I. Introduction
With the rapid development of high-speed railways all over the world, the demand for train safety monitoring, real-time scheduling, mobile ticketing and other services is increasingly urgent. This makes the railway mobile communication system GSM for Railways (GSM-R) which is based on the Global System for Mobile Communications (GSM) difficult to meet the system requirements on the data transmission service [1]. Meanwhile, Long Term Evolution (LTE) is gaining commercial use, and the mobile communication systems are faced with upgrading. Compared to GSM-R, LTE offers a wider bandwidth, a lower latency and an all-IP network architecture, which can effectively meet the current requirements of railway mobile communication systems for data transmission services. Therefore, LTE seems to be a good candidate to substitute the GSM-R as the basis technology for railway communications in the long term.