How to Deploy RIS to Minimize Delay Spread in HST Communications: Railroad Side, or Train Side?

High-speed train (HST) communications are challenging due to fast time-varying channels and Doppler effects. Recently, reconfigurable intelligent surface (RIS) has been envisioned to be one of the most promising technologies for tackling the difficult problems of HST communications. In this paper, we first propose a transmission model of the HST communication system using RIS on the train side. Then, different strategies for mitigating delay spread (DS) are compared from the perspectives of the phase shift set design and deployment of the RIS. In addition, we derive the spectral efficiency (SE) expression of the proposed system. Numerical evaluations validate the analytical results and reveal that the RIS on the train side not only increases SE but also reduces DS.