Understanding the RIS efficiency: from partial to full illumination.

Reconfigurable intelligent surfaces (RISs) are an attractive alternative to relays in high frequency wireless communications, as they can steer the beam towards the desired direction with low power requirements and without noise amplification. For successful incorporation in future wireless networks it is necessary to understand their efficiency, which depends on several parameters, such as the RIS distance and orientation relative to the transmitter and receiver, and size relative to the incident beam extent. In this work the efficiency of the RIS is studied in terms of the received power. Through simulations it is demonstrated how the RIS efficiency depends on the orientation of the RIS with respect to the transmitter and receiver, and also on the relative size of the RIS with respect to the footprint of the incident beam. The findings are quantified via a metric that expresses the percentage of the region within which a minimum received power is guaranteed, thus serving as a practical means for assessing the RIS efficiency particularly in mobile user environments.