Off-Center Optimization for Reconfigurable Intelligent Surface: Design and Experiments

Reconfigurable intelligent surface (RIS) has emerged as a promising solution to unify the electromagnetic and wireless communication spheres, enabling freedom from the conventionally uncontrollable wireless environments. Most of the current research assumes that the RIS will be placed on a fixed site, and the main beam of the transmit signal will be aligned towards the geometric center of the RIS. Some studies investigate RIS rotation strategy to maximize the received signal power. However, in the practical situation, precise rotation is difficult to be implemented and the RIS might be placed in an im-proper position where the transmit beam can not be aligned to the RIS center. To realize more flexible RIS configuration, we design an off-center optimization method for the RIS system. Specifically, the transmit beam can be directed toward a non-central position of the RIS to maximize the received signal power. The theoretical solution is derived and validation experiments are carried out. A 256-element RIS prototype operating in the C-band is developed. The congruity between the measured and simulated results confirms the correctness of the theoretical analysis. It is validated that, by using the proposed off-center optimization method, significant improvement can be obtained, and at least 15% increase of the received signal power can be obtained under the experimental conditions.