Analysis and Enhancement of Joint Security and Reliability in Cooperative Networks

In this study, we deal with a cooperative wireless communications system of security constraints in which there is no knowledge about channel state information (CSI) of the eavesdropper. In order to improve the tradeoff between security and reliability, we opportunistically employ relays via an incremental and selective cooperation with respect to the CSI of the main link. In this cooperation, named as opportunistic cooperative transmission (OCT), the first priority is communication through the direct link as far as possible. However, a relay that has the best link to the main destination is employed for retransmitting the source information when the direct link is so weak which cannot provide a specific quality. In fact, since improving the reliability of the favorable communications by use of the relays increases the risk of successful interception by the eavesdropper at the same time, the OCT scheme tries to avoid excess of rebroadcasting the secret information by the relays whenever the direct link is strong enough; otherwise it employs an opportunistically selected relayed link. Hence, it can improve the security for a specific level of reliability and vice versa. We derive closed-form expressions for the security-reliability tradeoff (SRT) and secrecy outage probability (SOP) of the OCT scheme. Moreover, we provide asymptotic analyses which are illustrative on the security behavior of the proposed scheme. Finally, simulation results are presented which confirm validity of the analysis and reveal the desired performance of OCT in terms of both SRT and SOP.