Thesis (M.Phil.)--University of Hong Kong, 2000. / Includes bibliographical references (leaves 116-121) Also available in print.

Thesis (M. Eng.)--Carleton University, 2001. / Includes bibliographical references (p. 86-91). Also available in electronic format on the Internet.

Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2009. / Full text also available. Scroll down for electronic link.

Thesis (M.S.)--University of Missouri-Columbia, 2004. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file viewed on (June 30, 2006) Vita. Includes bibliographical references.

In this dissertation, three multiuser detectors are developed for different application scenarios in direct-sequence code-division multiple access systems. The first detector is an overlapping widow decorrelating detector aimed at asynchronous reverse links. In companion with the design of this detector, a study on the decay property of the ideal decorrelating impulse response is presented, resulting in a quantitative description of the decay rate as a function of the Cholesky factors of the cross-correlation matrix of user signature signals. This result can serve as a guide for determining window length of decorrelating or minimum mean-squared error multiuser detection in asynchronous multiuser systems. Based on this result, a signal-adapted window-length determination algorithm is developed for the proposed detector. Several supporting utilities for efficient implementation of the proposed detector are also described. The second detector is a linear multiuser detector that is also aimed at the reverse links. Particularly, it is desirable for cases where the number of users is small and, thus, significant performance gain over the existing linear multiuser detectors is possible. Unlike in the decorrelating and MMSE detectors, minimizing the bit-error rate is taken as the optimization objective in the proposed detector. To avoid undesired local minima of the highly nonlinear BER cost function, a set of convex constraints is proposed for the optimization problem. It is shown that this constrained optimization problem has a unique solution once the decorrelating detector exists. It is also shown that the proposed detector achieves the best performance among linear detectors for most realistic situations. In addition, a Newton barrier method is developed for efficiently calculating the coefficient vector of the proposed detector (i.e., the solution of the constrained optimization problem). The third detector is an adaptive detector that is aimed at the forward link where information about interfering users is often unavailable. The proposed detector consists of a bank of blind adaptive filters, one for each resolvable path, followed by a channel estimator and a coherent diversity combiner. To allow blind adaptation, the impulse response of each filter is decomposed into two orthogonal parts: one part is fixed as the decorrelating coefficient vector for the path in the absence of interfering users and the other is free to be adapted according to the mean-squared error criterion. Assuming perfect adaptation, the performance of the proposed detector is shown to be between those of the decorrelating detector and the minimum mean-squared error detector. Other studies conducted include the effects of fading on the performance of the proposed detector and the behavior of the proposed blind adaptation algorithm. / Graduate

The initial code Acquisition Techniques of Direct Sequence Spread Spectrum Systems for two categories of serial and parallel search strategies is investigated. A simple and economic scheme for coarse code acquisition of Reverse Link for UMTS (FDD-WCDMA) application is presented. The emphasis is on the performance of a new scheme (using a Surface Acoustic Wave Matched Filter) as a term of probability of false detection (pf) in code division multiple access (CDMA) systems. Knowledge of initial code uncertainty phases help to reduce the overhead of preamble on the access channel and a very simple scheme for acquisition to be determined. In the reverse link this uncertainty is due to the cell radius only. Acquisition time required for a simple serial search scheme may therefore be unacceptably large. On the other hand, for parallel acquisition using parallel branches in accordance with the chip uncertainty time region leads to a lot of hardware complexity. Initially, the effect of multiple access interference and spreading sequence length are determined for models applicable to the reverse link of a mobile communications system. Then the acquisition performance is derived using a model of a cellular mobile communication channel, which includes the effects of multiple access interference, adjacent cell interference, frequency selective Rician channel, shadowing, power control error, and vehicle speed. It is shown that the most significant factors in determining the acquisition performance are the acquisition observation interval, the number of users, and the specular to diffuse power ratio. Numerical results based on analysis of acquisition performance in mobile channel show that the proposed acquisition scheme is efficient, robust, fast and suitable for real time low cost implementation.

