Abstract

Non-Orthogonal Multiple Access (NOMA) received significant attention in the field of wireless communications during the last ten years, particularly focusing on Power-Domain NOMA (PD-NOMA) that uses a successive interference cancellation (SIC) receiver. This review paper reexamines PD-NOMA from a pragmatic angle and sheds light on its virtues and limitations in uplink and downlink scenarios. We observe that the concept of PD-NOMA in the downlink can be seen as a pure signal constellation design and that a simple threshold detector is sufficient at the receiver end. For the uplink, we first address the fairness concerns arising from the mismatch between the users' data rates that can be achieved by the SIC receiver and the fair rates suggested by the power distribution among users. Furthermore, we discuss findings from a recent analysis comparing PD-NOMA to MultiUser MIMO under a common system framework. This study reveals that assigning the same power levels to users, thereby aligning PD-NOMA with MultiUser MIMO, optimizes performance in terms of average bit error rate. These insights raise questions about the fundamental utility of PD-NOMA and suggest that its best application is hierarchical multiple access, where devices with different power capabilities are paired together.