## 1 Introduction  
The balise transmission module (BTM) system is a subsystem of the train operation control system for high-speed trains. It is responsible for detecting the presence of a balise and obtaining the location information of the train. As a positioning sensor, BTM system is comprised of BTM, on-board antenna, balise and wayside equipment, as shown in Figure 1. The on-board antenna at the bottom of the train receives the RF signal from balise to get the location information. Due to the complex and harsh train bottom electromagnetic environment, the RF signal from balise is interfered with easily and the BTM system will not work properly. This could lead to serious consequences, such as location information decoding error or stopping the train [1]. Thus, it is of great significance to study the EMS of BTM for ensuring the normal work of BTM system under interference condition and the efficient operation of the train. There are several researches on the electromagnetic interference (EMI) of BTM systems. Most of them focus on analyzing the EMI coupling process in different balise allocated positions and different operating modes of the system [2; 3; 4; 5; 6]. The BTM system EMC issues are mentioned in European SUBSET 036 and European SUBSET 085, but not described in detail [7; 8]. In European SUBSET 116, EMC test methods are defined for analyzing the BTM system with disturbance [9], but it is still in draft. There are a few researches on the EMS of BTM system [10; 11] presented the test method in the laboratory. It is recommended for EMS research of manufactured equipment, but not suitable in the design phase of BTM system. A more detailed and practical method for EMS prediction and system analysis should be investigated. From function perspective, a high-speed train consists of several subsystems: train operation control, traction power supply, brake control and network communication systems. BTM system is a subsystem in the train operation control system. From EMC perspective, the EMI of BTM system could be from different coupling paths and other subsystems in the train, i.e., it is a system-level problem which could not be solved only in device-level within the subsystem. A device-level issue means an issue related to certain devices. For example, the influence from traction converter of the traction power supply system to the balise is a device-level issue. When the disturbance to a BTM system occurs and it is not clear which devices or subsystems are the sources and which devices are the victims, the analysis on the interaction between all the devices of BTM system and all the subsystem of the train is a system-level issue. In practice, almost all the EMC problems are system-level issues. However, most of EMS prediction methods solve the device-level EMS issue [12; 13; 14]. Even if electromagnetic topology method is regarded as a typical approach for system-level EMC analysis [15; 16; 17] the complicated huge system cannot be settled due to the increasing complexity of the topology structure with a rapid increase of nodes and harassment transmission paths. The concept of hierarchical method is proposed to simplify the system-level EMC analysis problem [18]. It is a \"flow down\" method, which means it can solve the EMI problem from device-level to system-level, but ignores the interaction between device and system. It is not suitable for predicting the EMI problem of the BTM system in practical engineering applications which is in the unstable electromagnetic environment. The working environment of BTM system would be affected by the changes of other subsystems in the train, e.g., the pantograph off-line disturbance, the emission from rail return current and other disturbances from the traction power supply system. In addition, the EMS of the BTM system should also consider the function of the system itself. Modular system-level modeling and prediction method is proposed in this paper based on the object-oriented analysis method (OOAM) [19].