Interference Study of 5G System on Civil Aircraft Airborne Beidou RDSS System in Takeoff and Landing Phase

Abstract

Radio Determination Satellite Service (RDSS) is a characteristic service of BeiDou, which can provide users with short message communication services. Since the working frequency of an RDSS system is close to that of a 5G system, the RDSS system is very susceptible to interference from 5G out-of-band radiation. This paper analyzes the compatibility of 5G interference with an RDSS system from the perspective of the signal and the system. Firstly, the compatibility assessment is carried out from the perspective of the signal, the impact of interference on the capture and tracking performance of BeiDou is illustrated, and the safe coexistence distance of the two systems from the perspective of capture probability is obtained from the perspective of the signal. Subsequently, based on the link budget criterion, the interference of 5G base stations and 5G terminals to RDSS receivers under different frequency isolation and the required distance isolation for safe coexistence are analyzed from the system perspective. Finally, from the perspective of civil aviation safety, the aggregate interference is used as an evaluation index to evaluate the interference suffered by the aircraft during takeoff and landing and to obtain the interference suffered by the ground-based 5G base station during the takeoff and landing of the aircraft on different routes and in different 5G propagation environments. The simulation results show that when the airplane is closer to the ground, the ground 5G base stations will cause harmful interference to the RDSS receiver. In this study, the real flight data are combined with the simulation model to obtain the exact influence range of 5G interference on the RDSS system under different viewpoints.

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 3GPP R16 TS38.104 protocol [4], 5G is divided into two main frequency bands (Frequency Range, FR), which are shown in

Table 1. The 5G frequency range.

Frequency Band Number	Frequency Range
FR1	410 MHz–7125 MHz
FR2	24,250 MHz–52,600 MHz

.

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. Frequency band distribution of China’s three major operators.

Operator	Frequency Range
China Mobile	2515–2675 MHz and 4800–4900 MHz
China Telecom	3400–3500 MHz
China Unicom	3500–3600 MHz

. 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, frequency planning, interference detection, and anti-interference issues surrounding GNSS are receiving more and more attention in the international arena. Since the interference brought about by the rapid development of mobile communication systems will have a practical impact on the ability of GNSS to provide high-precision and stable services in the long term, ensuring compatibility between the systems is a prerequisite for the realization of the synergistic development of the integration of air and space as the new generation of mobile communications is constructed and operated.

In recent years, scholars at home and abroad have studied ground communication and satellite navigation system interference events, with the use of different interference analysis indicators to determine the impact on the signal.

The U.S. terrestrial communication system optical cube system has appeared to have an impact on the navigation performance of GPS systems in the neighboring frequency [5]. For this reason, the U.S. government has analyzed the system’s ground modeling, operation scenarios, interference analysis of GPS systems, simulation and testing, and interference elimination measures to ensure the safe operation of the system. Baec M et al. used the spectral separation coefficient as an assessment of inter-system coexistence metrics to illustrate the impact of the optical cube system on the signals in the L1 and E1 frequency bands [6,7]. For the study of the compatibility issue between the two systems, Godet J et al. of the Galileo Architecture Support Team of the European Commission used the method of interference coefficients and equivalent carrier-to-noise ratios as metrics for the analysis of inter- and intra-system interference [8]. Marco Rao et al. from the Joint Research Center of the European Commission pointed out that interference from LTE signals may have a significant impact on pseudorange measurements and position calculations of GNSS. In the case of frequency band proximity, strong interference from LTE signals may lead to an increase in pseudorange error, thus affecting position accuracy [9].

