Secure communications in cognitive underlay networks over Nakagami-m channel

In this paper, the secure communication of a cognitive radio network (CRN) over Nakagami-m fading channel is investigated. An underlay protocol is used in the considered network, where the unlicensed users or secondary users (SUs) can operate simultaneously with the primary users (PUs) in the same spectrum bands providing that the transmit power of the SUs is constrained by not only the maximum tolerance interference at the PU's receiver but also the maximum transmit power at the SU's transmitter. The exact closed-form expressions of important secure performance metrics, i.e., secrecy outage probability (SOP) and secrecy capacity (SC), are derived. In addition, to give a deep insight into the secure performance trends, the asymptotic expression of the SOP is also obtained when the average signal-to-noise ratio (SNR) of the legitimate channel is high. It is proven that the considered system achieves full diversity gain regardless of the number of antennas at the eavesdropper. Finally, the correctness of our mathematical framework is verified by Monte Carlo simulations.