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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Wireless radio access technologies have been progressively evolving to meet the high data rate demands of consumers. The deployment and success of voice-based second generation networks were enabled through the use of the Global System for Mobile Communications (GSM) and the Interim Standard Code Division Multiple Access (lS-95 CDMA) networks. The rise of the high data rate third generation communication systems is realised by two potential wireless radio access networks, the Wideband Code Division Multiple Access (WCDMA) and the CDMA2000. These networks are based on the use of various types of codes to initiate, sustain and terminate the communication links. Moreover, different codes are used to separate the transmitting base stations. This dissertation focuses on base station identification aspects of the Frequency Division Duplex (FDD) WCDMA networks. Notwithstanding the ease of deployment of these networks, their asynchronous nature presents serious challenges to the designer of the receiver. One of the challenges is the identification of the base station identity by the receiver, a process called Cell Search. The receiver algorithms must therefore be robust to the hostile radio channel conditions, Doppler frequency shifts and the detrimental effects of carrier frequency offsets. The dissertation begins by discussing the structure and the generation of WCDMA base station data along with an examination of the effects of the carrier frequency offset. The various cell searching algorithms proposed in the literature are then discussed and a new algorithm that exploits the correlation length structure is proposed and the simulation results are presented. Another design challenge presented by WCDMA networks is the estimation of carrier frequency offset at the receiver. Carrier frequency offsets arise due to crystal oscillator inaccuracies at the receiver and their effect is realised when the voltage controlled oscillator at the receiver is not oscillating at the same carrier frequency as that of the transmitter. This leads to a decrease in the receiver acquisition performance. The carrier frequency offset has to be estimated and corrected before the decoding process can commence. There are different approaches in the literature to estimate and correct these offsets. The final part of the dissertation investigates the FFT based carrier frequency estimation techniques and presents a new method that reduces the estimation error. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2006.

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

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

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy