RIS-Assisted Transmission Scheduling for Train-to-Train Communication System

Millimeter wave (mmWave) has been considered as an enabler to alleviate the spectrum shortage problem while providing high data rates in high speed railway (HSR) communications. However, a mmWave channel has a larger path loss, and therefore is sensitive to blockage. In such a high speed mobile environment with fast-changing channels, considering the characteristics of mmWave, how to achieve reliable transmission scheduling in a train-to-train communication has become a critical issue, especially from the safety point of view. In this paper, we investigate a mmWave train-to-train communication system with the fixed frame structure, where two trains communicate with each other directly by mobile relays (MRs) deployed on top of cabin roofs. Due to the limited communication distance by MRs and the nature that mmWave is prone to blockage, reconfigurable intelligent surface (RIS) is placed between two trains to rebuild line of sight (LoS) link when original link is blocked or beyond the distance range. We aim to maximize the number of completely scheduled flows with different quality of service (QoS) requirements via optimizing the discrete phase shifts of RIS as well as flow scheduling. In particular, we consider multiple-frame scheduling to make use of the predictability of trains' trajectory. To solve this nonlinear integer programming problem, we decompose the joint scheduling problem into two subproblems. We first optimize the discrete phase shifts of RIS, and then build an interference-based graph of flows while transforming the transmission constraints into interference relationships. The goal of the problem can be realized through the proposed scheduling algorithm. Finally, simulation results show that the proposed algorithm outperforms three baseline schemes in the literature in terms of the number of the completely scheduled flows and system throughput.