Secure Transmission via a Wireless-Powered Untrusted Relay in Underlay CRNs

In this paper, we consider a underlay cognitive radio network (CRN), in which a secondary source (S) transmits information to a secondary destination (D) via an energy harvesting (EH) untrusted relay. Being an EH node, the relay harvests energy from the received RF signals based on power-splitting (PS) policy. Using the harvested energy, the relay not only help but also intercept the secondary transmission. To realize the positive secrecy rate, a secure scheme based on destination-aided jamming strategy is proposed. Jointly considering the secure requirement of the secondary network and the interference threshold of the primary network, the performance of the proposed secure scheme is evaluated in terms of probability of successfully secure transmission ($P_{SST}$). The closed-form expression of $P_{SST}$ is derived. Numerical results show that, the closed form expression of $P_{SST}$ coincides with the simulation result perfectly. Moreover, the proposed secure scheme outperforms an intuitive secure scheme under similar power consumption.