Joint Relay and Eavesdropper Selection Strategy Against Multiple Eavesdroppers over Nakagami- $m$ Fading Channels in Cooperative Decode-and-Forward Relay Networks

Physical layer security (PLS) in the presence of multiple eavesdroppers over Nakagami-
${m}$
fading channels for cooperative decode-and-forward (DF) relay network that consists of one source, one destination, and multiple relays is investigated. Different from the recent PLS system model that only considers one eavesdropper during eavesdropping attack, this paper extends the one-eavesdropper case to a multiple-eavesdropper scenario and investigates joint relay and eavesdropper selection (JRES) strategy against eavesdropping attack over Nakagami-
${m}$
fading channels. In the proposed strategy, the best relay is selected via the maximum relay forward channel capacity. In addition, since eavesdroppers are non-cooperative, the worst case is considered. Namely, the wiretap channel between the best relay and the chosen eavesdropper has the maximum capacity. Traditional direct transmission and opportunistic relay selection (ORS) scheme in the presence of one eavesdropper over Rayleigh channel are regarded as benchmarks. Moreover, a security–reliability tradeoff (SRT) performance is analyzed, where the reliability performance is expressed by outage probability (OP), while the security performance is measured by intercept probability (IP). Closed-form expressions of OP and IP are derived. The numerical results show that the proposed JRES scheme outperforms the traditional direct transmission and the ORS scheme in the presence of one eavesdropper over Rayleigh channel. The SRT performance is enhanced obviously with the increasing of relay numbers and Nakagami channel fading factor
${m}$
for a given number of eavesdroppers, which extends the PLS and SRT performance analysis to a more general case in a cooperative DF relay network.