A Varactor-Based Reconfigurable Intelligent Surface Concept for 5G/6G Mm-Wave Applications

A varactor-based reconfigurable intelligent surface (RIS) concept is presented for low-cost, high-precision beamforming of 5G/6G mm-waves. To enable this, we maximize the RIS unit cell (UC) phase coverage by (i) employing a varactor diode-tuned slotted-patch UC, and (ii) adding a metallic loop and via fence across the UC boundary, which also improves the bandwidth and angular insensitivity. In an infinite RIS environment, the UC exhibits a phase coverage of 330.7 degrees while maintaining the insensitivity of reflection coefficients up to 50-degree elevation angle. This UC design demonstrates a phase coverage that exceeds 315 degrees with under 0.7-dB reflection loss across a 11.6% bandwidth, where the capacitance tuning ratio of the considered commercial off-the-shelf varactor diode is only 1.8. A $15 \times 15$ RIS model with a 3-bit phase quantization is used for beam steering performance validation; it predicts the realized gain of 19.5 dBi for the 30-degree beam-steering direction at 28 GHz.