and handover traffic occurs.

No description available.

The frequency assignment problem involves the assignment of discrete channels (frequencies) to the transmitters of a radio network. A separation between the frequencies assigned to transmitters close to each other is required to avoid interference. Unnecessary separation causes an excess requirement for spectrum, which is a valuable resource. Consequently good assignments minimise both interference and the spectrum required. The subject of this thesis is the fixed spectrum frequency assignment problem, where the spectrum available is given and the target is to minimize the total interference of the system. Interference is modelled through binary constraints, and consequently the problem, which is treated as a combinatorial optimisation problem, can be represented by an undirected weighted graph. A summary of some of the integer programming formulations which model the problem is presented, together with a brief dimensional study of them. An efficient implementation of two well-known metaheuristic algorithms, adapted to the problem treated, is described. Some novel lower bounding techniques which, given a problem, work by combining lower bounds calculated for some of its clique-like subproblems are presented. The key idea is that it is quite easy to calculate tight lower bounds for problems represented by complete graphs (cliques). The lower bounds for clique-like subproblems are produced by two different methods, the first of which is based on the solution of a linear program, while the second is based on a closed formula. The most effective method to generate estimates for general problems is based on a linear program which is reinforced with inequalities derived from the lower bounds calculated on its clique-like subproblems. The last part of the thesis is dedicated to improvements to the lower bounding techniques, both for those working on general problems and for those developed for cliques only. Detailed computational results, obtained on a wide range of benchmarks, are reported.

Hydrogen technology has widely been acknowledged as a route to reduced carbon emissions and energy security. However, it is also recognised that technology development alone is not enough to cause a shift change in uptake and that public perception also needs to be considered. This paper identifies the public perception of hydrogen technology (with specific interest in biologically produced hydrogen) and determines whether it meets the public expectation in terms of its potential energetic output. The energetic viability of hydrogen (defined for this purpose as a net positive energy balance) is derived by determining the potential for agriculturally produced hydrogen using dark fermentative production techniques. A suitable range of energy crops have been identified using selection criteria including yield, harvest window and composition of the crops. Public perception of hydrogen energy was investigated through the use of two focus groups. This identified Welsh public opinions on the production and end use of hydrogen energy. Due to the rapid progress in wireless communication technologies and the increasing demands for new services; cutting edge techniques and research have developed wireless access delivery of broadband data. Such systems referred to as Broadband Fixed Wireless Access Systems (BFWA) or alternatively, Local Multipoint Distribution Systems (LMDS), are increasingly being regarded as a legitimate challenger to cable and digital subscriber lines (DSL), particularly in markets with poor cable or copper infrastructure. These systems have capabilities that go beyond the current WiFi and WiMAX technologies by providing users with higher bandwidths and faster data rates. The frequency of operation for such networks lies between 28 and 42 GHz according to the spectrum bands allocated for future LMDS networks. The presented work aims at proposing a generalized stochastic model for the LMDS urban/suburban propagation channel. The model is based on a physical electromagnetic representation of the millimetre wavelength channel, with particular emphasis on fading margins in line-of-sight (LOS) links. The study focuses on the analysis, modelling and measurements of the fading effects of signal scattering caused by building surfaces and the ground on the direct field in an LMDS link at millimetre­ waves. Outcomes have been validated against experimental results obtained in realistic propagation scenarios. More than 180 field measurement sets were taken at 40 GHz for model testing and validation purposes. The main innovation of this work is the solution proposed to address the problem of signal scatter at millimetre-wave lengths. The approach uses high-frequency approximations to the analytical solution given by Kirchhoff s tangent-plane representation of rough surfaces. The electromagnetic field and signal power at the receiver are considered random and are evaluated using the Physical Optics method over different possible realizations of the surface geometry and building architectural features. This has resulted in novel derivations of the mean field, mean power density and scatter distribution of the scattered field. In comparison to deterministic models, such as ray-tracing that require intensive computations and detailed (millimetric resolution) knowledge of topographical data that are almost impossible to obtain, the proposed model is rather efficient and yields accurate results. The model has also been employed in analysing the effects of building scatter on the variance of the main desired link and the adjacent- or co-channel interfering link arising from neighbouring cells in cellular LMDS networks. This has enabled predictions of signal-to-interference ratio statistics and distributions for a particular underlying propagation environment. Overall, compansons yielded very good agreements between measurements and predictions of the main statistical parameters, thus verifying the main assumptions relating to the received signal strength, as well as the validity of a Rician distribution in describing the signal envelope variability for both the main and interference links in complex millimetre-wave propagation environments.

Satellites will play an indispensable role in the deployment of commercial networks to meet an increasing demand for supporting multimedia services at high data rates. Next generation satellite systems, operating at high frequency bands offer large bandwidth and are able to provide broadband services. To interface satellite links with existing terrestrial networks for providing communication access to a variety of users directly, several performance issues need to be addressed. Current thesis presents a technically viable satellite-integrated network model that is efficient in carrying broadband services to users over a wide scattered area. Accurate prediction of attenuation level is necessary for a reliable network model to operate with required service availability. Long term rainfall data has been analysed to characterise attenuation level at a selected region such as Dhaka. It is shown that rainfall is highly seasonal and attenuation level is quite high during monsoon. However, the seasonal behaviour of rainfall can be exploited to improve the link availability. Radar and rain gauge measurements at Sparsholt are also used to find rain cell size distribution, which is an important factor in site diversity implementation to combat severe rain fade. It is found that convective rain cell has extension in the region of 10 km. The network model is designed with dimensioning the effective bandwidth to support a number of users over the satellite link by taking into account the multimedia traffic characteristics. Concatenated coding, a robust coding scheme is implemented to improve the link quality at a level required to deliver broadband services. The ITU-T performance objectives of 7.5×10 for CLR and 1.4×10 for CER over satellite links are met at a required Eb/No of 2.95 dB and 2.88 dB respectively. Different enhancement mechanisms for optimum TCP performance are implemented to combat the large propagation delay associated with a satellite link. It is revealed through the simulation that TCP performance over a satellite link is as efficient as terrestrial links with these enhancement mechanisms Finally, the overall performance of the designed network is evaluated through link budget analysis and simulation. An innovative downlink power control strategy has been implemented to maintain the link during the rainiest months. The interference level due to high power satellite transmission in the designed system is also calculated to protect other existing communication links sharing the same frequency bands. A feasible broadband network designed with characterising propagation as well as networking issues will efficiently deliver broadband communication services to a large population promptly and in a cost-effective manner. Such a network solution will be in the realm of current R & D towards broadband satellite networks.