Secrecy Analysis of NOMA System with Imperfect SIC in Overlay Cognitive Radio Networks

Cognitive radio (CR) and non-orthogonal multiple access (NOMA) can be regarded as two promising technologies which bestows with tremendous possible solutions for the next generation of wireless communications networks to maximise their usage of available spectrum. In this paper, we evaluate the performance of a overlay cognitive radio network (OCRN) for a downlink secure NOMA system, where the secondary transmitter (ST) is used as a decode-and-forward (DF) relay to assist the primary transmitter (PT) to transmit information to destination i.e., primary user (PU), while covertly transmitting their own information to the secondary user (SU) against the eavesdropper (Eve) of PT. The secrecy performance comparison between two users PU and SU are obtained under perfect and imperfect successive interference cancellation (SIC) assumptions, respectively. Furthermore, this paper investigates the secrecy performance of proposed downlink OCRN-NOMA system in terms of secrecy sum rate (SSR), the secrecy ergodic sum rate (SESR), and secrecy outage probability (SOP) by taking into consideration the existence of external or passive eavesdropper node. To characterize the secrecy performance of the proposed OCRN-NOMA system, this work derives the closed-form analytical expressions of the SOP/SESR metrics in the presence of both perfect SIC and imperfect SIC scenarios. Based on the analytical frameworks, the numerical and simulation results are obtained under different network parameters.