  Broader applications of the Internet of Things (IoT) are expected in the
forthcoming 6G system, although massive IoT is already a key scenario in 5G,
predominantly relying on physical layer solutions inherited from 4G LTE and
primarily using orthogonal multiple access (OMA). In 6G IoT, supporting a
massive number of connections will be required for diverse services of the
vertical sectors, prompting fundamental studies on how to improve the spectral
efficiency of the system. One of the key enabling technologies is
non-orthogonal multiple access (NOMA). This paper consists of two parts. In the
first part, finite block length theory and the diversity order of multi-user
systems will be used to show the significant potential of NOMA compared to
traditional OMA. The supremacy of NOMA over OMA is particularly pronounced for
asynchronous contention-based systems relying on imperfect link adaptation,
which are commonly assumed for massive IoT systems. To approach these
performance bounds, in the second part of the paper, several promising
technology directions are proposed for 6G massive IoT, including linear
spreading, joint spreading & modulation, multi-user channel coding in the
context of various techniques for practical uncoordinated transmissions,
cell-free operations, etc., from the perspective of NOMA.