On the domestic front, as the BeiDou system has been put into use, compatibility studies between it and mainstream mobile communication systems have been carried out one after another. People began to conduct compatibility studies between S signal and mobile communication systems, and the satellite receiving terminal was tested in different scenarios. The results showed that the 4G signal located at 2555 MHz–2575 MHz would cause the carrier-to-noise ratio of the BeiDou S signal to degrade, resulting in obvious interference effects [10]. Zhang Chunmei et al. used the carrier-to-noise ratio deterioration value as an evaluation index from the link budget point of view and analyzed the neighboring frequency interference caused by 4G to the BeiDou RDSS system. The results indicated that the interference would cause 5 dB–10 dB carrier-to-noise ratio degradation [11]. In 2019, the Beijing Satellite Navigation Center, from the point of view of the RF, proposed a response strategy for the interference of 4G to the BeiDou RDSS [12]. As 5G enters the stage of high-speed development, people begin to study the compatibility of 5G signals with satellite navigation signals. In 2021, Zhang Tianqiao et al. of Beijing Satellite Navigation Center analyzed the impact of 5G signals on BeiDou RDSS from the perspective of signals by using the equivalent carrier-to-noise ratio as an evaluation index, and gave the safe distance between the 5G base station and the RDSS receiver to be 40 m under the premise that the communication success rate is at 95% [13]. In 2022, Xi’an University of Posts and Telecommunications analyzed the interference results of 5G signals on BeiDou RDSS from the factors of positioning, communication, and bandwidth, and designed experimental scenarios for testing [14].

In summary, this paper focuses on the interference problem of the 5G system to the airborne BeiDou RDSS system, and carries out the interference analysis from the signal level and the system level, respectively, on the basis of which, from the perspective of civil aviation safety, evaluates the impact of the interference of the ground-based 5G system during the takeoff and landing phases of the aircraft. In Section 2, the structure of the BeiDou-3 RDSS outbound signal and the time-domain waveform of the 5G signal are introduced, and the interference problem caused by the serious out-of-band trailing of the 5G signal waveform is clarified. Section 3 mainly analyzes the interference from the signal level, and the performance impact of 5G as an interfering signal on BeiDou capture and tracking is investigated, and the critical distance of interference at the signal level is obtained. Section 4 mainly analyzes from the system level and obtains the theoretical distance isolation with different 5G frequency isolation. Based on this, the terrestrial 5G system interference suffered during aircraft takeoff and landing is evaluated using real data from flights and employing aggregate interference as an evaluation metric. Finally, in Section 5, Based on the simulation results, conclusions are drawn to provide a reference for the future planning and construction of ground-based 5G base stations and to promote the application of the BeiDou RDSS system in civil aircrafts.

2. System Models

2.1. RDSS Signal Model

The outbound signal bandwidth of BeiDou-3 is 2491.75 ± 8.16 MHz, which is twice as much as that of the old system. The outbound signal of BeiDou-2 uses convolutional code as the channel coding method, whereas BeiDou-3 adds Turbo code, and at the same time, it uses the combination of frequency-guided branch and data branch to replace the former dual data branch in terms of signal branching. The frequency-guide branch is used at the receiving end to track and demodulate the spread spectrum signal, ensuring that the receiver is able to correctly decode and process the navigation messages from the satellite. The outgoing BeiDou-3 RDSS signal adopts Unbalanced Quadrature Phase Shift Keying (UQPSK) with a direct-sequence spread spectrum. UQPSK is a deformation of Quadrature Phase Shift Keying (QPSK), and the UQPSK is two BPSK signals with different transmitting power and spreading gain. The block diagram of the outgoing signal modulation is shown in Figure 2.

The BeiDou RDSS signal, acquired by the receivers, can be expressed as follows:

$$S\left(t\right)=A_p{C}_p\left(t\right)d_p\left(t\right)cos(2\pi f_0+{\varphi }_0)+A_d{C}_d\left(t\right)d_d\left(t\right)sin(2\pi f_0+{\varphi }_0)$$

(1)

where $A_p$ and $A_d$ are the signal amplitudes of the p-branch and d-branch; $C_p$ and $C_d$ are the spreading code sequences of the p-branch and d-branch; $d_p$ and $d_d$ denote the data codes of the two branches; $f_0$ is the signal carrier frequency; and ${\varphi }_0$ presents the initial phase. From the above Figure 2, it can be seen that the P-branch does not use channel coding. The d-branch uses convolutional code coding in the common segment and Turbo coding in the dedicated segment. The fixed information rate of the public segment is 4 kbps, and the symbol rate is 8 kbps after channel coding, and the baseband information rate of the dedicated segment has various cases, and the symbol rate will be unified to 32 kbps after Turbo coding. The simulation obtains the spectrogram of the BeiDou S signal, as shown in Figure 3.

