High data rate, reliability and secrecy are fundamental pursuits in wireless communications systems. A multi-user wireless environment is particularly challenging because of interference and potential information wiretap. This dissertation proposes to utilize relay techniques to achieve high data rate, high reliability and absolute secrecy in multi-user wireless communications systems. First, a communication system with multiple source-destination pairs needing to communicate simultaneously is investigated. A relay beamforming scheme is proposed, in which a multi-antenna relay assists the communication. The proposed scheme enjoys spatial multiplexing gain and has a significant data rate advantage over widely studied orthogonal transmission schemes, e.g., Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA). The relay beamforming scheme also overcomes the difficulties of recently introduced interference alignment approaches by not requiring full cross-node Channel State Information (CSI). Beamforming schemes that maximize the system throughput and meet user Quality of Service (QoS) requirements are investigated, and both optimal and low-complexity suboptimal schemes are proposed. The system performance in terms of sumrate is analyzed and the rate limit is derived. The impact of imperfect CSI is analyzed and various approaches are proposed for mitigating the imperfect CSI effects in a practical system. Relay antenna selection is proposed to use in conjunction with the relay beamforming for more reliable communications. Second, physical layer secrecy in a two-slot communications system with one source, one destination, one eavesdropper and multiple relays is investigated. The goal is to achieve high secrecy rate without leaking any information to the eavesdropper. Different from the widely studied approaches in which the destination combines the listened signals in the first and second slots, we propose that the destination acts as a jammer in the first slot. This novel design effectively reduces the signal quality at the eavesdropper, and allows the secrecy rate to improve with higher power budget. Under this framework, a cooperative relaying scheme is proposed that targets at maximizing the secrecy rate. A set of novel approaches are proposed to realize this goal, namely, relay selection and optimal power allocation among the first/second slot data and jamming signals. Analysis of the scaling law of secrecy rate is conducted showing the secrecy rate trend at large transmit power and number of relays.
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Electrical and Computer Engineering
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Rutgers University Electronic Theses and Dissertations
Rutgers University. Graduate School - New Brunswick
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