  In this paper, we consider a multi-user multiple-input multiple-output (MIMO)
system aided by multiple intelligent reflecting surfaces (IRSs) that are
deployed to increase the coverage and, possibly, the rank of the channel. We
propose an optimization algorithm to configure the IRSs, which is aimed at
maximizing the network sum-rate by exploiting only the statistical
characterization of the environment, such as the distribution of the locations
of the users and the distribution of the multipath channels. As a consequence,
the proposed approach does not require the estimation of the instantaneous
channel state information (CSI) for system optimization, thus significantly
relaxing (or even avoiding) the need of frequently reconfiguring the IRSs,
which constitutes one of the most critical issues in IRS-assisted systems.
Numerical results confirm the validity of the proposed approach. It is shown,
in particular, that IRS-assisted wireless systems that are optimized based on
statistical CSI still provide large performance gains as compared to the
baseline scenarios in which no IRSs are deployed.