2.2. The 5G Signal Model

In 5G systems, CP-OFDM waveforms are widely used in the physical layer transmission of the air–port interface, providing high-speed and reliable data transmission capability for 5G. CP-OFDM waveforms stand for Cyclic Prefix Orthogonal Frequency Division Multiplexing (CP-OFDM) waveforms. CP-OFDM waveforms are a multicarrier modulation method used in 5G communication systems to achieve high-speed data transmission and resistance to multipath interference in wireless transmission. The CP-OFDM waveform is a multicarrier modulation method used in 5G communication systems to achieve high-speed data transmission and resistance to multipath interference in wireless transmissions. CP-OFDM employs the cyclic prefix technique to counteract inter-symbol interference due to multipath propagation by adding a repeating data point at the beginning of each OFDM symbol. This makes it easier to synchronize and demodulate the signal at the receiving end, thus improving the performance of the system. According to the literature [15], the CP-OFDM time domain signal can be expressed by the following equation:

$$s\left(t\right)={\sum }_{n=0}^{M-1}{\sum }_{k=0}^{N-1}{c}_{n,k}p(t-n(T_s+T_c))e^{-j2\pi k\frac{1}{N}(T_s+T_c)}$$

(2)

where $c_{n,k}$ is the nth OFDM symbol modulated on the kth subcarrier, $T_s$ and $T_c$ are the duration and protection interval, respectively, and $p_t$ presents the gate functions.

$$S\left(f\right)=\frac{P_s}{T_{tot}}\sum _{k=0}^{N-1}{\left|P(f-\frac{k}{T_s})\right|}^2$$

(3)

$p_s$ is the power of a single subcarrier of OFDM with subcarrier spacing $\frac{1}{T_s}$. The power spectral density of CP-OFDM is obtained by performing a Fourier transform on the gate function:

$$S^{CP}\left(f\right)=P_s{T}_{tot}\sum _{k=0}^{N-1}{\left\{\frac{sin\left[(f-\frac{k}{T_s})T_{tot}\right]}{\left[(f-\frac{k}{T_s})T_{tot}\right]}\right\}}^2$$

(4)

According to the literature [16], many different 5G candidate waveforms have been proposed, such as Filter Bank Multi-Carrier (FBMC), Universal Filtered Multi-Carrier (UFMC), Generalized Frequency Division Multiplexing (GFDM), etc., and application examples and performance comparisons of these waveforms are carried out. In this paper, the most commonly used OFDM waveforms for 5G are selected for power spectral density simulation, while FBMC waveforms are added for comparison, as shown in Figure 4 below. The blue part of the figure indicates the OFDM waveform and the red is the FBMC waveform PSD.

As can be seen in Figure 4, OFDM has higher out-of-band leakage and is more likely to cause interference to systems adjacent to it, so in this paper, the 5G waveforms are identified as OFDM waveforms for analysis.

3. Signal Level Compatibility Analysis

3.1. Analysis of the Effect of Interference on the Reception Performance of BeiDou

According to the working principle of the BeiDou receiver, the interference signals in BeiDou receiver reception performance can be divided into four aspects: code tracking, carrier tracking, capture, and data demodulation. Signal-to-Noise Ratio (SNR) is an index used to measure the signal quality, generally used to indicate the quality of the received signal containing interference and noise, but in order to facilitate the analysis of the interference, it will be expressed in terms of the carrier-to-noise ratio, so we introduce the equivalent carrier-to-noise ratio for the analysis. The carrier tracking error and code tracking error are used to respond to the carrier tracking performance and code tracking performance. The BeiDou receiver is a device used to receive and process the signals of the BeiDou satellite navigation system. As shown in Figure 5, the BDS receiver mainly includes an antenna, an RF front-end module, an IF signal processing module, and a navigation demodulation module, and the receiver antenna turns the received satellite signals into digital IF signals after processing by the RF front-end, in which the signal processing module mainly consists of three stages: capture, tracking, and data demodulation.

