  Reliability is becoming increasingly important for many applications
envisioned for future wireless systems. A technology that could improve
reliability in these systems is massive MIMO (Multiple-Input Multiple-Output).
One reason for this is a phenomenon called channel hardening, which means that
as the number of antennas in the system increases, the variations of channel
gain decrease in both the time- and frequency domain. Our analysis of channel
hardening is based on a joint comparison of theory, measurements and
simulations. Data from measurement campaigns including both indoor and outdoor
scenarios, as well as cylindrical and planar base station arrays, are analyzed.
The simulation analysis includes a comparison with the COST 2100 channel model
with its massive MIMO extension. The conclusion is that the COST 2100 model is
well suited to represent real scenarios, and provides a reasonable match to
actual measurements up to the uncertainty of antenna patterns and user
interaction. Also, the channel hardening effect in practical massive MIMO
channels is less pronounced than in complex independent and identically
distributed (i.i.d.) Gaussian channels, which are often considered in
theoretical work.
