  This paper considers an antenna structure where a (non-large) array of
radiating elements is placed at short distance in front of a reconfigurable
intelligent surface (RIS). This structure is analyzed as a possible emulator of
a traditional MIMO antenna with a large number of active antenna elements and
RF chains. Focusing on both the cases of active and passive RIS, we tackle the
issues of channel estimation, downlink signal processing, power control, and
RIS configuration optimization. With regard to the last point, an optimization
problem is formulated and solved, both for the cases of active and passive RIS,
aimed at minimizing the channel signatures cross-correlations and thereby
reducing the interference. Downlink spectral efficiency (SE) formulas are also
derived by using the popular hardening lower-bound. Numerical results,
represented with reference to max-fairness power control, show that the
proposed structure is capable of outperforming conventional non-RIS aided MIMO
systems even when the MIMO system has a considerably larger number of antennas
and RF chains. The proposed antenna structure is thus shown to be able to
approach massive MIMO performance levels in a cost-effective way with reduced
hardware resources.
