Analysis of FD-NOMA Cognitive Relay System With Interference From Primary User Under Maximum Average Interference Power Constraint

In this paper, we consider a non-orthogonal multiple access (NOMA) based underlay cognitive radio (CR) system consisting of a source, two destinations, and a relay in the secondary network. The source communicates with two destinations using the NOMA technique via the assistance of the relay operating in full-duplex (FD) mode. The operations of all secondary nodes are affected by the interference from a primary transmitter. Meanwhile, secondary transmitters must adjust their transmission powers so that the interference probability to a primary receiver is always less than a given value. Under this average interference power constraint, we propose the maximum average interference power (MAIP) constraint for the relay to achieve its highest possible average transmit power. Based on the MAIP constraint, we derive the exact closed-form expression of the outage probabilities and ergodic capacities at two destination users. Monte-Carlo simulations verify the accuracy of the obtained mathematical expressions. Numerical results show that the considered NOMA-FD-CR relay system’s performance is significantly affected by the interference from the primary transmitter and the maximum tolerable interference of the primary receiver. Additionally, using the MAIP constraint at the relay substantially improves the quality of the received signal at the far user with a slight reduction in the signal quality at the near user and fulfills the interference constraint without needing the instantaneous channel state information (CSI).