Joint Passive Beamforming and Deployment Design for Multi-Intelligent Refracting Surface-Aided High-Speed Railway Communication

This letter investigates a multi-intelligent refracting surface (IRS) aided high-speed railway communication system that can provide cooperative passive beamforming gains without incurring additional channel estimation overhead. A novel multi-IRS passive beamforming framework is designed to simultaneously track the time-varying channels between the remote radio head and a number of IRSs mounted on the top of the train. These IRSs cooperatively enhance the signal coverage within the train carriage. Specifically, the multi-IRS passive beamforming and deployment are jointly optimized to maximize the minimum average channel power across all discretized grids within the carriage, thereby ensuring fair coverage throughout the train. An alternating optimization algorithm, utilizing the semidefinite relaxation and one-dimensional line search techniques, is employed to solve this non-convex optimization problem efficiently. Numerical results reveal that the proposed distributed IRS deployment scheme outperforms its centralized deployment counterpart with the same total number of IRS elements. Moreover, the proposed multi-IRS passive beamforming and deployment joint design can significantly improve the signal coverage performance in the train carriage.