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 µm LC layer and features moderate physical length below 0.45 λ0. 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 µm. Nevertheless, a shorter physical size below 0.25 λ0 is accomplished and potential for a flat group delay is given.

With the rise of models describing the potential and limitations of Reconfigurable Intelligent Surfaces (RISs) in wireless communication and sensing channels, numerous hardware solutions are being developed. The most common solutions are based on varactors, Positive-Intrinsic-Negative (PIN) diodes, and Micro-Electro-Mechanical Systems (MEMS).  Recently, several works on 1-bit nematic Liquid Crystal (LC) RISs have been published. This paper presents the use of LC technology for the realization of continuously tunable mm-Wave RISs to address the increasing interest in this technology.  Starting from basic LC theory, we introduce two different realization methods, the reflectarray and the phased array method.  Comparing LC technology to the competing technologies, we see that the main advantage is the low cost of large panels, followed by a low-power consumption.  However, there are some challenges that the LC technology faces such as its impedance matching, its biasing, and tuning times in the order of 10s of ms. With this in mind, we analyze for which applications LC-based RISs are best suited.