Wireless Communication System Performance Enhancement using Adaptive Array Antennas

Al-Ka'bi, Amin Hamdallah Othman (2006). Wireless Communication System Performance Enhancement using Adaptive Array Antennas PhD Thesis, School of Information Technology and Electrical Engineering, University of Queensland.

       
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Author Al-Ka'bi, Amin Hamdallah Othman
Thesis Title Wireless Communication System Performance Enhancement using Adaptive Array Antennas
School, Centre or Institute School of Information Technology and Electrical Engineering
Institution University of Queensland
Publication date 2006
Thesis type PhD Thesis
Supervisor Professor Marek Bialkowski
Abstract/Summary The application of antenna arrays has been suggested in recent years for mobile communications systems to overcome the problem of limited channel bandwidth, thereby satisfying an ever growing demand for a large number of high bit rate mobiles on communications channels. It has been shown by many studies that when an array is appropriately used in a mobile communications system, it helps in improving the system performance by increasing channel capacity and spectrum efficiency. It also extends the range of coverage, reduces multipath fading, and co-channel interference. These benefits are achieved by adjusting the antenna radiation pattern for a given wireless environment. The main objective of the adaptive antenna array is to reduce the received interference by forming nulls towards the co-channel interference sources, while tracking the desired signal by steering a beam towards it. The sensitivity pattern of the array is adjusted to suit these requirements by combining signals from different antenna elements with appropriate weighting. Here, the knowledge of a reference signal, a training signal, or the direction of the desired signal source is necessary to differentiate the desired signal from the interferers. The term adaptive antenna is used when the weights (gains and phases) applied to the signals induced on the array elements, are regularly updated before combining. The adaptation/updating is used to control the radiation pattern of the array dynamically, according to the requirements of the system. There are many adaptive methods and algorithms to modify the array weights, and to estimate the direction of arrival (DOA) of the desired and interference signals, each with its advantages and disadvantages. In an optimal adaptive antenna array system the gain and phase of each antenna element is adjusted to achieve the optimal performance of the array in some sense. For example, in steered beam adaptive arrays, the basis for adjusting gains and phases of each element is to obtain maximum output Signal-to-Interference-plus-Noise Ratio (SINR) by cancelling undesired interferences while receiving the desired signal. Such an arrangement is referred to as “optimal combining” in the literature. This thesis aims at developing novel analytical and simulation models for assessing the performance of a steered-beam adaptive antenna array with an arbitrary number of variably spaced dipole elements operating with incident narrow-band signals of various bandwidths and powers. The outline of the thesis is as follows: Chapter 1 presents a general overview of the usage of adaptive array antennas in wireless communication systems, including their adaptive algorithms, advantages/disadvantages and performance comparisons. In chapter 2, the performance of the uniformly and non-uniformly spaced adaptive array antennas is investigated. Analytical expressions for assessing their performance are introduced and then computer simulations are performed to illustrate the performance of the array under various operating conditions. In this chapter, the presence of mutual coupling between the array elements is ignored. Chapter 3 presents a treatment of direction-of-arrival estimation algorithms covering a review of many existing direction-of-arrival (DOA) estimation methods, including their capabilities and limitations. Chapter 4 presents theoretical and experimental investigations into performances of narrowband uniformly and non-uniformly spaced steered beam adaptive linear dipole array antennas that are subjected to pointing errors and mutual coupling. It is shown that the array's tolerance to pointing errors can be enhanced by controlling the inter-element spacing. Chapter 5 describes the effect of polarized signals (desired and interference) on the performance of uniformly spaced steered beam adaptive array antennas. A comparison between adaptive arrays with single dipole and cross-dipole elements is conducted to show the effect of polarization on their behaviour. It is shown that the cross-dipole array antennas have better performance in terms of the output SINR than the single dipole antenna array antennas if the polarization of the desired signal is unknown, while the single dipole arrays give better performance if the polarization of the desired signal is known. In chapter 6, a new method for computing the parameters of the mobile fading channel is introduced. Investigations into the effect of the mobile fading environment on the performance of the uniformly and non-uniformly spaced steered beam adaptive array are presented. In chapter 7, a novel method of incorporating steered beam adaptive arrays in Direct Sequence-Code Division Multiple Access (DS-CDMA) spread spectrum systems is presented. The performance of the system is investigated in terms of Bit Error Probability (BER). Chapter 8 presents the conclusions of the thesis and suggestions for future work.

 
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