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.

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In this paper, we consider an active eavesdropping scenario in a cooperative system consisting of a source, a destination, and an active eavesdropper with multiple decode-andforward relays. Considering an existing assumption in which an eavesdropper is also a part of network, a proactive relay selection by the eavesdropper is proposed. The best relay which maximizes the eavesdropping rate is selected by the eavesdropper. A relay selection scheme is also proposed to improve the secrecy of the system by minimizing the eavesdropping rate. Performances of these schemes are compared with two passive eavesdropping scenarios in which the eavesdropper performs selection and maximal ratio combining on the relayed links. A realistic channel model with independent non-identical links between nodes and direct links from the source to both the destination and eavesdropper are assumed. Closed-form expressions for the secrecy outage probability (SOP) of these schemes in Rayleigh fading channel are obtained. It is shown that the relay selection by the proactive eavesdropper is most detrimental to the system as not only the SOP increases with the increase in the number of relays, but its diversity also remains unchanged.Index Terms-Active eavesdropping, cooperative system, Decode-and-forward relay, relay selection, secrecy outage probability.

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