An RIS Unit Cell With an Independently Controllable Phase-Frequency Slope and Phase Center

Reconfigurable intelligent surface (RIS) has emerged as an important technology to optimize the multi-path environment in the next generation of wireless communication systems. RIS is typically a 2D array of reconfigurable unit cells that forms a controllable beam by creating a phase gradient across the array surface. In this work, we design an electrically tunable linearly polarized RIS at 2.5 GHz that yields a resonance profile with both a controllable phase at the band-center frequency and a controllable phase-frequency slope. In other words, we add the tunability of the phase-frequency slope to the tunability of a resonance center frequency to increase the achievable rate of wideband communication systems. The proposed design consists of three layers of unit cells with dog-bone-shaped elements in the top layer, patch elements in the middle layer, and a ground plane in the bottom layer. Each patch and dog-bone-shaped element is loaded with a varactor. The reverse bias voltages are then controlled to provide a phase-frequency profile with +90°, 0°, and −90° phase shift at 2.5 GHz and a slope that varies between 0.39°/MHz and 6.56°/MHz. Measurements at 2.5 GHz using a prototype array of 4 × 5 dog-bone elements overtop of 5 × 5 patch elements show phase-frequency profiles with +90°, 0°, and −90° phase shift and slope values between 0.42°/MHz and 9.35°/MHz, thus validating the design.