Massive Unsourced Random Access for NGMA: Architectures, Opportunities, and Challenges

Multiple access technology plays an important role in communication systems, which can enable different users to access the system efficiently and reliably. However, the conventional multiple access technologies are not suitable for future sixth generation (6G) systems as ultra-high reliability, extremely low latency, and massive connectivity should be supported. Unsourced random access (URA), where all potential users share a common codebook and the BS only needs to decode a list of messages instead of active users' identities (IDs), is a promising multi-access technology for next generation multiple access (NGMA). This article summarizes and compares existing random access (RA) schemes and determines the advantages of URA for 6G systems. Moreover, considering the special unsourced property of URA, URA applications are discussed and summarized as user-specific and user-independent scenarios. Finally, potential research and future directions are pointed out.