Physical layer security of maximal ratio combining in underlay cognitive radio unit over Rayleigh fading channels

In this paper, we investigate the secrecy outage performance of maximal ratio combining (MRC) in cognitive radio networks (CRNs) over Rayleigh fading channels. In a single input multiple output wiretap system, we consider a secondary user SU-TX transmits confidential messages to another secondary user SU-RX equipped with K (K>1) antennas while MRC technique is adopted to improve its received signal-to-noise ratio. Meanwhile, an eavesdropper exists to overhear the information between SU-TX and SU-RX. SU-TX adopts the underlay strategy to meet the quality of service of the primary user without spectrum sensing. In this paper, we derive the closed-form expressions for the secrecy outage probability and the probability of nonzero secrecy capacity of MRC scheme in CRNs over Rayleigh fading channels. Simulations are conducted to validate the accuracy of our proposed analytical models.