Massive machine-type communications (mMTC) is one of the main three focus
areas in the 5th generation (5G) of mobile standards to enable connectivity of
a massive number of internet of things (IoT) devices with little or no human
intervention. In conventional human-type communications (HTC), due to the
limited number of available radio resources and orthogonal/non-overlapping
nature of existing resource allocation techniques, users need to compete for
connectivity through a random access (RA) process, which may turn into a
performance bottleneck in mMTC. In this context, non-orthogonal multiple access
(NOMA) has emerged as a potential technology that allows overlapping of
multiple users over a radio resource, thereby creating an opportunity to enable
more autonomous and grant-free communication, where devices can transmit data
whenever they need. The existing literature on NOMA schemes majorly considers
centralized scheduling based HTC, where users are already connected, and
various system parameters like spreading sequences, interleaving patterns,
power control, etc., are predefined. Contrary to HTC, mMTC traffic is different
with mostly uplink communication, small data size per device, diverse quality
of service, autonomous nature, and massive number of devices. Hence, the
signaling overhead and latency of centralized scheduling becomes a potential
performance bottleneck. To tackle this, grant-free access is needed, where mMTC
devices can autonomously transmit their data over randomly chosen radio
resources. This article, in contrast to existing surveys, comprehensively
discusses the recent advances in NOMA from a grant-free connectivity
perspective. Moreover, related practical challenges and future directions are
discussed.

本文全面讨论了来自免授权连接角度的非正交多址接入 (NOMA) 的最新进展，探讨了相关的实际挑战和未来方向。