Wideband Beamforming Employing Fully Spatial Signal Processing

Uthansakul, Monthippa (2007). Wideband Beamforming Employing Fully Spatial Signal Processing PhD Thesis, School of Information Technology and Electrical Engineering, University of Queensland.

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Author Uthansakul, Monthippa
Thesis Title Wideband Beamforming Employing Fully Spatial Signal Processing
School, Centre or Institute School of Information Technology and Electrical Engineering
Institution University of Queensland
Publication date 2007
Thesis type PhD Thesis
Supervisor Prof Marek Bialkowski
Abstract/Summary In the past, wireless communication systems were deployed using antennas with fixed radiation pattern which were meeting desired coverage characteristics. Such systems could not adapt to varying traffic environments. Smart antennas are a new technology that uses array antennas with adaptable radiation patterns. This varying radiation pattern can be accomplished with the use of RF hardware or digital signal processing. The smart antenna concept has mostly been demonstrated for the case of narrowband wireless systems. As the future demands high speed data transmission, a logical step is to consider a smart antenna operating over a wide frequency band. Such an antenna system is alternatively named a wideband beamformer. In this thesis, theoretical and experimental investigations into a wideband beamformer are carried out. The considerations commence with a conventional smart antenna employing a narrowband weighting scheme. The results reveal that this weighting scheme is unsuitable for a wideband array antenna, as the resulting radiation pattern varies with frequency. Next, various remedies are considered to overcome this shortcoming. From a literature review, one can gather that a wideband beamformer can be realized through space-time, space-frequency or fully spatial signal processing. The first two approaches are not considered here because they require a large number of tapped-delay lines or frequency filters. As a result of these considerations, a wideband beamformer using only spatial signal processing, also named a wideband spatial beamformer, is studied in this thesis. The focus is on the wideband spatial beamformer employing a rectangular array with real-valued weights. The advantage of this beamformer is that the weights can be realized by attenuators or amplifiers. As a result, the use of tapped-delay lines and frequency filters is avoided. The thesis points out some shortfalls of the original spatial beamformer, which was reported in the literature, and proposes suitable solutions. It is shown that the original algorithm fails for small size arrays. The thesis proposes a remedy to this problem. The shortfall is overcome through oversampling of the assumed radiation pattern. Having done modifications of the original beamforming algorithm the thesis looks at the issues of beamforming, null steering, and determining the direction of arrival of a wideband signal. The presented solutions form novel contributions of the thesis. In theoretical investigations, the beamforming performances are investigated through own computer programs developed in MATLAB®. The developed algorithms neglect mutual coupling effect in antenna arrays. Next, the full EM analysis (FEKO®) based on the Method of Moments is applied to study the performance of the beamformer when the mutual coupling between the array elements is taken into account. As the original, as well as the newly proposed beamforming algorithm, neglect mutual coupling effects in the antenna array, the radiation patterns show increased side lobes, when mutual coupling is included in full EM simulations. The thesis proposes a suitable solution to this problem. The adverse effect of mutual coupling is reduced by initially assuming the radiation pattern with low side lobes. As a result of this action, when the mutual coupling is present, the computed side lobes are increased to the level which can be tolerated. The next stage concerns the development of a full prototype of 4×4-element beamformer. In the original work concerning the use of microstrip antennas to form a wideband spatial beamformer was postulated. The candidate points out that such an array would be difficult to accomplish in practice. The reason is that microstrip patch antennas usually operate over a narrow band and they radiate in broadside direction which is perpendicular to the array’s plane. The investigated wideband spatial beamformer requires antenna elements to radiate in the endfire direction (that is parallel to the array plane). Because of this requirement the thesis postulates the use of planar monopole antennas. These antenna elements produce their main beam in the direction parallel to the array’s plane. In contrast to microstrip patches these antenna elements exhibit wideband impedance and radiation pattern operation. Also they are compact in size so they can meet tight spacing, as is required in the investigated beamformer. In addition to the appropriate choice of antenna elements, the thesis also provides a suitable solution to the feeding network of the planned prototype. The feeding network is constituted by attenuators, power splitters and a rat2race hybrid. The constructed prototype is tested in an anechoic chamber with respect to its beamforming capability over a wide frequency band. The obtained experimental results show a good agreement with the simulation results, as accomplished the full EM analysis and simulation software FEKO®. In addition to (i) the development of a beamforming algorithm which works for an arbitrary size of the investigated spatial beamformer (ii) the reduction of the radiation pattern degradation caused by mutual coupling and (iii) the development of a fully operational prototype, the novel contributions of this thesis include (iv) the development of a null steering, and (v) direction of arrival algorithms for wideband signals.

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