Modulation and Coding for NOMA and RSMA

The next-generation multiple access (NGMA) serves as an umbrella term encompassing transmission schemes distinct from conventional orthogonal methods. As a prominent candidate of NGMA, nonorthogonal multiple access (NOMA) emerges as a promising solution, enhancing connectivity by allowing multiple users to concurrently share time, frequency, and space. However, NOMA faces challenges in practical implementation, particularly in canceling interuser interference (IUI). In this article, first, we discuss the principles behind NOMA and review the conventional NOMA methods and results. Then, to address the above challenges, we present asynchronous transmission and interference-aware modulation techniques, leading to decoding free from successive interference cancellation (SIC). The goal is to design constellations that dynamically adapt to interference, minimizing bit error rates (BERs) and enhancing user throughput in the presence of IUI, intercarrier interference, and intercell interference (ICI). The traditional linkage between minimizing BER and increasing spectral efficiency is addressed, with the exploration of deep autoencoders (AEs) for end-to-end (E2E) communication as a new concept with significant potential for improving BERs. Interference-aware modulation techniques can revolutionize constellation design and communication over nonorthogonal channels. rate-splitting multiple access (RSMA) is another promising interference management technique in multiuser systems. Beyond addressing existing challenges and misconceptions in finite-alphabet NOMA, this article offers fresh insights into the field and provides an overview of code-domain NOMA (C-NOMA) schemes, trellis-coded NOMA (TC-NOMA), and RSMA as other potential candidates for NGMA. Additionally, we discuss the evolution of channel coding toward low-latency communication and examine the modulation and coding schemes (MCSs) in fifth-generation (5G) cellular networks. Finally, we examine future research avenues and challenges, highlighting the importance of addressing them for the practical realization of NOMA from a theoretical concept to a functional technology.