Security Analysis of Overlay Cognitive Wireless Networks with an Untrusted Secondary User

In this article, we study the transmission secrecy performance of primary user in overlay cognitive wireless networks, in which an untrusted energy-limited secondary cooperative user assists the primary transmission to exchange for the spectrum resource. In the network, the information can be simultaneously transmitted through the direct and relay links. For the enhancement of primary transmission security, a maximum ratio combining (MRC) scheme is utilized by the receiver to exploit the two copies of source information. For the security analysis, we firstly derive the tight lower bound expression for secrecy outage probability (SOP). Then, three asymptotic expressions for SOP are also expressed to further analyze the impacts of the transmit power and the location of secondary cooperative node on the primary user information security. The findings show that the primary user information secrecy performance enhances with the improvement of transmit power. Moreover, the smaller the distance between the secondary node and the destination, the better the primary secrecy performance.