Filtering Reconfigurable Intelligent Surface With Out-of-Band Electromagnetic Transparency

This article presents a reconfigurable intelligent surface (RIS) featuring both filtering response and out-of-band electromagnetic (EM) transparency. First, a theoretical method that simultaneously enables the functionalities of filtering and out-of-band EM transparency is established. Second, an equivalent circuit model and RIS element are designed based on the theoretical solutions. Full-wave simulations show agreement with the circuit calculations, confirming the targeted responses of out-of-band near-lossless transmission, in-band tunable phase shifts, and filtering. Finally, an RIS prototype is fabricated and measured for further validation. The measured results closely match the simulations, exhibiting the desired gain-filtering response. In addition, in-band beams achieve scanning up to 60°. Moreover, out-of-band radiation patterns transmitted through the RIS exhibit little distortion, verifying the EM transparency. To the best of the authors’ knowledge, this is the first RIS capable of achieving filtering and out-of-band EM transparency simultaneously. It offers a promising solution to address adjacent-band interference in RIS-assisted multiband wireless networks.