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By combining the concepts of superposition coding at the transmitter(s) andsuccessive interference cancellation (SIC) at the receiver(s), non-orthogonalmultiple access (NOMA) has recently emerged as a promising multiple accesstechnique for 5G wireless technology. In this article, we first discuss thefundamentals of uplink and downlink NOMA transmissions and outline their keydistinctions (in terms of implementation complexity, detection and decoding atthe SIC receiver(s), incurred intra-cell and inter-cell interferences). Later,for both downlink and uplink NOMA, we theoretically derive the NOMA dominantcondition for each individual user in a two-user NOMA cluster. NOMA dominantcondition refers to the condition under which the spectral efficiency gains ofNOMA are guaranteed compared to conventional orthogonal multiple access (OMA).The derived conditions provide direct insights on selecting appropriate usersin two-user NOMA clusters. The conditions are distinct for uplink and downlinkas well as for each individual user. Numerical results show the significance ofthe derived conditions for the user selection in uplink/downlink NOMA clustersand provide a comparison to the random user selection. A brief overview of therecent research investigations is then provided to highlight the existingresearch gaps. Finally, we discuss the potential applications and keychallenges of NOMA transmissions.

Non-Orthogonal Multiple Access (NOMA) in Cellular Uplink and Downlink: Challenges and Enabling Techniques