A high flexibility enabled single layered tunable and polarization insensitive RIS for mm-Wave dynamic beamforming applications

A single-layered and polarization-insensitive tunable Reconfigurable Intelligent Surface (RIS) with a simple biasing circuit model is proposed. The optimized RIS unitcell utilizes a varactor diode with a suitable variable capacitance for providing a reflection phase shift to the incident wave. The RIS unitcell structure consists of a symmetric circular patch layer and a slotted ground layer with grounded via for biasing the varactor diode. This structure provides five operating states with reflection phase shifts of 0 ⁰ , -30 ⁰ , -60 ⁰ , -120 ⁰ , and − 180 ⁰ at 28 GHz with a simple biasing configuration. The beamforming with a sidelobe level tuning range of -5.5 to -12.7 dB is achieved and demonstrated using an 8x8 RIS array model with high flexibility in reconfiguring the phase shift values. The RIS model withholds a polarization angle insensitivity of up to 60 ⁰ . The simulation-based approach evaluates the RIS array configuration to assess the beamforming ability that provides a gain of 14.12 dB under normal incidence. The 8x8 RIS array model is fabricated, and an experiment is carried out for a 1.15 pF (0 V biasing) condition as a proof of the concept demonstration. The simulated and experimental results proved its key feature of high flexibility, suitable for mm-wave dynamic beamforming applications.