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State of the Art , Taxonomy , and Open Issues on NOMA in Cognitive Radios

The explosive growth of mobile devices and the rapidly increasing demand of wideband wireless services enable an urgency for advanced communication techniques that can achieve high spectrum efficiency and satisfy the massive connectivity requirement. Cognitive radio (CR) and non-orthogonal multiple access (NOMA) techniques are envisioned to be adopted in the fifth generation wireless networks. Integrating NOMA techniques into CR networks has the tremendous potential to improve spectrum efficiency and increase the number of users. However, there are many technical challenges due to the severe interference caused by using non-orthogonal resources. Many efforts have been conducted to facilitate NOMA techniques into CR networks and investigate the effect of NOMA on them. This article aims to survey the latest research efforts made to enable NOMA techniques in CR networks. A taxonomy is devised to categorize and classify the literature based on operation paradigms, enabling techniques, objectives and optimization characteristic. Moreover, the key challenges are outlined to provide guidelines for the domain researchers and designers to realize the application of NOMA techniques into CR networks. Finally, we discuss the open research issues associated with NOMA in CR networks.