Coordinated Reconfigurable Intelligent Surfaces: Non-Diagonal Group-Connected Design

Reconfigurable intelligent surfaces (RIS) constitute a promising technology for future wireless communications in terms of improving the spectral-efficiency and energy-efficiency. In this context, a novel RIS structure, which we refer to as coordinated RIS architecture, is formulated, where different RIS elements can be connected by configurable impedances to eliminate the channel fading. In the proposed RIS architecture, both the RIS element connection pattern and the configurable impedances can be optimized, based on the channel state information (CSI). The proposed architecture exhibits higher optimization flexibility than the state-of-the-art single-connected RIS architecture and group-connected RIS architecture, where only the configurable impedances can be optimized. Specifically, when considering base stations (BS) having a single antenna, the maximal ratio combining (MRC) criterion may be harnessed for designing the RIS element connection pattern, while in the case of multiple BS antennas, the alternating optimization algorithm may be employed for iteratively optimizing the BS's active beamforming vector and the RIS's passive beamforming matrix. Our numerical results show that the proposed coordinated RIS architecture achieves higher power gain than the group-connected RIS architecture having the same number of configurable impedances. Furthermore, the power gain in our proposed RIS architecture tends to that of the fully-connected architecture upon increasing the number of RIS elements, while requiring significantly fewer configurable impedances.