## 4 EMC Evaluation Method for Communication Receiver  
According to [43], the BER of BDS is closely related to its working performance. Whether it works normally can be judged by the number of BDS stable tracking satellites (in this paper, it can be observed by the upper computer). According to the system data released by the China Satellite Navigation System Management Office, when the BER is lower than \(10^{-5}\), BDS is not disturbed and can work normally. At this time, the number of satellites that BDS can stably track is more than 10. When the number of satellites that BDS can stably track is 8-10, the interference to BDS is not apparent at this time and BDS can still work normally. When the BER is higher than \(10^{-5}\) and lower than \(10^{-3}\), BDS is apparently disturbed. At this time, the number of satellites that BDS can stably track is 5-8, and BDS cannot work normally. When the BER is higher than \(10^{-3}\), BDS stops working. At this time, the number of satellites that BDS can stably track is less than 5. Therefore, we can evaluate the performance of the BDS receiver through the value of the BER. In the experimental verification, we choose BDS receiver as the test object, on the one hand, because the minimum power level of the B3I signal transmitted by the satellite to the antenna of the receiver is \(-163\) dBw, which is \(-133\) dBm according to (25). Therefore, the sensitivity of receiver is very low, which leads to the disturbance phenomenon, and its EMC problem needs to be solved urgently. On the other hand, through a large number of experiments, it is found that when the BDS receiver is interfered, with the increase of the degree of interference, the number of satellite signals received by BDS receiver will decrease. Therefore, the interference degree of BDS receiver can be quantified through this phenomenon. \[P_{\text{dBm}}=P_{\text{dBw}}+30 \tag{25}\]  
Equation (25) gives the relationship between power values in different units, where \(P_{\text{dBm}}\) is the power value in dBm and \(P_{\text{dBw}}\) is the power value in dBw. According to the system parameters disclosed in the B3 section interface document [44] published by the Beidou Navigation Satellite Office of China, the code rate of B3I signal ranging code is 10.23 Mcps and the ranging code length \(L=10.230\). Therefore, when calculating the BER of the receiver, the bit rate of the receiver needs to take the chip rate of 10.23 Mcps after spread spectrum, that is, \(T_{chip}=1/10.23\)Mcps, each information symbol period in the receiver is \(T_{b}\), the ranging code period is \(T_{c}\), and the spread spectrum multiple is \(N=T_{b}/T_{c}\), and the physical meaning is the anti-interference gain obtained after expanding the frequency band. It can also be explained that the received signal of the navigation satellite is very weak, and the spread spectrum gain is very large. Compared with other communication systems, it uses very low bit rate for high anti-interference performance. The BER of spread spectrum communication can be expressed as:  
\[P_{e}=Q(\sqrt{4\frac{W_{L}/R}{P_{j}/P_{av}}})=Q(\sqrt{4\frac{W_{L}}{R\times SIR }}) \tag{26}\]  
\[Q(x)=\frac{1}{\sqrt{2\pi}}\int\limits_{x}^{\infty}e^{-i^{2}/2}dt,x\geq 0 \tag{27}\]where \(R\) is the information transmission rate, \(W_{L}=W/2=1/T_{c}=R_{p}\), \(R_{p}\) is the pseudo code rate, \(P_{j}\) is the average power of interference signal, \(P_{av}\) is the average power of useful signal.