RIS-Aided Highly Directional Millimeter-Wave Communications: Power and Beamwidth Control

Reconfigurable Intelligent Surface (RIS) is considered as a technology capable of artificially modify the wireless channel, then the communication performance might be improved in terms of the coverage and the signal strength. Most studies focus on the omni-directional communication systems operating in the sub-6 GHz band and assume that the RIS can be fully illuminated. However, in future highly directional millimeter-wave communication systems, the beamwidth emitted by the base station (BS) is very narrow, and only a part of the RIS elements, instead of full of them, will be illuminated. As a result, the signal power and the RIS reflected beamwidth will decrease. In addition, random failures of RIS units may introduce phase shift errors, which will degrade the received signal power. To address these problems, in this paper, the RIS illuminated area will be optimized through rotation. Specifically, the illuminated area as a function of the rotation angle is explicitly analyzed, and closed-form expressions for the optimal rotation angle and the critical rotation angle are derived. The effectiveness of the proposed scheme is verified by simulation results, which demonstrate that, the received signal power can be improved. The RIS reflected beamwidth can be increased to four times compared to its original size in our simulation setup, while maintaining the same received power, thus allowing an enlarged receiving area.