Rotatable Block-Controlled RIS: Bridging the Performance Gap to Element-Controlled Systems

Abstract

The passive reconfigurable intelligent surface (RIS) requires numerous elements to achieve adequate array gain, which linearly increases power consumption (PC) with the number of reflection phases. To address this PC problem, this letter introduces a rotatable block-controlled RIS (BC-RIS) that preserves spectral efficiency (SE) while reducing power costs. Unlike the element-controlled RIS (EC-RIS), which necessitates independent phase control for each element, the BC-RIS uses a single phase control circuit for each block, substantially lowering power requirements. In the maximum ratio transmission, utilizing statistical channel state information (CSI) to rotate blocks and coherently superimpose signals with optimized reflection phase of blocks, the BC-RIS achieves the same averaged SE as the EC-RIS. Rotating RIS blocks with the statistical CSI allows for slow mechanical rotation speeds. To counteract the added power demands from rotation, influenced by block size, we have developed a segmentation scheme to minimize overall PC. Furthermore, constraints for rotation power-related parameters have been established to enhance the energy efficiency of the BC-RIS compared to the EC-RIS. Numerical results confirm that this approach significantly improves energy efficiency while maintaining performance.