In this paper, we study opportunistic relay selection in cooperative networks with secrecy constraints, where a number of eavesdropper nodes may overhear the source message. To deal with this problem, we consider three opportunistic relay selection schemes. The first scheme tries to reduce the overheard information at the eavesdroppers by choosing the relay having the lowest instantaneous signal-to-noise ratio (SNR) to them. The second scheme is conventional selection relaying that seeks the relay having the highest SNR to the destination. In the third scheme, we consider the ratio between the SNR of a relay and the maximum among the corresponding SNRs to the eavesdroppers, and then select the optimal one to forward the signal to the destination. The system performance in terms of probability of non-zero achievable secrecy rate, secrecy outage probability and achievable secrecy rate of the three schemes are analyzed and confirmed by Monte Carlo simulations.
Index Terms—Rayleigh fading, security constraints, achievable secrecy rate, secrecy outage probability, Shannon capacity, relay selection.