  Massive MIMO, a key technology for increasing area spectral efficiency in
cellular systems, was developed assuming moderately sized apertures. In this
paper, we argue that massive MIMO systems behave differently in large-scale
regimes due to spatial non-stationarity. In the large-scale regime, with arrays
of around fifty wavelengths, the terminals see the whole array but
non-stationarities occur because different regions of the array see different
propagation paths. At even larger dimensions, which we call the extra-large
scale regime, terminals see a portion of the array and inside the first type of
non-stationarities might occur. We show that the non-stationarity properties of
the massive MIMO channel changes several important MIMO design aspects. In
simulations, we demonstrate how non-stationarity is a curse when neglected but
a blessing when embraced in terms of computational load and multi-user
transceiver design.
