RIS‐Aided Massive MIMO Antennas*

In this chapter, we propose and study an antenna structure where a (not large) array of radiating elements is placed at short distance in front of a passive reconfigurable intelligent surface (RIS). In this new structure, we detail a channel estimation procedure using different configurations of the RIS elements and derive a closed-form expression for the hardening lower-bound of the downlink performance. Next, we formulate an optimization problem, with respect to the phase shifts of the RIS, aimed at minimizing the channel cross-correlations while preserving the channel individual norms. The numerical analysis shows that the proposed structure is capable of overcoming the performance of a conventional massive MIMO system without the RIS.