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Khan, Ammar

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Beamforming for OFDM based hybrid terrestrial satellite mobile system

The thesis research concerns an integrated framework of terrestrial and satellite networks based on Orthogonal Frequency Division Multiple Access (OFDM) air interface which we call Hybrid Terrestrial-Satellite Mobile System (HTSMS). HTSMS which enables frequency reuse amongst the two networks serves users in urban areas via terrestrial Base Stations whilst satellite links provide service in rural areas in a transparent and seamless manner. The thesis focuses on mitigation of Co-Channel Interference on the uplink of the satellite using Least Mean Squares beamforming onboard the satellite. We propose a preamble transmission strategy based on pilot re-allocation for superior Co-Channel Interference mitigation, specific to HTSMS. Within the preamble framework, we further propose Fully-Dense Preamble, Partially-Dense Preamble and Reduced-Length Preamble as possible schemes and analyse their performance as compared to receiver side alternatives such as Variable Step Size-Least Mean Squares and Normalised-Least Mean Squares beamforming. Results show that the approach not only gives superior convergence but it enables better system performance with less pilot transmissions. Exploiting the susceptibility of the beamforming process to pilots, we further propose Novel Iterative Turbo Beamforming for the HTSMS with a Bit Interleaved Coded Modulation-OFDM. The proposed technique is based on improving a priori information of the soft decoded data and uses both soft data and pilots to perform adaptive beamforming in a turbo-like recursive manner. Results show that proposed approach exhibits significant bit error rate gains with only 1 iteration. Finally, to reduce the associated complexity of onboard beamforming, we first quantify performance advantages of adaptive beamforming against non-adaptive. For the non-adaptive case, we propose onboard based semi-static beamforming where the required beam orientation computed at the ground is transmitted to the satellite at which beamforming weights are calculated. The proposed mechanism is a practical and attractive alternative to existing non-adaptive beamforming approaches, especially for satellite systems offering broadcasting/fixed services. On comparison, results show that adaptive beamforming is superior, however semi-static has comparable performance in specific scenarios. In light of this, we propose a novel-semi adaptive beamformer. The proposed technique is a switch-type beamforming, where a novel switching mechanism enables adaptive and non-adaptive processing to coexist. The algorithm is also robust to both spurious switching as well as other disturbances in the system. For HTSMS, results show that semi-adaptive beamformer can save up to 98% of the filtering computing power without degradation to system performance

Beamforming for OFDM based hybrid terrestrial satellite mobile system

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University of Surrey