Exploiting the Temporal Dimension of Reconfigurable Intelligent Surfaces for Multiuser Downlink

Until now, researchers in wireless communications have mainly focused their attention on space-domain designs of reconfigurable intelligent surfaces (RISs), where the main concern is to engineer the spatial-phase gradient across the RIS to decouple incidence and reflected/refracted directions. However, such designs do not exploit temporal dimension of the RIS. We show that leveraging the temporal dimension of the RIS might be beneficial for downlink from a communication perspective depending on the considered network utility function and the available channel state information at the transmitter (CSIT). Moreover, we highlight that, regardless of the available amount of CSIT, the design of temporal variations across the RIS is instrumental for developing scheduling algorithms aimed at maximizing the network capacity subject to some fairness constraints. Numerical results corroborate our main findings.