Nonlocal Dual-Band Reconfigurable Intelligent Surfaces for Precise Full-Space Beamforming

This paper introduces a nonlocal, dual-band reconfigurable intelligent surface (RIS) designed for full-space beam synthesis at 4.0 GHz and 6.3 GHz. The constituent unit cells comprise a pair of interleaved sub-cells that are specifically engineered to operate independently at their respective target frequencies. This hardware-level decoupling facilitates an efficient synthesis framework based on microwave network theory (MNT) that rigorously accounts for mutual coupling within both bands. Under this framework, the optimal biasing for sub-cells is determined to achieve precise full-space beam synthesis at both frequencies. The proposed method is numerically and experimentally validated with an RIS comprising 14 X 14 varactor-loaded unit cells that can be individually biased. We experimentally demonstrate arbitrary beam profile synthesis beyond simple beam steering, including dual-beam and sector patterns in full space. Experimental and simulation results show good agreement with the MNT model, confirming the effectiveness of the proposed method.