Cooperative NOMA: State of the Art, Key Techniques, and Open Challenges

In this article, we investigate the integration of NOMA into cooperative relaying. We first introduce several fundamental cooperative NOMA network structures, and then classify them into two categories: user relaying and dedicated relaying. After that, we discuss the combination of cooperative NOMA with various advanced transmission technologies, such as full-duplex, cognitive radio, energy harvesting and multiple-input multiple-output. Following this, we focus on system evaluation and resource allocation of cooperative NOMA. Finally, major challenges and open issues are highlighted.