Outage Performance of Cooperative Underlay CR-NOMA With Imperfect CSI

In this letter, a decode-and-forward cooperative underlay cognitive radio (CR) non-orthogonal multiple access (NOMA) network is studied. Exact closed-form expressions for the outage probability (OP) of two NOMA secondary destination users (U1and U2) are derived in the case of imperfect channel state information. Then, we find optimal power allocation factors for different distances of U1to satisfy the OP fairness for both users. Moreover, the proposed system model shows the superiority of the CR-NOMA compared with cooperative orthogonal multiple access. The derived analytical expressions are corroborated through Monte Carlo simulations.