  The unlicensed spectrum, although free, has become an invaluable resource
toward enabling massive Internet-of-things (IoT) applications, where
Internet-enabled devices are deployed at a large scale. However, realizing
massive IoT connectivity over unlicensed bands requires efficient spectrum
sharing among IoT devices and fair coexistence with other wireless networks. In
this article, we discuss several spectrum sharing methods to address these
intra- and inter-network sharing issues. To this end, we first consider
ALOHA-based networks that avoid spectrum sensing, yet rely on diversity to
improve connection density of random access. Then, we present sensing-based
solutions such as unlicensed cellular access that help support a wider range of
applications with different rate requirements and connection densities.
Finally, we highlight future research directions for massive IoT connectivity
over the unlicensed spectrum.
