## 2 EMI Theoretical Mechanism  
In practical cases, a vehicular communication system usually includes several electronic devices, such as HF radios, VHF radios, and other devices. The mutual influence of these devices will inevitably lead to complex electromagnetic environments in these systems and may generate various types of EMI, such as fundamental wave interference, high-order harmonic wave interference, and high-order intermediation interference [29; 30].  
Assuming that the original transmitted signal in the system is \(s(t)\), it is expressed as in Equation (1). Because of the mutual influence among the electronic devices in the system, \(s(t)\) is subjected to the mutual effect gain \(G[s(t)]\), and its form is shown in Equation (2), where \(a_{i}\) denotes the power coefficients of the devices. Under the mutual influence of \(G[s(t)]\), \(s(t)\) can be modified to \(u(t)\), as expressed in Equation (3).  
\[s(t)=A\cos(\omega_{1}t)+B\cos(\omega_{2}t) \tag{1}\]  
\[G[s(t)]=\sum_{i=1}^{\infty}a_{i}s^{i-1}(t)\ \ \ i=1,2,3\ldots \tag{2}\]  
\[u(t)=s(t)G[s(t)] \tag{3}\]  
By taking the power series expansion of \(u(t)\) in terms of \(s(t)\), the final signal can be expressed as the sum of various additional signal components, which is shown in Equation (4) and Table 1[31; 32].  
\[\begin{array}{lll}u(t)&=&a_{1}s(t)+a_{2}s^{2}(t)+a_{3}s^{3}(t)+\ldots\ldots \\ &=&a_{1}[A\cos(\omega_{1}t)+B\cos(\omega_{2}t)]+a_{2}[A\cos(\omega_{1}t)+B\cos (\omega_{2}t)]^{2}\\ &&+a_{3}[A\cos(\omega_{1}t)+B\cos(\omega_{2}t)]^{3}+\ldots\\ &=&a_{1}[A\cos(\omega_{1}t)+B\cos(\omega_{2}t)]+a_{2}[A^{2}\cos(\omega_{1}t)+ B^{2}\cos(\omega_{2}t)+2 AB\cos(\omega_{1}t)\cos(\omega_{2}t)]\\ &&+a_{3}[A^{2}\cos^{2}(\omega_{1}t)+B^{2}\cos^{2}(\omega_{2}t)+2 AB\cos(\omega _{1}t)\cos(\omega_{2}t)][A\cos(\omega_{1}t)+B\cos(\omega_{2}t)]+\ldots\\ \end{array} \tag{4}\]  
As can be seen in Table 1, some additional signal components, such as a fundamental wave component, some high-order harmonic wave components, and some high-order intermediation components, are generated beyond the original signal \(s(t)\)[33]. The amplitudes of the additional signal components are determined by the amplitudes \(A\) and \(B\) of \(s(t)\) and the power coefficients \(a_{i}\) of the mutual effect gain. The values of \(|a_{i}|\) decrease with increasing \(i\). The trend of the amplitudes of the high-order signal components is the same as the trend of \(|a_{i}|\). In general, the power density of the additional signal components is mainly centered around lower orders. Hence, only some additional signal components of second and third orders are listed in Table 1. If one of these additional signal components falls into the received bandwidth of the communication system, EMI will be generated.  
\begin{table}
\begin{tabular}{c c} \hline
**Formula** & **Signal Component** \\ \hline \(A\cos(\omega_{1}t)+B\cos(\omega_{2}t)\) & original signal \\ \(a_{1}[A\cos(\omega_{1}t)+B\cos(\omega_{2}t)]\) & fundamental wave \\ \(a_{2}AB[A\cos(\omega_{1}+\omega_{2})t+B\cos(\omega_{1}-\omega_{2})t]\) & second-order intermediation \\ \(a_{2}[A^{2}\cos(2\omega_{1}t)+B^{2}\cos(2\omega_{2}t)]/2\) & second-order harmonic wave \\ \(3a_{3}A^{2}B[\cos(2\omega_{1}+\omega_{2})t+\cos(2\omega_{1}-\omega_{2})t]/4\) & third-order intermediation \\ \(3a_{3}AB^{2}[\cos(\omega_{1}+2\omega_{2})t+\cos(\omega_{1}-2\omega_{2})t]/4\) & third-order intermediation \\ \(a_{3}[A^{3}\cos(3\omega_{1}t)+B^{3}\cos(3\omega_{2}t)]/4\) & third-order harmonic wave \\ \hline \end{tabular}
\end{table}
Table 1: The main additional components of the signal.