## 4 System Perspective Compatibility Analysis
### Compatibility Analysis Methods
In addition, due to the airport's terminal buildings, terminals, and other architectural facilities, the signal propagation process may undergo reflection, bypassing, and scattering to reach the receiving end, so the signal transmission mode is determined to be a non-line-of-sight (NLOS) transmission. Therefore, this paper mainly selects two 5G signal propagation environments for subsequent comparative simulation, one is the RMa specified in the 3 GPP standard, and the other is the free-space propagation loss model assuming that the interfering signals in the propagation environment are not blocked by the buildings, and the propagation paths are also line-of-sight (LOS) transmission. The NLOS path loss model for the RMa scenario is calculated as follows [20]:  
\[PL_{NLOS} =(3.2(\log(11.75h_{UT}))^{2}-4.97)+(43.42-3.1\log(h_{BS}))(\log(d_{ 3D})-3)\] \[+20\log(f_{c})-(24.37-3.7(\frac{h}{h_{BS}})^{2})\log(h_{BS})+161.0 4+7.5\log(h)-7.1\log(W) \tag{15}\]  
where \(f_{c}\) represents the signal frequency in GHz; \(d_{2D}\) shows the horizontal distance from the antenna at the transmitting end of the base station to the antenna at the receiving end of the user; \(d_{3D}\) indicates the spatial distance between the two systems; \(h_{UT}\) and \(h_{BS}\) are, respectively, the actual user antenna height and actual base station antenna height; \(h\) represents the average height of the buildings; and \(W\) indicates the width of the street. Figure 11: The 5G signal propagation schematic. Note that all of the above height quantities are measured in m. The free-space propagation loss model is:  
\[PL_{free}=20\log_{10}f+20\log_{10}d+32.44 \tag{16}\]  
where \(f\) represents the frequency in MHz and \(d\) indicates the distance from the aircraft to the receiver in m. After determining the 5G signal propagation environment, this paper focuses on the ground base station interference to the onboard RDSS receiver during the takeoff and landing of an airplane. Since the number of 5G base stations constructed near the airport needs to meet the demand of passengers as well as ground services, it is necessary to simulate the aggregate interference from ground 5G base stations. First, the interference from a single 5G base station needs to be determined. The interference generated by a single 5G macro base station at the airborne RDSS system can be calculated as follows:  
\[I_{n}=P_{tx}+G_{rx}-PL+G_{tx}-ACLR-L_{rx} \tag{17}\]  
where \(P_{tx}\) represents the transmit power of the scrambled system (measured in dBm); \(G_{tx}\) and \(G_{rx}\) are, respectively, the transmit antenna gain of the scrambled system and the receive antenna gain of the disturbed system (measured in dBi); \(L_{rx}\) shows the feeder loss of the RDSS receiver in dB; \(PL\) presents the propagation path loss of the Interference signal; and \(ACLR\) is the adjacent channel leakage ratio. The aggregate interference caused by 5G base stations to airborne RDSS systems can be calculated as follows:  
\[I_{agg}=10\log(\sum_{n=1}^{N}10^{I_{n}/10})\leq I_{Max} \tag{18}\]  
\(I_{Max}\) is the maximum acceptable interference power level of the receiver, considered equal to \(-127\) dBm according to the BD420007-2015 [21].