Optimal Position and Orientation Study of Reconfigurable Intelligent Surfaces in a Mobile User Environment

Reconfigurable intelligent surfaces (RISs) have been often proposed as a means for restoring nonline-of-sight links in wireless communications. Because the path loss changes with the relative distance of the RIS from the transmitter and the receiver and their relative orientations, the quality of the communication is expected to depend strongly on the RIS placement in an RIS-aided link. In this work, the optimal placement of the RIS is studied with respect to both position and orientation and it is shown that even for an ideal RIS, i.e., lossless, polarization preserving, and with a uniform response with respect to the incident angle, the quality of the communication is greatly affected by the RIS orientation. To provide insight into the optimal choice, the analysis is based on an analytical model that treats the RIS as a continuous surface of finite size that steers the incident beam toward a prescribed direction. By imposing a desired minimum threshold on the received power, it is studied how the efficiency of the RIS depends on both its position and orientation. The decisions that have to be made concerning the placement of the RIS are illustrated with examples of D-band indoor scenarios.