Ergodic Secrecy Capacity of Cooperative NOMA System with Untrusted User

Non-orthogonal multiple access (NOMA) technique can potentially increase spectral efficiency and improve the network capacity of wireless networks. Ergodic secrecy capacity (ESC) is an important parameter for evaluating the security of wireless systems. In this paper, we investigate the secrecy performance of a cooperative NOMA system with an untrusted user using a half-duplex amplify-and-forward relay in terms of ESC over Rayleigh fading environments. A satellite source uses the NOMA technique to communicate with the near and far users, which is assisted by a relay for downlink communication, and an untrusted user, i.e., an eavesdropper, also interacts with this relay. We derive the lower bound ESC of the proposed system and analyze its performance in terms of power allocation coefficient, variance between relay and user, variance between relay of the eavesdropper, and average signal-to-noise ratio of the eavesdropper link. Simulation results demonstrate that the cooperative NOMA system outperforms the cooperative orthogonal multiple access system in terms of ESC. Finally, Monte Carlo simulations are used to validate the analytical results.