| In view of limited spectral resources, the explosive growth in the number of wireless users accompanied by a shift to more sophisticated broadband services, makes mutual interference among users an increasingly pressing problem. Despite the fact that antenna arrays and beamforming are widely recognized as vital tools in suppressing interference and increasing the system capacity, research in beamforming for wireless communications to date has been limited to arrays whose interelement spacing is small, typically half a wavelength of the operating frequency or less. Due to the element clustering and the entailing farfield conditions, these beamformers are in essence directional filters, sensitive only to differences in the direction of arrival (DOA) and blind to the user's bearing, and thus severely limited in their ability to identify and suppress signals arriving from similar directions.; This dissertation introduces the concept of nearfield spot-beamforming with distributed arrays to wireless communications. Distributed arrays (DAs) are arrays, whose elements are spatially dispersed to synthesize a nearfield for their users. The superior performance of DAs accrues from the exploitation of the wavefront curvature in the nearfield to form range and direction sensitive spot-beams. Being a true spatial filtering operation, spot-beamforming is inherently more flexible than conventional farfield pencil-beamforming, affording improved interference suppression and making it possible, in particular, to suppress interferences with the same DOA as the desired signal.; This dissertation offers a theoretical justification of spot-beamforming by showing that DAs arise as the array class that maximizes both, the array resolution in single user systems, and the array gain in stationary multiple user systems. It discusses spatial aliasing issues associated with DAs and introduces the class of distributed linear arrays whose elements are of both, theoretical and practical significance. Furthermore, this dissertation proposes a semi-adaptive algorithm for optimal spot-beamforming in stationary multiple user systems. It finally extends the concept of spot-beamforming to broadband systems and demonstrates its superior performance in fixed wireless systems, where gain improvements of 20 dB are recorded. |
