## 4 System Perspective Compatibility Analysis  
### Compatibility Analysis Methods  
This section focuses on the system angle compatibility analysis of the BeiDou RDSS system and 5G system. Firstly, based on the link budget criterion, the deterministic analysis method is adopted to determine the maximum permissible interference intensity, and the distance isolation in the case of interference with the RDSS system by the 5G base station and 5G terminal is obtained. According to the 3 GPP standard to determine the different environments of 5G signal propagation, the aggregate interference is used as the evaluation index to analyze the interference suffered by the airborne RDSS system of the aircraft in the takeoff and landing phase. In this paper, the Adjacent Channel Interference Ratio (ACIR) ) is chosen to measure the degree of influence of 5G signals on BeiDou RDSS signals. ACIR is commonly used to represent the interference of the interfering signals emitted by the scrambling system on the receivers of the scrambled system, and it is an important parameter for evaluating the inter-system interference. The ACIR is usually determined by the Adjacent Channel Loss Ratio (ACLR) and Adjacent Channel Selectivity (ACS) are jointly determined, and the relationship between ACLR, ACS, and ACIR can be obtained according to the literature [18]:  
\[ACIR^{-1}=ACLR^{-1}+ACS^{-1} \tag{13}\]  
In Equation (13), ACS represents the suppression ability of the receiver of the disturbed system to the interference signal from the adjacent frequency system. ACLR denotes the ratio of the transmit power of the scrambling system to the adjacent frequency leakage power from the scrambling system received in the band of the adjacent frequency system. Since the BDS system terminal has not formed a unified standard, this paper uses ACLR to calculate the interference, and the results are not out of generality. Figure 10 gives a schematic of the power leakage when the 5G system coexists with the adjacent frequency of the BeiDou RDSS receiver. Figure 10: Adjacent channel power leakage schematic. \begin{table}
\begin{tabular}{c c} \hline
**Propagation Distance (m)** & **Loss Value (dB)** \\ \hline
8 & 58.5295 \\ \hline
8.5 & 59.0560 \\ \hline
9 & 59.5525 \\ \hline
9.5 & 60.0221 \\ \hline
10 & 60.4677 \\ \hline \end{tabular}
\end{table} 
Table 3: _Cont._The black box in Figure 10 represents the RDSS operating center band, and the red, blue, and black curves represent the bands of the 5G system under different frequency isolation, where the black curve represents the worst-case out-of-band leakage, at which time the frequency isolation of the two systems is 0 MHz. According to the above figure of leakage when the two systems coexist in adjacent frequencies, the relationship between the adjacent channel leakage power ratio and the isolation bandwidth can be calculated by using the out-of-band fouling template of the base station specified in the standard 3 GPP TR36.104, as well as Equation (14). \[ACLR=P-10\log_{10}\left(\int_{f_{c}+\Delta f-B/2}^{f_{c}+\Delta f +B/2}10^{P(f)/10}df\right) \tag{14}\]  
where \(P\) represents the 5G base station transmit power, \(f_{c}\) is the center carrier frequency of BeiDou RDSS, \(\Delta f\) is the frequency spacing between the 5G system and the RDSS system, \(B\) and \(P(f)\) represent the channel bandwidth of the BeiDou RDSS system and power spectral density, respectively. In this paper, when determining the 5G signal transmission environment, the 5G signal propagation is divided into Uma and RMa scenarios according to the path loss model specified in the 3 GPP TR 36.873 and ITU-R M.2412 [19] standards. The transmission schematic is shown in Figure 11. Combined with the airport construction environment, the airport construction area needs to be selected as a flat, obstacle-free area with stable wind direction to ensure safety during takeoff, landing, and flight, so the 5G signal propagation model for the airport field is selected as RMa for the comprehensive airport construction considerations.