I. Introduction
As the fifth-generation (5G) cellular network is being deployed worldwide, the research community is investigating key technologies towards the sixth-generation (6G), which is expected to be standardized in the late 2020s [1], [2], [3]. Among the key enablers, we count the introduction of reconfigurable intelligent surfaces (RISs), which are large planar arrays of configurable small meta-atoms [4], [5], [6]. Such RISs can be placed on regular surfaces and through their configuration enable the modification of the radio propagation channel far beyond what was previously possible. An important canonical use case is to overcome the line-of-sight (LOS) blockage between a base station (BS) and a user equipment (UE), which is especially relevant in millimeter wave (mmWave) and sub-terahertz (THz) frequency bands (30 GHz - 300 GHz) [6], [7], [8].