A 3D Geometry-Based Reconfigurable Intelligent Surfaces-Assisted mmWave Channel Model for High-Speed Train Communications.

In this paper, a 3D geometry-based millimeter-wave channel model is proposed for reconfigurable intelligent surface (RIS)-assisted high-speed train communications. Remarkably, the RIS is attached to surface of train window to guarantee the channel between RIS and receiver is quasi-static, and radiation pattern of RIS element and penetration loss caused by train carriage are taken into account. Furthermore, two RIS phase optimization cases are presented and compared, and a heuristic vector method is applied to further describe RIS phase optimization algorithms for LoS and NLoS scenarios. Based on the proposed model, channel statistical characteristics, namely, spread function and space-time correlation function are derived and analyzed, and impacts of RIS phase optimization cases and channel parameters on channel statistical properties are investigated. The results demonstrate that effective RIS phase optimization schemes increase amplitude of channel impulse response and mitigate small-scale fading caused by high-speed movement and multipath effect, and the phase optimization case that phase of sum of SBR and MBR components to align with the phase of BS-Rx link can achieve higher received power. Moreover, RIS phase optimization cases enhance correlation coefficients in space and time domains by adjusting phase in real time. It is found that correlation functions of RIS phase aligned with LoS components are more stable than case that phase of SBR plused MBR components aligned with phase of BS-Rx link for arbitrary carrier frequencies, angle spread, time instant, number of RIS elements, and moving speed. These observations can be used to provide a foundation for designing and developing RIS-assisted high-speed train communication systems.