Grant-Free Random Access in Machine-Type Communication: Approaches and Challenges

Massive machine-type communication (MTC) is expected to play a key role in supporting Internet of Things (IoT) applications such as smart cities, factories, and connected vehicles through cellular networks. MTC characterized by large number devices their sparse activities, which are difficult be supported conventional approaches motivate the design new access technologies. In particular, 5th generation (5G), grant-free (GF) or 2-step random schemes introduced for more efficient reducing signaling overhead. this article, we first introduce GF discuss how it can modified with massive multiple-input multiple-output (MIMO) exploit high spatial multiplexing gain. We then explain preamble designs that improve performance variations based on notions semi-GF non-orthogonal multiple (NOMA). Finally, challenges toward next systems presented.