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Abstract
Due to the limited wireless spectrum and the increasing demands for reliable high throughput data transmission, research on methods to maximize spectral efficiency without sacrificing the transmission quality becomes an important issue. In order to provide a robust transmission system, problems encountered in mobile wireless system such as multipath fading and co-channel interference need to be solved. Adaptive beamforming, sometimes known as smart antenna is a promising technique to mitigate the interference problem. Meanwhile, adaptive modulation has the ability to avoid the detrimental fading effect and maximizes the average throughput. In order to gain the benefits of both techniques, this thesis considers the combination of these two adaptive techniques.
The study begins by investigating the potential benefit of utilizing adaptive modulation mode selector at the output of adaptive beamformer. By applying this combination approach, the adaptive modulator is designed to assist the adaptive beamformer in determining the need to recalculate the adaptive beamforming weight vector. The novelty of this approach lies in the ability of the joint technique to reduce the weight recalculation rate, thus saving the processing power. Computer simulation was carried out to compare the performance of the proposed joint adaptive beamforming adaptive modulation (ABF-AM) with the conventional adaptive beamforming fixed modulation (ABF-FM) for the case of one and two interferers. Results for both cases show that the proposed ABF-AM is able to fulfil the target bit error rate (BER) of 10 -2, while improving performance by reducing the required average received signal-to-interference-plus-noise ratio (SINR) as compared to the ABF-FM technique.
Motivated by the fact that the BER performance of an adaptive beamformer depends on the interfering signal’s direction of arrival (DOA), this thesis proposes another joint adaptive approach. Different from conventional adaptive modulation that adapts to the time varying channel condition, this thesis proposes an adaptive modulation scheme that adapts to the interfering signal’s DOA at the adaptive beamforming receivers. The performance of the proposed technique is analyzed in terms of the average throughput and the outage probability for the target BER of 10-2 and 10 -4. The trend is observed as the number of antenna elements is increased. The initial study of the proposed joint technique involves temporal reference based adaptive beamforming algorithm, which is Sample Matrix Inversion algorithm. The study of the proposed joint technique is then extended by applying it with Minimum Symbol Error Rate (MSER) beamforming. The performance of the proposed joint MSER beamforming adaptive modulation is compared with the traditional Minimum Mean Square Error (MMSE) beamforming. Simulation results show that the joint MSER beamforming adaptive modulation is superior to the MMSE counterpart.
