Secrecy outage of dual-hop regenerative multi-relay system with relay selection

Relay selection is considered to enhance the secrecy of a dual-hop regenerative multi-relay system with an eavesdropper. Without assuming perfect decoding at the relays, the secrecy outage probability of a single relay system is obtained first. Secrecy outage of optimal, traditional, and suboptimal relay selection schemes is then evaluated. To reduce the power consumption, partial relay selection schemes based only on either of the source-relay or relay-destination instantaneous channel state information (ICSI) are introduced. The secrecy outage is evaluated and compared with the other schemes. Secrecy outage of all the selection schemes are obtained in closed-form. An optimal relay selection scheme is proposed using secrecy outage, which does not require any ICSI. Asymptotic and diversity gain analysis of the secrecy outage is presented when source-relay and relay-destination average SNRs are the same or different. We observe that the improvement in eavesdropper link quality affects the secrecy outage more when required secrecy rate is low as compared to the case when rate is high. We also observe that relay selection improves performance more when number of relays are more. It is important to note that either of the source-relay or the relay-destination link quality can equally limit the secrecy outage performance even if the other link quality is infinitely good.