## 2 Modular System-Level Modeling Approach
### Prediction Procedure  
To analyze EMS for BTM system, the only thing should be considered is the input and output of the modules in this prediction model. The prediction procedure for EMS prediction is shown in Figure 4.  
\(S_{1}\), \(S_{2}\), \(S_{3}\) represent the radiated disturbance, inducted disturbance, and conducted disturbance respectively. The value of disturbance \(S_{1}\), \(S_{2}\), \(S_{3}\) depend on the train operation state or other subsystem working state. \(C_{1}\), \(C_{2}\), \(C_{3}\) are the coupling function of radiation, induction, and conduction respectively. The gate function set is determined according to the relationship between the modules in different layers. Here, the gate function set consists of two types: \(\mathbf{F_{\mathrm{s}}}\) and \(\mathbf{F_{\mathrm{c}}}\). \(\mathbf{F_{\mathrm{s}}}\) indicates the connection between the electromagnetic disturbance source layer and EMI coupling layer, while \(\mathbf{F_{\mathrm{c}}}\) is used to establish the connection between EMI coupling layer and sensitive equipment layer. \[ \begin{array}{l}F_{s}=\left\{\begin{array}{ll}F_{s1},&F_{s2},&F_{s3}\\ F_{c1},&F_{c2},&F_{c3}\end{array}\right\}\,\end{array}  \tag{1}\]  
Due to the flexibility of object-oriented module modeling method, the module could be used as the requirement of system-level analysis. Every element of gate function set \(\mathbf{F}\) consists of 0 or 1. \"0\" represents there is no connection between the two modules, and \"1\" is the opposite. For example, if induction disturbance module and inductive coupling module interact with each other, \(F_{s2}=1\), otherwise \(F_{s2}=0\). \(S_{\mathrm{r1}}\) is the disturbance signal produced from \(S_{1}\) and went through the on-board antenna by radiation coupling. Meanwhile, \(S_{\mathrm{r2}}\) is the disturbance signal produced from \(S_{2}\), effected by \(F_{s2}\) and goes through the antenna cable by inductive coupling like cables crosstalk or antenna by inductive coupling like  
Figure 4: Prediction Process of Module System-level Model. near field coupling. And disturbance signal \(S_{\tau 3}\) is produced from \(S_{3}\) and goes through the cables in the BTM system by conductive coupling. All these three disturbance signals \(S_{\tau 1}\), \(S_{\tau 2}\), and \(S_{\tau 3}\) would act on the input port of BTM at the same time. \(S_{n}\) at the input port of BTM is composed of \(S_{\tau 1}\), \(S_{\tau 2}\) and \(S_{\tau 3}\). EMS prediction of BTM system is to analyze what kind of disturbances will lead to inevitable performance degradation for the system. \(T\) is defined as the critical condition for sensitive equipment normal operation. That means \(T\) is the basic requirements of routine operation for sensitive equipment. The critical condition \(T\) relies on the three functions of the system mentioned at the beginning of Section 2. \"\(S_{n}=T\)\" stands for the system could work well with the disturbance \(S_{n}\). Increase the voltage of \(S_{1}\), \(S_{2}\) or \(S_{3}\), until the disturbance \(S_{n}\) just make the system could not work properly. For easy manipulation on-site, voltage is chosen to assess the value of the disturbance \(S_{n}\) in EMS prediction. Thus, the disturbance voltage \(V_{N}\) is considered as the sensitivity level of BTM system. And \(V_{N}\) could be expressed as the summary of \(V_{\tau 1}\), \(V_{\tau 2}\) and \(V_{\tau 3}\) the voltages of \(S_{\tau 1}\)\(S_{\tau 2}\) and \(S_{\tau 3}\). \[V_{N}=V_{\tau 1}+V_{\tau 2}+V_{\tau 3}, \tag{2}\]  
where,  
\[ \begin{array}{l}V_{\tau 1}=V_{1}F_{s1}C_{1}F_{c1}\\ V_{\tau 2}=V_{2}F_{\delta 2}C_{2}F_{\delta 2}\\ V_{\tau 3}=V_{2}F_{s3}C_{3}F_{c3}\end{array} , \tag{3}\]  
where \(V_{1}\), \(V_{2}\), and \(V_{3}\) are the voltages of disturbance signal \(S_{1}\), \(S_{2}\), and \(S_{3}\) respectively. If the sensitivity level of BTM system is confirmed, the value of \(V_{N}\) is a determined value. According to this prediction procedure, the sensitivity level of BTM system is a key factor and needed to be calculated for different BTM system functions.