  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. Its 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 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.
