Title: Shedding Light on Non-Orthogonal Multiple Access

Abstract: Non-Orthogonal Multiple Access (NOMA) has been a hot research topic in wireless communications over the past decade. Although there are other versions of NOMA, the literature on the subject has been heavily focused on Power-Domain NOMA (PD-NOMA), which imposes a power imbalance between user signals and employs a successive interference cancellation (SIC) receiver to detect them. The interest in this technique was driven by an information theoretic argument that orthogonal multiple access is not optimum in general and that superposition coding coupled with SIC provides an optimum solution for multiple access. In this talk, we revisit the concept of PD-NOMA, discuss it from a pragmatic angle, and we make several rather surprising observations. In particular, we highlight the fact that the basic principle of PD-NOMA on the downlink is a pure signal constellation design and that a threshold detector is actually all that is needed at the receiver. For the uplink, we first point out the fairness issue related to the disparity between the users’ data rates achieved by the SIC receiver and the fair rates suggested by the power distribution among users. Next, using a unified system model that covers both PD-NOMA and Multi-User MIMO, we summarize the results of a recent study on the power imbalance in PD-NOMA, which revealed that the optimum in terms of the average bit error rate is achieved when the power imbalance is reduced to 0, i.e., when this technique coincides with Multi-User MIMO. This result tends to question the concept of PD-NOMA in general, and it suggests that this technique is only suitable for hierarchical multiple access, i.e., in scenarios where high-profile user equipment are paired with low-power devices like sensors. We also briefly recall NOMA-2000, which avoids the basic problems of PD-NOMA.

Bio: Hikmet Sari is a Professor at Nanjing University of Posts and Telecommunications, Nanjing, China. From 2003 to 2016, he was Professor and Department Head at Supelec, near Paris, and Chief Scientist of Sequans Communications. Prior to this, he held various research and managerial positions in industry including Philips, SAGEM, Alcatel, Pacific Broadband Communications, and Juniper Networks. He holds an Engineering Diploma and a Ph.D. from the ENST, Paris, France. He was elevated to the IEEE Fellow Grade in 1995 for his contributions during the 1980s to advanced signal processing for digital microwave radio systems, but he is best known today for his pioneering work in the 1990s on OFDM, OFDMA, and Single-Carrier Transmission with Frequency-Domain Equalization (SC-FDE), which significantly influenced the IEEE 802.16e and the 3GPP LTE standards. He also published the first papers on Non-Orthogonal Multiple Access (NOMA) back in the year 2000. His distinctions include election to the IEEE Fellow Grade (1995), the Andre Blondel Medal (1995), the Edwin H. Armstrong Achievement Award (2003), the Harold Sobol Award (2012), election to the European Academy (2012), election to the Science Academy of Turkey (2012), and the Heinrich Hertz Award (2021).