When NOMA Meets Multiuser Cognitive Radio: Opportunistic Cooperation and User Scheduling

This correspondence paper investigates a novel non-orthogonal multiple access (NOMA) assisted overlay spectrum sharing framework for multiuser cognitive radio networks toward an enhanced spectrum utilization. In particular, one secondary user is scheduled to help forward the primary signal and convey its own signals as well by applying the NOMA principle. A reliability-oriented secondary user scheduling (R-SUS) scheme is first proposed with a target at minimal primary and secondary outage probabilities. Then, a fairness-oriented secondary user scheduling (F-SUS) scheme is proposed, such that all the candidate secondary users have an equal opportunity to be scheduled for the cooperation. Expressions of primary and secondary outage probabilities are derived in closed form to evaluate the resultant network reliability performance. The results reveal that: 1) the proposed R-SUS and F-SUS schemes can achieve a full diversity order for the primary and secondary transmissions, and 2) although the F-SUS scheme enhances user fairness, it suffers a higher secondary outage probability compared with the R-SUS scheme.