Novel FSS Based Multibit Passive 8 × 8 RIS for NextGen Wireless Communication

This research presents a novel frequency selective surface (FSS) based reconfigurable intelligent surfaces (RISs) for next-generation wireless communication at 3.8 GHz and 4.2 GHz, respectively. The unit element of the FSS is built over a low-loss dielectric substrate and made of split convoluted square ring (CSR) geometry. Four varactor diodes per unit cell are placed within the gap of the CSR to actively tune its capacitance by adjusting the bias voltages. This, in turn, alters the resonant frequencies of the CSR and, consequently, the associated reflection phase. For the 2-bit RIS a conducting copper plate of thickness 0.5 mm is placed at an air-spacer height of 1 mm. One of the ends of each varactor has been shorted to the ground plane using a pair of vias. Within the operational BW of 3.753.85 GHz, the RIS exhibits a low-loss feature with a reflection magnitude > -2.1 dB. In another improved version, a 3-bit RIS is realized at 4.2 GHz by placing a thin (0.035 mm) ground plane at the back face of the dielectric. Reduction in profile height improves the angular stability of the RIS. Also reflection amplitude within the operating range (4.15-4.25 GHz) is found > -4.3 dB. Later, a 64 elements 2D array of 2-bit RIS is demonstrated for actively tuning beam steering ability by changing the reverse bias voltages across the diode.