In this paper, we investigate the physical layer security performance of a buffer-aided cognitive relay network over Rayleigh fading channels, where an underlay cognitive radio system is employed. In this network, a cognitive transmitter communicates with a cognitive receiver through a buffer-aided relay, in the presence of an eavesdropper overhearing the transmitted information. To evaluate the secrecy performance of the system, secrecy outage probability (SOP) has been used as a popular performance metric, where a closed-form expression is obtained. Numerical results obtained from the derived SOP expression, along with the Monte-Carlo simulation results are provided to evaluate the secrecy performance of the system under different system parameters. In addition, the secrecy performance of the system without buffer is evaluated and compared with that of the buffer-aided system. Assuming that the delay caused by buffer is negligible, it is shown that increasing the buffer size from L=2 to L=20 leads to a decrease in SOP from   10^-2 to 10^-3, indicating a performance improvement in the order of about 10. Moreover, buffer-aided relay system achieves 3 dB SNR gain compared to the performance of the system without buffer, at the SOP of 10^-3.  
 

Keywords: Cognitive Radio Networks, PHY Security, Relay Networks, Buffer-Aided Relay, Secrecy Outage Probability, Rayleigh Fading

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