Reconfigurable intelligent surface (RIS) emerges as an efficient and promising technology for the next wireless generation networks and has attracted a lot of attention owing to the capability of extending wireless coverage by reflecting signals toward targeted receivers. In this paper, we consider a RIS-assisted high-speed train (HST) communication system to enhance wireless coverage and improve coverage probability. First, coverage performance of the downlink single-input-single-output system is investigated, and the closed-form expression of coverage probability is derived. Moreover, travel distance maximization problem is formulated to facilitate RIS discrete phase design and RIS placement optimization, which is subject to coverage probability constraint. Simulation results validate that better coverage performance and higher travel distance can be achieved with deployment of RIS. The impacts of some key system parameters including transmission power, signal-to-noise ratio threshold, number of RIS elements, number of RIS quantization bits, horizontal distance between base station and RIS, and speed of HST on system performance are investigated. In addition, it is found that RIS can well improve coverage probability with limited power consumption for HST communications.

翻译：可重构智能表面（RIS）作为一种高效且前景广阔的技术，被应用于下一代无线通信网络，因其能够通过向目标接收器反射信号来扩展无线覆盖范围而备受关注。本文研究了一种RIS辅助的高速列车（HST）通信系统，旨在增强无线覆盖并提高覆盖概率。首先，探讨了下行单输入单输出系统的覆盖性能，并推导出覆盖概率的闭式表达式。此外，为了优化RIS的离散相位设计和RIS布局，我们构建了旅行距离最大化问题，并受到覆盖概率约束的限制。仿真结果验证了部署RIS能够实现更好的覆盖性能和更高的旅行距离。本文还研究了包括发射功率、信噪比阈值、RIS单元数量、RIS量化位数、基站与RIS之间的水平距离以及HST速度在内的关键系统参数对系统性能的影响。此外，研究发现RIS能够在有限功耗下有效提高HST通信的覆盖概率。