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-multipleoutput (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.INDEX TERMS Beyond 5G/6G, coverage hole, intelligent reflecting surface, liquid crystal, millimeter wave, polarized MIMO, channel capacity
I. INTRODUCTION
A. BACKGROUNDIn fifth-generation mobile communication systems (5G), millimeter-wave bands with wide bandwidths are utilized to deliver high data rates and low-latency mobile communication services [1]. These high-frequency bands are also expected to be utilized in beyond 5G/6G mobile networks to satisfy the increasing demand for communication quality. On the other hand, these frequency bands suffer from high blockage loss due to their rectilinear propagation characteristics. This creates coverage holes where a signal from a base station (BS) is insufficient to provide high communication quality in the communication area [2].To overcome the coverage holes, ...
