NOMA for Next-Generation Massive IoT: Performance Potential and Technology Directions

Broader applications of the Internet of Things (loT) are expected in the forthcoming 6G system, although massive loT is already a key scenario in 5G, predominantly relying on physical layer solutions inherited from 4G LTE and primarily using orthogonal multiple access (OMA). In 6G loT, supporting a massive number of connections will be required for diverse services of the vertical sectors, prompting fundamental studies on how to improve the spectral efficiency of the system. One of the key enabling technologies is non-or-thogonal multiple access (NOMA). This article consists of two parts. In the first part, finite block length theory and the diversity order of multi-user systems are used to show the significant potential of NOMA compared to traditional OMA. The supremacy of NOMA over OMA is particularly pronounced for asynchronous contention-based systems relying on imperfect link adaptation, which are commonly assumed for massive loT systems. To approach these performance bounds, in the second part of the article, several promising technology directions are proposed for 6G massive loT, including linear spreading, joint spreading and modulation, multi-user channel coding in the context of various techniques for practical uncoordinated transmissions, cell-free operations, and so on, from the perspective of NOMA.