Development of a Dual-Polarized Direction-Variable Liquid-Crystal Meta-Surface Reflector for Intelligent Reflecting Surface

Intelligent reflecting surfaces (IRSs) have been attracting attention as a solution to coverage hole problems in millimeter-wave communication areas. They are considered to be one of the key technologies of next-generation mobile communication systems. Since polarized multiple-input-multiple-output (MIMO) is utilized in millimeter-wave communication systems to deliver high-speed data transmission, an IRS is also required to be applied for polarized MIMO and reflect signals in a wide angle range. To realize an IRS for the polarized MIMO, the IRS is required to reflect the signal of each polarization in the same direction with high isolation. In addition, the IRS is required to control the reflection phase over a wide range. In recent research, many proposals related to the development of IRSs have been proposed. However, since the configuration of a conventional IRS with diodes controls the reflection characteristics by controlling the electric length or shape of the reflecting element, it becomes asymmetrical in the direction of each polarization and it is difficult to apply polarized MIMO. To solve this problem, in this paper, we propose a new design of a dual-polarized liquid-crystal-based IRS for 28 GHz mobile communication systems. Since the proposed IRS controls the reflecting phase by controlling the electric thickness of the substrate, the IRS can control the reflecting phase without changing the shape of the reflecting element. By optimizing the design of the reflecting element, the IRS can achieve a wide reflection phase control of 260 degrees for both vertical and horizontal polarizations. The IRS can also control the reflection direction of each polarization in the range of ±60 degrees with a high isolation of more than 20 dB. The main contributions of this paper are 1) a proposal for the design of a liquid crystal IRS for polarized MIMO; 2) a proposal of a design method for the liquid-crystal IRS using unit cell analysis; 3) verification of the proposed IRS through electromagnetic field analysis; 4) development of the proposed liquid-crystal IRS; 5) experimental verification of the proposed IRS in an anechoic chamber, and 6) derivation of the MIMO channel capacity of the proposed dual-polarized IRS.