UBC Theses and Dissertations
Beamforming techniques for user capacity improvements of IS-95 cellular CDMA systems Chan, Lester Kwok-Hung
In this thesis, the use of adaptive beamforming antennas for enhancing the system capacity of IS-95 Code Division Multiple Access (CDMA) digital cellular systems is investigated. Adaptive beamforming is a Spatial Division Multiple Access (SDMA) technology which can be used to reduce interference in the spatial dimension. By using beamforming antennas, the signals can be transmitted and received in selective directions, giving rise to significantly reduced interference and therefore improved system capacities. In order to estimate the capacity improvements that can be achieved, several antenna models including the omnidirectional, 3-sectored antennas and multi-element (4, 6 and 8) beamforming arrays are considered in this thesis. Furthermore, the extended Hashemi multipath model is modified explicitly for the IS-95 systems by generating multipath signals which are separated by one chip period and therefore readily resolvable by the IS-95 RAKE receivers. A detailed IS-95 system was considered, and a bit error rate (BER) performance model was developed. In this BER model, the performance of the multipath combining RAKE receiver is investigated for the considered multipath Rayleigh fading environment by employing coherent and non-coherent maximum ratio combining for the downlink and the uplink, respectively. An important feature of the proposed model is that it ensures for each mobile communication link an average BER of 10 ⁻³ is maintained. This implies that, each receiver, based upon its multipath power distributions, can experience a different signal-to-interference ratio (SIR) per bit threshold. Based upon these models, a sophisticated and very generic capacity simulation software platform was developed as an effective approach to accurately simulate the IS-95 system capacities under realistic system conditions, including more practical channel models. In this simulation platform, a 3-tier hexagonal cell structure with 19 cells is considered and the mobile users are assumed to be uniformly distributed across the 19-cell region. User voice activity and imperfect power control are modelled according to their individual statistical properties. In order to accurately simulate the capacities of the IS-95 systems, each user was introduced on an individual basis, until system saturation condition occurred during the capacity simulations. Based upon the numerous computer simulated performance evaluation results obtained, the advantages of using adaptive beamforming antennas, as compared to conventional antennas, are presented in terms of IS-95 system capacity improvements. These performance results have shown that using adaptive beamforming antennas can significantly improve the capacities of the IS-95 systems as compared to conventional antennas. In addition, the simulation results have also shown that the capacities of the IS-95 systems are heavily dependent upon the signal propagation loss index, power control performance and the urbanization characteristics of the geographical area.
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