3.2. Analysis of the Effect of Interference on the Tracking Performance of BeiDou

3.2.1. Carrier Tracking Performance

The phase measurement error sources of Phase-Locked Loop (PLL) include phase jitter and dynamic stress error. The dynamic stress error is very small compared to the phase jitter error when the equivalent carrier-to-noise ratio satisfies the signal capture, so it is negligible.

The error sources that cause phase jitter are mainly categorized into thermal noise, jitter in the oscillation frequency caused by mechanical chatter, and Allan squared error. In practice, since the source of PLL phase-locked loop chattering may be instantaneous, only the carrier tracking error caused by thermal noise is generally considered. The carrier tracking error due to thermal noise and interference can be expressed as:

$${\sigma }_{PLL}=\frac{{180}^{\circ }}{\pi }\sqrt{\frac{B_L}{C/N_0}\left(1+\frac{1}{2TC/N_0}\right)}$$

(5)

$B_L$ represents the phase-locked loop noise bandwidth; T indicates the pre-detected integration time; $C/N_0$ is the carrier-to-noise ratio.

Figure 6 shows the carrier tracking error plot versus the equivalent carrier-to-noise ratio for different pre-detection integration times where the receiver carrier loop noise bandwidth BL is constant and equal to 20 Hz. From the above simulation, it can be seen that the carrier tracking error decreases with the increase in the equivalent carrier-to-noise ratio. When the tracking error threshold is 15°, under the simulation conditions with different pre-detection integration times, the longer the pre-detection integration time, the smaller the carrier tracking error, and the smaller the interference on the carrier tracking performance.

3.2.2. Code Tracking Performance

In the tracking process of the BeiDou receiver, the tracking loop continues to complete the tracking of the code phase after the carrier tracking is finished. The code tracking loop commonly used in the receiver is the Delayed Phase-Locked Loop (DLL), and the measurement error sources of the code loop mainly include the code phase jitter due to thermal noise and the dynamic stress error in two parts. Representing the mean square error of the code phase measurement error due to thermal noise, the formula for the measurement error for the code loop of the normalized early minus late power law discriminator is as follows:

$${\sigma }_{DLL}=\left\{\begin{array}{c}\sqrt{\frac{B_L}{2·C/N_0}d(1+\frac{2}{(2-d)T·C/N_0})},d\ge \frac{\pi }{W{T}_c}\hfill \\ \sqrt{\frac{B_L}{2·C/N_0}(\frac{1}{W{T}_c}+\frac{W{T}_c}{\pi -1}{(d-\frac{1}{W{T}_c})}^2)}(1+\frac{2}{(2-d)T·C/N_0}),\frac{1}{B_{fe}{T}_c}<d<\frac{\pi }{W{T}_c}\hfill \\ \sqrt{\frac{B_L}{2·C/N_0}\frac{1}{W{T}_c}(1+\frac{1}{T·C/N_0})},d\le \frac{1}{W{T}_c}\hfill \end{array}\right.$$

(6)

where $B_L$ represents the code loop noise bandwidth, $B_{fe}$ represents the RF front-end bandwidth, $T_c$ represents the pseudo-distance code width, D and T represent the correlator spacing and pre-detection integration time, respectively, and $C/N_0$ represents the equivalent carrier-to-noise ratio.

When the receiver code loop noise bandwidth is 2 Hz, and the correlator spacing D is 1 code chip, the relationship between the code tracking error and the equivalent carrier-to-noise ratio can be calculated as shown in Figure 7, and it can be seen that the longer the pre-detection integration time, the smaller the code tracking error. Increasing the pre-detection integration time can reduce the effect of interference on the code tracking performance.

