  Future wireless networks must provide ever higher data rates. The available
bandwidth increases roughly linearly as we increase the carrier frequency, but
the range shrinks drastically. This paper explores if we can instead reach
massive capacities using spatial multiplexing over multiple-input
multiple-output (MIMO) channels. In line-of-sight (LOS) scenarios, therank of
the MIMO channel matrix depends on the polarization and antenna arrangement. We
optimize the rank and condition number by identifying the optimal antenna
spacing in dual-polarized planar antenna arrays with imperfect isolation. The
result is sparely spaced antenna arrays that exploit radiative near-field
properties. We further optimize the array geometry for minimum aperture length
and aperture area, which leads to different configurations. Moreover, we prove
analytically that for fixed-sized arrays, the MIMO rank grows quadratically
with the carrier frequency in LOS scenarios, if the antennas are appropriately
designed. Hence, MIMO technology contributes more to the capacity growth than
the bandwidth. The numerical results show that massive data rates, far beyond 1
Tbps, can be reached both over fixed point-to-point links. It is also possible
for a large base station to serve a practically-sized mobile device.
