Comparison of Compact Liquid Crystal-based Phase Shifting Topologies for Reconfigurable Intelligent Surfaces

This paper presents and compares two promising approaches for realizing liquid crystal (LC)-based phase shifting topologies for Reconfigurable Intelligent Surfaces (RIS). Main requirements for the phase shifters include high Figure of Merit (FoM), compact size, low response times and constant group delay. The first approach is based on defective ground structure inverted microstrip lines (DGS-IMSL) and shows high FoM above 74° /dB between 25 and 30 GHz. In addition, it promises low response times due to its thin 4.6 $\mu$m LC layer and features moderate physical length below 0.45 $\lambda_{0}$. However, it is limited by a non-constant group delay. The second approach, the bandpass inverted microstrip line (BP-IMSL), has a lower FoM of approximately 26° /dB between 28 to 30 GHz with a thicker LC layer of $20\mu \mathrm{m}$. Nevertheless, a shorter physical size below 0.25 $\lambda_{0}$ is accomplished and potential for a flat group delay is given.