## 3 The Objective Electromagnetic Environmental Effects Assessment  
### Basic Four-Domain Method  
The four-domain method refers to time, frequency, space, and energy domains. In the objective quantitative assessment of simple electromagnetic environmental effects, it is necessary to first choose evaluation indexes with macro and micro aspects [23] according to the electromagnetic signal feature in the four domains. In the macro view, more than ten indicators are used to evaluate the complexity of the electromagnetic environment. Generally, researchers adopt signal intensity, background noise intensity, frequency threat degree, frequency occupancy degree, frequency coincidence degree, electromagnetic spectrum control degree, and so on. In the micro view, the work efficiency (WE) of equipment or the changing rate of the relevant macro indexes are used to reflect the complexity of the electromagnetic environment. The comprehensive electromagnetic environmental effects assessment model is established based on the proportion of the resources occupied by the related index exceeding the threshold level of the electronic system. Figure 2 illustrates the research design of the four-domain method for electromagnetic environmental effects assessment. [PERSON] et al. [24] adopted spatial occupancy (SO), time occupancy (TO), spectrum occupancy (FO), and the average power spectral density (AP) to build the evaluation model. Meanwhile, they used the general evaluation standard [25] to evaluate the electromagnetic environmental complexity from the macroscopic view, and the performance loss of specific equipment was used to define the complexity of the same environment from the microscopic view. The D-S evidence theory and fuzzy mathematics were used to quantify the qualitative result of electromagnetic environmental complexity. It effectively reduces the error of the evaluation. Figure 1: The research architecture of the electromagnetic environmental effects assessment. Based on the traditional TO, FO, and SO, [PERSON] et al. [26] combined macro- and micro-assessment factors to quantify the complexity of the electromagnetic environment using the weighting method. For the macroscopic aspect, the statistical law of the electromagnetic environment in sensing data is quantified based on the average energy value (AE), standard deviation (SD), and skewness (SK) of the power spectral density. For the micro aspect, the relative variance ratio (RV), the average variance ratio (AV), and the maximum variance ratio (MV) of the power spectral densities are used to quantify the microscopic changes of the electromagnetic environment. The quantitative evaluation of the electromagnetic environment's complexity is more refined and accurate. However, the weighting coefficient of each evaluation index is obtained empirically, which will lead to overly subjective models. Except for the characteristics of the four-domain occupancy degree, this domain's characteristic correlation degree [27] is also used to evaluate the electromagnetic environmental complexity and its threat [28]. It seems that the four-domain method can comprehensively evaluate the complexity of the electromagnetic environment. However, it has two shortcomings. First, it needs to define the threshold value of the electromagnetic environment. However, there is no unified definition of the threshold; therefore, it is not easy to ensure the reliability of the evaluation results. Second, the weightings in the assessment model are more subjective. If a mathematical weighting is adopted, the iterative calculation process of the weighting is complicated. Therefore, the traditional four-domain models are not optimistic enough in electromagnetic environment evaluations, especially for complex electromagnetic environments. Table 1 shows the commonly used classifications and evaluation criteria for electromagnetic environmental complexity using the basic four-domain method.