3.3. Analysis of the Effect of Interference on the Capture Performance of BeiDou

The presence of interfering signals may make it difficult for the receiving equipment to accurately capture the target signal. In a low signal-to-noise ratio environment, the interfering signal may mask the target signal, making it difficult for the receiving equipment to correctly identify and capture the target signal, thus affecting communication quality and reliability. This section focuses on the analysis of the impact of interference on the performance of BeiDou capture, according to the literature [17] to establish a binary signal detection model. $H_0$ expresses the condition that only noise exists; $H_1$ expresses the condition that interference and noise exist at the same time, then the false alarm probability of the signal can be expressed as:

$$P_f={\int }_{V_t}^{\infty }p(z\mid H_0)dz$$

(7)

The signal detection probability can be expressed as:

$$P_d={\int }_{V_t}^{\infty }p(z\mid H_1)dz$$

(8)

where $V_t$ is a detection threshold; $p(z\mid H_1)$ is the probability density of detection of the input signal power z in the presence of both interference and noise; and $p(z\mid H_0)$ is the probability density of detection of the input signal power z in the presence of only noise.

When the condition $H_1$ holds, z follows a non-central Cartesian distribution with 2 M degrees of freedom, and when the condition holds $H_0$, z follows a central Cartesian distribution with 2 M degrees of freedom. The conditional probability density of z is given by the following expression [17]:

$$P(z\mid H_1)=\frac{1}{2}{\left(\frac{z}{\lambda }\right)}^{\frac{1}{2}(M-1)}exp(-\frac{1}{2}(z+\lambda ))I_{M-1}\left(\sqrt{z\lambda }\right)$$

(9)

$$P(z\mid H_0)=\frac{1}{2^M\Gamma \left(M\right)}{z}^{M-1}exp(-\frac{z}{2})$$

(10)

$$\lambda =2M(C/N_0)T_L$$

(11)

where $\lambda$ denotes the non-central parameter of the non-central Cartesian distribution, $T_L$ is the pre-detection equivalent carrier-to-noise ratio, $C/N_0$ is the pre-detection integration time, $\Gamma \left(M\right)$ is the gamma function, defined as $\Gamma \left(M\right)=(M-1)!$, and $I_{M-1}(\sqrt{z\lambda })$ is the first class $M-1$-order Bessel function.

Firstly, the false-alarm probability $P_f$ of signal detection is determined; then, subsequently, the judgment threshold $V_t$ can be obtained according to Equation (7), and finally the detection probability is obtained. The false alarm probability of signal detection should be minimized as much as possible during the capture process. In this paper, the capture performance is simulated in the following simulation by taking the pre-detection integration time of 20 ms and 10 ms for the case of false alarm probability as $1\times {10}^{-6}$.

The signal detection probability is usually used to measure the accuracy of the receiver in detecting signals and is one of the important indicators for evaluating the performance of a communication system. From Figure 8, it can be seen that there is a positive correlation between the signal detection probability and the equivalent carrier-to-noise ratio, and the signal detection probability increases as the equivalent carrier-to-noise ratio increases. In order to obtain the effect of 5G transmit power on the BeiDou signal, 5G is introduced as an interfering signal into the interference-to-signal ratio ISR, and the relationship between the equivalent carrier-to-noise ratio and the interference-to-signal ratio is:

$$ISR=G_{SVi}-G_J+101g\left[Q{R}_c({10}^{-\frac{{\left(C{N}_0\right)}_{eq,dN}}{10}}-{10}^{-\frac{{\left(C{N}_0\right)}_{dN}}{}})\right]$$

(12)

where $G_{SVi}$ is the antenna gain pointing to the first satellite, $G_J$ is the antenna gain, and $C{N}_0$ is the receiver carrier-to-noise ratio in the case of no interference. According to the relationship between detection probability and equivalent carrier-to-noise ratio from Equation (12), the effect of 5G signal transmit power on detection probability is obtained by simulation, as shown in Figure 9 below.

