  One of the use cases for 5G systems and beyond is ultra-reliability
low-latency communication (URLLC). An enabling technology for URLLC is massive
multiple-input multiple-output (MIMO), which can increase reliability due to
improved user separation, array gain and the channel hardening effect.
Measurements have been performed in an operating factory environment at 3.7 GHz
with a co-located massive MIMO array and a unique randomly distributed array.
Channel hardening can appear when the number of antennas is increased such that
the variations of channel gain (small-scale fading) is decreased and it is here
quantified. The cumulative distribution function (CDF) of the channel gains
then becomes steeper and its tail is reduced. This CDF is modeled and the
required fading margins are quantified. By deploying a distributed array, the
large-scale power variations can also be reduced, further improving
reliability. The large array in this rich scattering environment, creates a
more reliable channel as it approaches an independent identically distributed
(i.i.d.) complex Gaussian channel, indicating that one can rethink the system
design in terms of e.g. channel coding and re-transmission strategies, in order
to reduce latency. To conclude, massive MIMO is a highly interesting technology
for reliable connectivity in reflective and heavily shadowed industrial
environments.
