  If we assume line-of-sight propagation and perfect channel state information
at the base station -- consistent with slow moving terminals -- then a direct
performance comparison between single-cell Massive MIMO at PCS and mmWave
frequency bands is straightforward and highly illuminating. Line-of-sight
propagation is considered favorable for mmWave because of minimal attenuation,
and its facilitation of hybrid beamforming to reduce the required number of
active transceivers. We quantify the number of mmWave (60 GHz) service antennas
that are needed to duplicate the performance of a specified number of PCS (1.9
GHz) service antennas. As a baseline we consider a modest PCS deployment of 128
antennas serving 18 terminals. We find that, to achieve the same per-terminal
max-min 95%-likely downlink throughput, 10000 mmWave antennas are needed. To
match the total antenna area of the PCS array would require 128000
half-wavelength mmWave antennas, but a much reduced number is adequate because
the large number of antennas also confers greater channel orthogonality. The
principal alleged benefit of mmWave technology--vast amounts of inexpensive
spectrum--is at least partially offset by the complexity of possibly unwieldy
amounts of hardware.