As can be seen from the simulation results in Figure 9, when the 5G signal power is −50.54 dBw, the signal detection probability is less than 1, which indicates that, in this case, the signal may not be fully captured or detection failure may occur, and when the 5G signal power is lower than −50.54 dBw, it will not have an impact on the BeiDou signal capture performance. According to the research scenario of this paper, the safety distance analysis is performed from the signaling perspective, and the transmit power of the 5G base station is determined. According to the regulations in 3GPP TS38.104, 5G base stations can be categorized into Type 1-C, Type 1-H, etc., based on the operating frequency range and the type of reference port connection. Since the research object of this paper is the FR1 band, a 5G base station of type 1-C is selected as the research object. Based on the 3GPP document, the transmit power of this type of 5G base station is 8 dBw, so from the simulation results, the propagation loss of the 5G signals is about 58 dB when there is an incomplete capture situation. If the 5G signal propagation environment is regarded as free space propagation, the relationship between the propagation distance and the loss value is shown in

Table 3. Propagation distance versus loss value.

Propagation Distance (m)	Loss Value (dB)
4.5	53.5319
5	54.4471
5.5	55.2749
6	56.0307
6.5	56.7259
7	57.3696
7.5	57.9689
8	58.5295
8.5	59.0560
9	59.5525
9.5	60.0221
10	60.4677

below. When the free space propagation loss of the 5G signal is 58 dB, the propagation distance corresponds to about 8 m. Therefore, when the two systems are more than 8 m apart, the BeiDou S signal can utilize its pseudo-code gain to achieve normal capture, and the two systems can be compatible.

4. System Perspective Compatibility Analysis

4.1. 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 3GPP 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]:

$$ACI{R}^{-1}=ACL{R}^{-1}+AC{S}^{-1}$$

(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.

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 3GPP TR36.104, as well as Equation (14).

$$ACLR=P-10{log}_{10}({\int }_{f_c+\Delta f-B/2}^{f_c+\Delta f+B/2}{10}^{P\left(f\right)/10}df)$$

(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 $P\left(f\right)$ 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 3GPP 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. 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 3GPP 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]:

$$\begin{array}{cc}\hfill P{L}_{NLOS}& =(3.2{(log\left(11.75h_{UT}\right))}^2-4.97)+(43.42-3.1log\left(h_{BS}\right))(log\left(d_{3D}\right)-3)\hfill \\ & +20log\left(f_c\right)-(24.37-3.7{\left(\frac{h}{h_{BS}}\right)}^2)log\left(h_{BS}\right)+161.04+7.5log\left(h\right)-7.1log\left(W\right)\hfill \end{array}$$

(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. Note that all of the above height quantities are measured in m. The free-space propagation loss model is:

$$P{L}_{free}=20{log}_{10}f+20{log}_{10}d+32.44$$

(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:

$$I_n=P_{tx}+G_{rx}-PL+G_{tx}-ACLR-L_{rx}$$

(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:

$$I_{agg}=10log(\sum _{n=1}^N{10}^{I_n/10})\le I_{Max}$$

(18)

$I_{Max}$ is the maximum acceptable interference power level of the receiver, considered equal to −127 dBm according to the BD420007-2015 [21].

4.2. The 5G Interference with BeiDou RDSS Receiver Analysis Results

This section focuses on the analysis of the distance isolation required for different frequency isolation between the 5G system and the RDSS system, which is mainly summarized by dividing the 5G system into the 5G base station and the 5G terminal interfering with the RDSS receiver. According to the relationship between ACLR and system frequency isolation in the first two columns of

Table 4. Deterministic analysis results of 5G base station interference to RDSS receivers.

