Relay Selection for Improved Physical Layer Security in Cognitive Relay Networks Using Artificial Noise

In this paper, the physical layer security of underlay cognitive decode-and-forward (DF) relay networks is investigated, wherein one secondary transmitter (ST) exchanges confidential information with one secondary destination with the assistance of multiple cognitive DF relays by sharing the licensed spectrum of an primary user, and the relayed transmission is intercepted by multiple passive eavesdroppers. To provide security guarantee for secondary transmission, we propose a novel cooperative scheme aided by artificial noise. In the absence of the eavesdroppers' channel state information (CSI), two opportunistic relay selection schemes, namely best relay selection (BRS) and conventional relay selection (CRS), are proposed as an incentive to further enhance the physical layer security. To be specific, only the CSI of the main channel is employed in the CRS scheme, whereas the BRS scheme invokes both the CSI of the main channel and interference channel. We address the secrecy performance in terms of secrecy outage probability, for which the closed-form expressions for both the relay selection schemes are derived. Simulation results are provided to verify our analysis and show that the proposed BRS scheme significantly achieves a better secrecy outage performance compared with the CRS scheme as well as the conventional scheme without jamming.