A Shared-Aperture Dual-Band Sub-6 GHz and mmWave Reconfigurable Intelligent Surface With Independent Operation

A novel dual-band independent reconfigurable intelligent surface (DBI-RIS) design that combines the functionalities of millimeter-wave (mmWave) and sub-6-GHz bands within a single aperture is proposed. This design aims to bridge the gap between current single-band RISs and wireless systems utilizing sub-6-GHz and mmWave bands that require RIS with independently reconfigurable dual-band operation. The mmWave element is realized by a double-layer patch antenna loaded with 1-bit phase shifters, providing two reconfigurable states. An 8\times 8 mmWave element array is selectively interconnected using three RF switches to form a reconfigurable sub-6-GHz element at 3.5 GHz. A suspended electromagnetic bandgap (EBG) structure is proposed to suppress surface waves and ensure sufficient geometric space for the phase shifter and control networks in the mmWave element. A low-cost planar spiral inductor (PSI) is carefully optimized to connect mmWave elements, enabling the sub-6-GHz function without affecting mmWave operation. Finally, the prototypes of the DBI-RIS are fabricated, and experimental verification is conducted using two separate measurement testbeds. The fabricated sub-6-GHz RIS successfully achieves beam steering within the range from -35^{\circ} to 35^{\circ} for DBI-RIS with 4\times 4 sub-6-GHz elements, while the mmWave RIS demonstrates beam steering between -30^{\circ} and TEXPRESERVE5 for DBI-RIS with 8\times 8 mmWave elements, and has good agreement with simulation results.