Secrecy Rate Maximization at Near User in Untrusted NOMA with Trusted DF Relay

In this paper, we consider a cooperative non-orthogonal multiple access (NOMA) system with two untrusted users and a trusted decode-and-forward (DF) relay. We formulate two optimization problems to maximize the secrecy rate of near user under two scenarios, namely joint and individual power budget constraints on source and relay. Closed-form expressions are obtained for power sharing between source and relay, along with energy-efficient power allocation coefficients for both the users. Numerical results are provided, which verify the exactness of the theoretical analysis and present insights on the design of a secure cooperative communication network.