  Multiple access technology played an important role in wireless communication
in the last decades: it increases the capacity of the channel and allows
different users to access the system simultaneously. However, the conventional
multiple access technology, as originally designed for current human-centric
wireless networks, is not scalable for future machine-centric wireless
networks.
  Massive access (studied in the literature under such names as massive-device
multiple access, unsourced massive random access, massive connectivity, massive
machine-type communication, and many-access channels) exhibits a clean break
with current networks by potentially supporting millions of devices in each
cellular network. The tremendous growth in the number of connected devices
requires a fundamental rethinking of the conventional multiple access
technologies in favor of new schemes suited for massive random access. Among
the many new challenges arising in this setting, the most relevant are: the
fundamental limits of communication from a massive number of bursty devices
transmitting simultaneously with short packets, the design of low complexity
and energy-efficient massive access coding and communication schemes, efficient
methods for the detection of a relatively small number of active users among a
large number of potential user devices with sporadic transmission pattern, and
the integration of massive access with massive MIMO and other important
wireless communication technologies. This paper presents an overview of the
concept of massive access wireless communication and of the contemporary
research on this important topic.
