Robust Beamforming and Secure Transmission for Wireless Relay Networks with Direct Links and Bounded Channel Uncertainties by Nanmiao Wu Thesis Supervisor: Prof. Shaodan Ma Department of Electrical and Computer Engineering Cooperative beamforming has received significant attention due to its ability to improve the spatial diversity and extend the coverage of wireless relay networks. Various cooperative relaying schemes have been widely used, e.g., the amplify-and-forward (AF) scheme, the decode-and-forward (DF) scheme, the compress-and-forward (CF) scheme, etc. Among them, AF-based relaying scheme is most popular due to its simplicity and low complexity. On the other hand, owing to the broadcast nature of wireless signals, the concerns of privacy and security in wireless relay networks have received growing interests. Recently, physical-layer security for relay communication has been investigated. The relay-assisted cooperative transmission has been shown to enhance transmission security greatly. According to the role of relays, they can be basically classified into cooperative beamforming (CB), cooperative jamming (CJ), mixed CB and CJ, and joint CB and CJ. For CB, all relays forward the information from the source to the legitimate receiver. For CJ, relays do not forward the information to the legitimate receiver, but just cooperatively emit artificial noise (AN) to confuse the eavesdroppers for security purpose. For mixed CB and CJ, partial relays are involved in cooperative forwarding of the confidential information from the source to the legitimate receiver, while the rest ones perform CJ to confuse the eavesdroppers. For joint CB and CJ, all the relays not only cooperatively forward information, but also emit AN such that the legitimate receiver can correctly decode the confidential information while preventing the eavesdroppers from interception. In most of prior relay beamforming and secure design for wireless relay networks, either perfect channel state information (CSI) for all links is assumed or direct link between the source and the destination is ignored. However, direct link is usually available and can offer additional diversity, and perfect CSI is hard to obtain at the legitimate receiver due to limited length of training sequences, quantization errors and feedback errors, etc. As a result, it is beneficial and practical to take the direct link and imperfect CSI into consideration when designing the relay beamformer. In this thesis, we will particularly investigate robust and secure designs for two popular relay networks. In each network, we will design different robust beamforming vectors to minimize the total relay transmission power while guaranteeing various constraints. Different from prior cooperative transmission designs, direct link between the source and the receiver is considered and no perfect channel state information is assumed. Considering bounded channel uncertainties in all links, the worst cases of the total relay transmission power, quality of service (i.e., the received signal-to-noise ratio) and security are first identified. Then two iterative algorithms are proposed to design the relay beamforming for AF relay networks. The effectiveness and practicality of the proposed robust secure designs are demonstrated by computer simulations. 1 TABLE OF CONTENTS TABLE OF CONTENTS........................................................................