## 3 Construction of the Model  
The system EMC can be classified into three states: compatibility, incompatibility, and critical compatibility. Whether or not a system has interference can be defined by the interference margin (IM). When \(\text{IM}>0\), a potential EMI is present, and there is electromagnetic incompatibility in the system. When \(\text{IM}=0\), a potential EMI is uncertain, and there is critical electromagnetic compatibility in the system. When \(\text{IM}<0\), a potential EMI is absent, and there is electromagnetic compatibility in the system. These definitions are shown in Table 2. The evaluation content, methods, and criteria of each level in the model are introduced in detail below. EMI may be generated between subsystems within a system (in-system EMI) or between the system and the external environment (inter-system EMI). According to the practical requirements of EMC evaluation and the analysis of armored vehicular communication systems, the integrity and accuracy of EMC evaluation for vehicular communication systems are mainly considered when constructing an evaluation model. Therefore, the evaluation process is divided into five levels. The first, second, and fourth level evaluate and analyze the EMI that may be present in a system, which includes the working frequency and the signal spectrum of the vehicular radio and the degree of the vehicle's antenna isolation. The third and fifth levels evaluate and analyze the EMI that may be present between the system and the external environment, which includes the sensitivity of the vehicular receiver and the vehicle's communication performance. The flowchart of the proposed five-level evaluation model is presented in Figure 1 and shows the five levels of the hierarchical architecture. Based on the proposed model, an EMC evaluation can be performed, as follows:  
Step 1: Choose all transmitters and receivers that will make up the set of devices in the vehicular communication system. \begin{table}
\begin{tabular}{c c c} \hline \hline
**IM** & **Potential EMI** & **EMC** \\ \hline \(>\)0 & Present & incompatibility \\ =0 & Uncertain & critical compatibility \\ \(<\)0 & Absent & compatibility \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Three grades of potential EMI and EMC. Figure 1: Flowchart of the proposed evaluation model. Step 2: Perform a Level 1 evaluation of all the device sets, focusing on the four kinds of interference margins between them. The equations for identifying the presence of the four types of interference margins in the system are used to define the IM criterion during Level 1 evaluation. If any of the equations are satisfied, then the IM will be identified as \(\text{IM}>0\), which indicates that a potential EMI is present for all device sets, and the EMC evaluation will then proceed to the next level. Otherwise, the evaluation will be finished. Step 3: When \(\text{IM}>0\) during Level 1 evaluation, a second-level evaluation should be performed on all device sets; this evaluation focuses on the signal spectrum interference between the device sets. The IM criterion is defined in the form of the equations for identifying the presence of three types of spectral EMI based on the working frequencies in the system. If any of the equations are satisfied, then the IM will be identified as \(\text{IM}>0\), which indicates that a potential EMI is present for all device sets, and the EMC evaluation will then proceed to the next level. Otherwise, the evaluation will be finished. Step 4: When \(\text{IM}>0\) at Level 2, a third-level evaluation should be performed on all receivers. This EMC evaluation focuses on the decrease in receiver sensitivity based on digital maps. The IM criterion at Level 3 is defined based on the threshold of the receiver sensitivity in the system. If the decrease in receiver sensitivity is higher than the threshold, then the IM will be identified as \(\text{IM}>0\), which indicates that a potential EMI is present in the system, and the EMC evaluation will then proceed to the next level.