Secure Cooperative Half-Duplex Cognitive Radio Networks With $K$ -th Best Relay Selection

In this paper, we study the secrecy performance of a half-duplex cognitive relay network in the presence of multiple eavesdroppers and multiple primary users. In particular, generic K th best relay selection schemes for opportunistic relay selection (ORS) and partial relay selection (PRS) are proposed. Exact closed-form and asymptotic expressions for secrecy outage probability (SOP) of the considered schemes are derived. The outcome shows that ORS is able to achieve full secrecy diversity order while PRS obtains unit diversity order. Besides, the order of the selected relay is proved to affect the secrecy diversity order in the ORS scheme but not the PRS scheme, while the number of primary users does not have influence on the diversity order. Additionally, the significance of the change in the number of relays on the SOP enhances at higher orders of selected relay. Nevertheless, increasing the number of eavesdroppers reduces the secrecy diversity gain of the considered system in both schemes.