Channel-Aware Power Allocation and Decoding Order in Overlay Cognitive NOMA Networks

Guoxin Li, Deepak Mishra, Hai Jiang

Abstract: To further improve spectral efficiency, this paper considers a non-orthogonal multiple access (NOMA)-assisted overlay cognitive radio network consisting of a pair of primary users and a pair of secondary users. In this network, communication between the pair of primary users is achieved with the help of the secondary transmitter (ST). In return, ST can communicate with the secondary receiver by sending a superimposed signal consisting of its own message and the received primary message. We intend to minimize the outage probability of the secondary system under quality-of-service constraints of the primary system by jointly optimizing the power allocation (PA) factor at ST and the decoding order (DO) at the receivers. When full channel state information (CSI) is available at ST, the optimal PA and DO are derived in closed form under the constraint that the target rate of the primary system should be guaranteed. When only statistical CSI is available at ST, the optimal PA and DO are derived in closed form or by using a bisection method, under the constraints that the outage probability of the primary system is not more than a predetermined threshold and is not larger than that of the secondary system. For the proposed strategies, outage probability expressions of the primary and secondary systems are derived in closed form or semi-closed form. Numerical results are shown to verify our analysis and demonstrate the superiority of the proposed strategies in different CSI-availability scenarios.