Coexisting RIS Induced Multipath Fading in High-Speed Train Systems

Reconfigurable Intelligent Surfaces (RIS) have recently emerged as a promising and cost-effective technology for enabling stable and reliable communications in high-speed train (HST) environments. However, the coexistence of multiple RIS-assisted networks in confined train environments poses significant theoretical and practical challenges. This study focuses on the RIS coexistence problem in HST systems, specifically investigating the impact of multipath fading induced by coexisting RIS (C-RIS) on interfered users. First, we develop a simplified mutual-coupling channel model for C-RIS-induced multipath (CR-MP), applicable to operational frequency bands above 5 GHz. Second, we evaluate the effects of CR-MP on interfered users using the multipath power ratio (MPR), defined as the ratio of the received signal power to the desired signal power. The MPR formulation is derived as a function of the user's spatial position, operational frequency, and penetration loss. Finally, we analyze the spatial and temporal characteristics of the CR-MP channel through the MPR radiation pattern and the received signal envelope, respectively. Simulation results reveal that the spatial characteristics are time-varying and strongly influenced by the frequency difference and penetration loss, while the temporal characteristics depend on the user's position. When the target user is located in specific positions, the fading effects induced by CR-MP can be negligible with a reduced frequency difference, while the received signal power gain can be significantly enhanced with an increased frequency difference. The findings of this study provide practical insights for RIS deployment in real-world systems, particularly for mitigating C-RIS-induced interference in coexisting RIS-assisted networks.