Performance analysis of the link selection for secure device-to-device communications with an untrusted relay

In this paper, we study the performance of the selection combining and switch and stay combining methods in secure cooperative device-to-device (D2D) communications underlying a cellular network where an untrusted amplify and forward relay helps D2D communications. In our model, we specifically consider the interference from the cellular system over the D2D pair and the interference from the D2D pair over the cellular system. We assume that D2D communication is performed in such a way that the required outage probability of the cellular system is preserved. We obtain an exact closed-form expression for the lower bound of secrecy outage probability of the D2D pair, as well as an asymptotic expression for the outage probability for both the selection combining and switch and stay combining schemes. Using simulations, we study and compare the performance behavior of the selection combining and switch and stay combining schemes as the system parameters change.