Security and Reliability Analysis of Relay Selection in Cognitive Relay Networks

Improving security and reliability is a main concern in many wireless transmission systems. In this paper, we explore the security and reliability in cognitive relay networks with an eavesdropper and multiple relays where the decode-and-forward (DF) protocol is considered. We propose a novel cognitive relay selection to improve the security and reliability of the considered system. The most important contribution is that we obtain the cumulative distribution functions (CDFs) of the channels from the selected relay to the destination and to the eavesdropper, respectively, which makes other related theoretical research possible. Furthermore, based on the proposed relay selection criterion, we derive the accurate closed-form outage probability and intercept probability of the considered system. At last, simulation results verify the correctness of our derivations. Interestingly, although the relay selection is based on the relay-destination channel and the relay-eavesdropper channel, it can be seen from the theoretical results that the reliability performance is completely irrelevant to the relay-eavesdropper channel's statistical characteristics, whereas the security performance is entirely independent of statistical characteristics of the relay-destination channel. In addition, the security performance is always improved with the increasing number of relays, but the reliability performance is not proportional to the number of relays any more when the number of relays reaches a certain value.