  The Multi-User downlink, particularly in a Multi-Cell Massive MIMO setting,
requires enormous amounts of instantaneous CSIT (Channel State Information at
the Transmitter(s)), iCSIT. Here we focus on exploiting channel covariance CSIT
(coCSIT) only. In particular multipath induced structured low rank covariances
are considered that arise in Massive MIMO and mmWave settings, which we call
pathwise CSIT (pwCSIT). The resulting non-Kronecker MIMO channel covariance
structures lead to a split between the roles of transmitters and receivers in
MIMO systems. For the beamforming optimization, we consider a minorization
approach applied to the Massive MIMO limit of the Expected Weighted Sum Rate.
Simulations indicate that the pwCSIT based designs may lead to limited spectral
efficiency loss compared to iCSIT based designs, while trading fast fading CSIT
for slow fading CSIT. We also point out that the pathwise approach may lead to
distributed designs with only local pwCSIT, and analyze the sum rates for iCSIT
and pwCSIT in the low and high SNR limits.
