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