Non-orthogonal multiple access (NOMA) is one of the most promising radio access techniques in next-generation wireless communications. Compared to orthogonal frequency division multiple access (OFDMA), which is the current de facto standard orthogonal multiple access (OMA) technique, NOMA offers a set of desirable potential benefits, such as enhanced spectrum efficiency, reduced latency with high reliability, and massive connectivity. The baseline idea of NOMA is to serve multiple users using the same resource in terms of time, frequency, and space.
The available NOMA techniques can broadly be divided into two major categories, i.e., power-domain NOMA and code-domain NOMA. Code-domain NOMA can further be classified into several multiple access techniques that rely on low-density spreading and sparse code multiple access. Other closely related multiple access schemes in this context are lattice-partition multiple access, multi-user shared access, and pattern-division multiple access.
Recent studies demonstrate that NOMA has the potential to be applied in various fifth generation (5G) communication scenarios, including Machine-to-Machine (M2M) communications and the Internet-of-Things (IoT). Moreover, there are some existing evidence of performance improvement when NOMA is integrated with various effective wireless communications techniques, such as cooperative communications, multiple-input multiple-output (MIMO), beamforming, space–time coding, network coding, full-duplex, etc. Given all advancements and experimental outcomes, standardization of NOMA has been established for the next-generation American digital TV standard (ATSC 3.0) under the term layered-division multiplexing (LDM), and has been initiated for the third generation partnership project (3GPP) under the name multi-user superposition transmission (MUST).
Since the principle of NOMA allows multiple users to be superimposed on the same resource, this leads to interference for such systems. Consequently, existing resource management and interference mitigation techniques, especially for ultra-dense networks, need to be revisited due to the incorporation of additional interference this new technology brings. For the similar reason, beamforming and the resultant other problems (e.g., precoding) in massive-MIMO systems introduce additional challenges and need to be solved in order to achieve full utilization of these technologies. From the perspective of physical layer, existing channel coding, modulation and estimation related problems need to be revised as well. Cognitive, cooperative, and visible light communications all are conducive paradigms under NOMA systems compared to conventional systems. However, the resultant evolved problems due to the incorporation of this new technology need to be solved before acquiring benefits from these paradigms. Although NOMA technique offers numerous advantages, the enhanced information sensing ability of more users via this technique, leads to higher security and privacy threat. Therefore, a series of security problems from the physical to application layers need to be solved in order to build a robust, efficient and effective system via this technique.
Edited by: Rukhsana Ruby, Derrick Wing Kwan Ng, Victor C.M. Leung and Taneli Riihonen