Beam-forming and beam-focusing capabilities in RIS-aided communications

High frequency communications are a promising technology for addressing the increasing demand for spectrum resources in 6G, and reconfigurable intelligent surfaces (RISs) are expected to be a significant enabler in both line-of-site and nonline-of-site scenarios. Owing to their wavefront engineering capabilities, RISs can offer channel flexibility and reconfigurability via advanced operations that extend beyond simple beam-forming. Beam-focusing, a characteristic with great potential for controllable power maximization at desired hotspots, has been so far largely unexplored. In this paper, the conditions for efficient RIS beam-focusing are studied and are discussed with respect to the conditions for conventional RIS beam-forming. The merits of both capabilities are assessed analytically and are illustrated with numerical examples. It is shown that the size of the beamfootprint on the RIS is a key parameter to control the efficiency of both beam-forming and beam-focusing, with direct implications on crucial operations, such as power maximization, tracking and localization.