  The use of base stations (BSs) and access points (APs) with a large number of
antennas, called Massive MIMO (multiple-input multiple-output), is a key
technology for increasing the capacity of 5G networks and beyond. While
originally conceived for conventional sub-6GHz frequencies, Massive MIMO
(mMIMO) is ideal also for frequency bands in the range 30-300 GHz, known as
millimeter wave (mmWave). Despite conceptual similarities, the way in which
mMIMO can be exploited in these bands is radically different, due to their
specific propagation behaviors and hardware characteristics. This paper reviews
these differences and their implications, while dispelling common
misunderstandings. Building on this foundation, we suggest appropriate signal
processing schemes and use cases to efficiently exploit mMIMO in both frequency
bands.
