  Massive machine-type communication (MTC) is expected to play a key role in
supporting Internet of Things (IoT) applications such as smart cities, smart
factory, 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
or 2-step random access schemes are introduced for MTC to be more efficient by
reducing signaling overhead. In this article, we first introduce grant-free
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 notion of non-orthogonal multiple access (NOMA). Finally, design
challenges of grant-free random access towards next generation cellular systems
are presented.
