Physical-Layer Security Enhancement via Relay-Aided D2D Communications Underlaying Cellular Networks

In this paper, we propose a relay-assisted device-to-device (D2D) communication as an underlay in a cellular network, where two sequential best relay nodes based on different selection criteria are chosen. Novel analytical expressions of the secrecy outage probability (SOP) and the probability of nonzero secrecy capacity (PNSC) are derived to evaluate the secrecy performance of the cellular network. In addition, we derive a simple and explicit approximation of the SOP in the high signal-to-noise ratio (SNR) to gain some insights on how various system parameters affect the aforementioned secrecy performance. Also, for the sake of comparison, the analytical SOP expression when only the cellular network exists, i.e., without the interference of the D2D communication, is obtained. Furthermore, to assess the usefulness of the cooperation between the D2D communication and the cellular network, we derive an analytical expression for the mutual outage probability (MOP) which represents the true outage probability across both the cellular and D2D networks. An asymptotic analysis of the MOP in the high SNR regime is obtained and the diversity order is derived. Finally, our proposed mathematical framework is compared with Monte-Carlo simulations to verify the accuracy of the derivations.