Secrecy Rate Maximization in Relay-Assisted NOMA with Untrusted Users

Non-orthogonal multiple access (NOMA) has been considered as a promising solution to provide spectrally efficient communications in the 5th generation and beyond networks. Cooperative communication, on the other hand, helps in improving the coverage and reliability of the communications. In this paper, we consider a cooperative NOMA communication system with two untrusted users and a trusted amplify-and-forward relay. Maximization of the secrecy rate of the near user while considering the service requirements of the far user is tackled. Closed-form expressions are obtained for the optimal power sharing between the source and the relay, along with the optimal power allocation for both users. Numerical results are also provided, which verify the exactness of the theoretical analysis and provide insights on the design of secure NOMA-based cooperative communication networks.