Secrecy outage probability of maximal ratio combining and optimum combining in interference-limited wiretap channels

This paper investigates the security performance of maximal ratio combining (MRC) and optimum combining (OC) in single-input multi-output (SIMO) wiretap channels in an interference-limited environment. We assume that the legitimate receiver (Bob) and eavesdropper (Eve) are equipped with M and N antennas, respectively, and the received signals at Bob and Eve are corrupted by L interferences with equal transmission power. We consider a worst-case scenario where the channel state information (CSI) of the interference channels is available at Eve, so OC is applied at Eve for eavesdropping. Two different scenarios for Bob are considered: if the CSI of the interference channels is available, OC is applied for secure reception, otherwise, MRC is used instead. Exact and asymptotic expressions for the secrecy outage probability are derived in these two scenarios. Our results show that MRC and OC can achieve the same secrecy diversity order that depends on M only; however, OC can achieve higher array gain over MRC and the performance gap between them increases with M while decreases with L.