Liquid Crystals: The way to Scalable and Practical Reconfigurable Intelligent Surfaces in 6G

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.