Unveiling the Potential of NOMA: A Journey to Next-Generation Multiple Access

The revolutionary sixth-generation wireless communications technologies and applications, notably digital twin networks (DTN), connected autonomous vehicles (CAVs), space-air-ground integrated networks (SAGINs), zero-touch networks, industry 5.0, healthcare 5.0, agriculture 5.0, and more, are driving the evolution of next-generation wireless networks (NGWNs). These innovative technologies and groundbreaking innovative applications will generate a sheer volume of data that requires the swift transmission of massive data across wireless networks and the capability to connect trillions of devices, thereby fueling the use of sophisticated next-generation multiple access (NGMA) schemes. In particular, NGMA strives to cater to the massive connectivity in the 6G era, enabling the smooth and optimized operations of NGWNs compared to existing multiple access (MA) schemes. This survey showcases non-orthogonal multiple access (NOMA) as the frontrunner for NGMA, spotlighting its novel contributions within the existing literature in terms of “What has NOMA delivered?”, “What is NOMA currently providing?” and “What lies ahead for NOMA?”. We present different variants of NOMA in this comprehensive survey, detailing their fundamental operations. In addition, this survey highlights NOMA’s applicability in a broad range of wireless communications technologies such as multi-antenna systems, machine learning, reconfigurable intelligent surfaces (RIS), cognitive radio networks (CRN), integrated sensing and communications (ISAC), terahertz networks, autonomous aerial vehicles (AAVs), etc. This survey delves deeper by providing a comprehensive literature review of NOMA’s interplay with various state-of-the-art wireless technologies. Furthermore, despite the numerous perks and advantages of NOMA, we also highlight several technical challenges of NOMA, which NOMA-assisted NGWNs may encounter. Next, we unveil the research trends of NOMA in the 6G era, offering reliable, robust, and swift communications. Finally, we offer design recommendations and insights along with the future perspectives of NOMA as the leading choice for NGMA within the realm of NGWNs.