## 1 Introduction  
BeiDou Radio-Determination Satellite Service (RDSS) is an advantageous service of the BeiDou system, which can provide users with fast positioning, position reports, and short message communication services [1, 2]. When the BeiDou RDSS system is used for flight tracking, it can monitor and track the position and status of the aircraft in the air in real time, and use its short message function to send the aircraft's current position, flight altitude, speed, and other information to the ground monitoring center, so that when the flight deviates from the route and other abnormalities, ATC center staff can obtain the relevant information in a timely manner and take the necessary measures to improve the flight safety and efficiency of the flights [3]. Fifth-Generation Mobile Networks (5G) have higher data transmission speeds, lower latency, greater network capacity, and better connection stability. According to 3 GPP R16 TS38.104 protocol [4], 5G is divided into two main frequency bands (Frequency Range, FR), which are shown in Table 1.  
According to the ITU's RDSS business regulations, the BeiDou RDSS uplink frequency band is 1610 MHz-1626.5 MHz, and the downlink frequency band is 2483.5 MHz-2500 MHz, which can be seen that the uplink/downlink of the BeiDou RDSS system is located in the range of the FR1 frequency band of the 5G. This paper focuses on the research of the FR1 frequency band of the 5G, and the FR2 frequency band is out of the scope of this paper. The FR2 frequency band is not within the scope of this paper. The three major operators of the main frequency band distribution of China's deployment of the 5G frequency band are shown in Table 2. It can be seen that China Mobile's 2515-2675 MHz band is only 15 MHz away from the downlink band of the RDSS system, and because of the close frequency interval, it is necessary to consider whether the two systems produce interference problems when running. When the 5G signal is separated from the adjacent frequency isolation of the communication system, due to the characteristics of the 5G waveform, there is often a long out-of-band trailing so that it is easy to fall into the adjacent frequency of the communication system to cause interference. As shown in Figure 1, the portion entering the downlink of the BeiDou RDSS consists of out-of-band radiation or stray radiation from the 5G signal. Both types of radiation are continuous broadband interferences, with out-of-band radiation typically resulting from inadequate modulation methods or unreasonable filter design. These forms of radiation will both have a certain impact on the stability of communication system operation. With the rapid development of satellite navigation systems and mobile communication systems, the allocation and utilization of spectrum resources will become tighter due to the limited spectrum resources, and there may be overlapping or spectrum interference and competition in the frequency use of satellite navigation systems, mobile communication systems, and other communication systems. Therefore, the spectrum management, \begin{table}
\begin{tabular}{c c} \hline
**Frequency Band Number** & **Frequency Range** \\ \hline FR1 & 410 MHz–7125 MHz \\ \hline FR2 & 24,250 MHz–52,600 MHz \\ \hline \end{tabular}
\end{table} 
Table 1: The 5G frequency range. \begin{table}
\begin{tabular}{c c} \hline
**Operator** & **Frequency Range** \\ \hline China Mobile & 2515-2675 MHz and 4800–4900 MHz \\ \hline China Telecom & 3400–3500 MHz \\ \hline China Unicom & 3500–3600 MHz \\ \hline \end{tabular}
\end{table} 
Table 2: Frequency band distribution of China’s three major operators. Figure 1: The 5G signal interference schematic. frequency planning, interference detection, and anti-interference issues surrounding GNSS are receiving more and more attention in the international arena.