Non-Orthogonal Multiple Access: A Unified Perspective

Non-orthogonal multiple access (NOMA) is a promising technique for future mobile communication systems, which can approach multiuser channel capacity by sharing the same time-frequency resources with multiple users. In this article, we provide a unified framework for NOMA and review the principles of various NOMA schemes in different domains with the objective of creating a unified framework. A systematic performance comparison of different NOMA schemes regarding their peak-to-average power ratio, receiver complexity, latency, grant-free access, user load and peak throughput is also provided for different application scenarios. Relying on our unified framework, we generalize the current understanding of NOMA principle from the conventional code and power domains to the spatial domain as well as to their hybrids and to the networking domain. Finally, the challenges in terms of resource allocation, channel estimation, security, system flexibility and implementation issues are also discussed.