User and Passive Beam Scheduling Scheme for Liquid Crystal IRS-assisted mmWave Communications

Intelligent reflecting surfaces (IRS) have been considered a strong solution for coverage holes in millimeter wave communication. By controlling the reflection phase of the IRS, radio waves can be reflected toward user equipment (UE) within the coverage hole, enabling communication with the base station (BS). We focus on liquid crystal (LC) IRS, which uses liquid crystals to control the reflection phase. Compared to other IRSs, LC IRS has the advantage of lower power consumption. However, it has a longer response time to change the reflection direction than the symbol length in new radio (NR). Generally, user scheduling at an NR BS is performed on a time-division basis, assuming instantaneous beam switching. However, during the response time of the LC IRS, the radio waves are not reflected in the desired direction, resulting in a decrease in throughput. This paper proposes a UE selection and reflection direction control method to improve the throughput reduction in an environment with an LC IRS. The proposed method formulates the problem to optimize the reflection pattern and switching timing, considering the loss due to switching. This can reduce the number of switches while addressing multiple UEs. Simulation evaluations demonstrate the improvement in throughput in an environment with an LC IRS using the proposed method.