| Summary: | Due to the open nature of the medium, wireless communications systems are highly vulnerable to security attacks. In recent years, security within the physical layer has gained attention, since the underlying techniques can add to those found in traditional cryptographic approaches. In this thesis, new mathematical models were developed for the analysis of secrecy capacity and outage probability in different scenarios, following which new optimisation problems were formulated and new algorithms devised to solve those problems. The process began by analysing secrecy performance for various cooperative communication scenarios in the presence of single and multiple eavesdropper(s). A multicast cooperative system was also analysed, based on distributed Alamouti space-time coding. Furthermore, the secrecy performance of a relay selection scheme was analysed in an independent but non-identically distributed (i.n.i.d) Rayleigh fading scenario. The second part of the thesis concentrated on optimisation methods for cooperative relaying and jamming techniques. For a dual-hop system, a joint cooperative beamforming and jamming scheme was created, considering both a perfect and an imperfect eavesdropper's channel state information (CSI). Optimal solutions to degrade the eavesdropper's interception by minimising its received signal to interference and noise ratio (SINR) were also presented whilst ensuring the legitimate receiver's SINR requirement. For the multi-hop scenario, the secrecy rate, with and without transmitting artificial noise, was considered for maximisation, and an optimal power splitting solution under limited power constraints at the transmitters was also proposed. In addition, an iterative solution for the joint optimisation of transmit power and power splitting coefficient at each transmitter was posited. The analyses and optimisation algorithms developed provide new insights into secrecy performance and optimal transmission schemes in various practical scenarios.
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