QoS-Aware Secrecy Rate Maximization in Untrusted NOMA With Trusted Relay

Abstract

This letter considers a trusted decode-and-forward relay-assisted two-user non-orthogonal multiple access (NOMA) communication system, and tackles the secure rate maximization problem at the near user. The problem is solved optimally using KKT conditions, while satisfying the quality of service (QoS) requirements of the far user. Particularly, closed-form expressions for the power sharing between the source and the relay, and the power allocation coefficients corresponding to power-domain NOMA for both users are obtained. An upper-bound on the threshold rate of the far user is also derived, which indicates the best QoS that can be ensured. Numerical results showed that the relay-user link acts as the bottleneck and provide design insights on optimal power allocation satisfying the tradeoff between the rate secrecy at the near user and the QoS requirements of the far user.