High levels of interference between satellite constellation systems, fading and shadowing are a major problem for the successful performance of communication systems using the allocated L/S frequency bands for Non-Geostationary Earth Orbit (NGEO) satellites. As free spectrum is nonexistent, new systems wishing to operate in this band must co-exist with other users, both satellite and terrestrial. This research is mainly concerned with two subjects. Firstly, band sharing between different systems Code Division Multiple Access (CDMA) and Time Division Multiple Access (TDMA) has been evaluated for maximizing capacity and optimising efficiency of using the spectrum available. For the case of widened channel bandwidth of the CDMA channel, the overlapping was tested under different degrees of channel overlap and different orders of filters. The best result shows that at the optimum degree of channel overlap, capacity increases by up to 21%. For the case of fixed channel bandwidth, the optimum overlapping between CDMA systems depends on the filtering Roll-off factor and achieves an improvement of the spectrum efficiency of up to 13.4%. Also, for a number of narrowband signal users sharing a CDMA channel, the best location of narrowband signals to share spectrum with a CDMA system was found to be at the edge of the CDMA channel. Simulation models have been constructed and developed which show the combination of DS- CDMA techniques, forward error correction (FEC) code techniques and satellite diversity with Rake receiver for improving performance of interference, fading and shadowing under different environments. Voice activity factor has been considered to reduce the effect of multiple access interference (MAI). The results have shown that satellite diversity has a significant effect on the system performance and satellite diversity gain achieves an improvement up to 6dB. Further improvements have been achieved by including concatenated codes to provide different BER for different services. Sharing the frequency band between a number of Low Earth Orbit (LEO) satellite constellation systems is feasible and very useful but only for a limited number of LEOS satellite CDMA based constellations. Furthermore, satellite diversity is an essential factor to achieve a satisfactory level of service availability, especially for urban and suburban environments.

Adaptive antenna array technology will undoubtedly form a vital part of third generation cellular systems owing to not only confining the radiated energy associated with a mobile to a small volume on the downlink but also reducing the interference due to cochannel users on the uplink. The objective of this thesis is to develop signal enhancement schemes for adaptive antenna arrays for the purpose of enhancing the quality and capacity of direct sequence code division multiple access (DS-CDMA) systems. Firstly, The signal enhancement scheme using a real symmetric array covariance matrix (RSACM) method was proposed. This proposed scheme is composed of a unitary and persymmetric transformation methods. A real symmetric array covariance matrix has the same Toeplitz-plus-Hankel matrix structure that is produced by almost total noiseless data sequence. The second proposed signal enhancement scheme consists of the rotation of signal subspace (RSS) and Toeplitz matrix approximation (TMA) methods. This proposed scheme improved the system performance by reducing the interference- plus-noise effect from the complex array covariance matrix of the pre-correlation received signal vector. The third proposed signal enhancement scheme is a modified linear signal estimator (MLSE) which involves the rank N approximation by reducing total noise eigenvalues (RANE) and TMA methods. The aim of this proposed scheme is to improve the system performance by effectively reducing the interference-plus-noise effect from the post-correlation received signal. Secondly, the computational complexity and the performance for all proposed signal enhancement schemes in this thesis are investigated and compared. The signal enhancement scheme using the RSS+TMA and MLSE methods was also proposed for a multi-rate and multicell DS-CDMA systems. The relative other-cell interference factor was analysed for a multicell condition. Finally, the performance of all proposed signal enhancement schemes is shown to be much better than that of no signal enhancement method under a single cell, multicell, single rate, and multirate conditions.

Approved for public release; distribution is unlimited. / The Wideband Code Division Multiple Access is a third generation air interface, initiated in European Union research projects at the start of the 1990s. The standard emerged by the end of 1999 as part of the 3GPP standardization process. It was designed to support multiple simultaneous services with high quality services through an increased data rate. This research examines the properties and parameters of the WCDMA system to determine the feasibility of intercepting and exploiting this technology with known assets. It explores this possibility by looking at link analysis, adaptive antennas and co-channel interference canceling techniques to determine if the interception of WCDMA signals is possible. / Lieutenant Junior Grade, Hellenic Navy

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