Frequency Isolation (MHz)	ACLR (dB)	5G Base Station Transmit Power (dBm)	5G Base Station Antenna Gain (dBi)	BeiDou Terminal Antenna Gain (dBi)	Interference Received by BeiDou Terminals (dBm)	Allowable Maximum Interference Power (dBm)	Required Path Loss (dB)	Required Distance Isolation (km)
0	30.07	38	18	0	22.93	−127	152.93	4.2693
1	31.09	38	18	0	21.91	−127	151.91	3.8019
2	31.98	38	18	0	21.02	−127	151.02	3.3884
3	33.07	38	18	0	19.93	−127	149.93	3.0199
4	34.09	38	18	0	18.91	−127	148.91	2.6915
5	35.00	38	18	0	18.00	−127	148.00	2.3988
6	35.97	38	18	0	17.03	−127	147.03	2.1379
7	37.21	38	18	0	15.79	−127	145.79	1.8620
8	38.96	38	18	0	14.04	−127	144.04	1.5135
9	41.92	38	18	0	11.08	−127	141.08	1.0715
10	58.88	38	18	0	−5.88	−127	124.12	0.1548
11	58.88	38	18	0	−5.88	−127	124.12	0.1548
12	58.88	38	18	0	−5.88	−127	124.12	0.1548

, the required distance isolation for different frequency isolation can be calculated as shown in the last column of Table 4.

From the data in Table 4, it can be seen that different degrees of 5G out-of-band leakage will result in different frequency isolation than that of the RDSS system, and the smaller the frequency isolation, the more severe the out-of-band leakage of 5G, and the greater the distance isolation required. When the frequency isolation is greater than 10 MHz, the value of ACLR will no longer change. To summarize, the distance isolation required for the 5G base station to interfere with the RDSS receiver is between 0.154 km and 4.26 km.

Similar to the analysis of the 5G base station interfering with the BeiDou RDSS receiver, according to the out-of-band emission template of the 5G terminal specified in the standard 3GPP TR36.101 [22] and the Equation (14), the relationship between the ACLR and the frequency isolation when neighboring frequency coexistence can be obtained, the limit of out-of-band spurious radiation of the terminal will no longer change when the frequency isolation of the two systems is more than 25 MHz. Therefore, the ACLR value will also no longer change. The analysis results of the 5G terminal interfering with the RDSS receiver are shown in

Table 5. Deterministic analysis results of 5G terminal interference with RDSS receivers.

Frequency Isolation (MHz)	ACLR (dB)	5G Base Station Transmit Power (dBm)	5G Base Station Antenna Gain (dBi)	BeiDou Terminal Antenna Gain (dBi)	Interference Received by BeiDou Terminals (dBm)	Allowable Maximum Interference Power (dBm)	Required Path Loss (dB)	Required Distance Isolation (km)
0	21.84	21	0	0	−0.84	−127	126.16	0.1949
1	23.16	21	0	0	−2.16	−127	124.84	0.1659
2	23.53	21	0	0	−2.53	−127	124.47	0.1584
3	23.79	21	0	0	−2.79	−127	124.21	0.1548
4	24.38	21	0	0	−3.88	−127	123.62	0.1445
5	24.88	21	0	0	−3.88	−127	123.62	0.1445
6	24.88	21	0	0	−3.88	−127	123.62	0.1445
7	24.88	21	0	0	−3.88	−127	123.62	0.1445
8	24.88	21	0	0	−3.88	−127	123.62	0.1445
9	24.88	21	0	0	−3.88	−127	123.62	0.1445
10	24.88	21	0	0	−3.88	−127	123.62	0.1445
11	24.88	21	0	0	−3.88	−127	123.62	0.1445
12	24.96	21	0	0	−3.96	−127	123.04	0.1349
13	25.50	21	0	0	−4.50	−127	122.50	0.1288
14	26.12	21	0	0	−5.12	−127	121.88	0.1174
15	26.84	21	0	0	−5.84	−127	121.16	0.1096
16	27.70	21	0	0	−6.70	−127	120.30	0.1000
17	28.79	21	0	0	−7.79	−127	119.21	0.0870
18	30.31	21	0	0	−9.31	−127	117.69	0.0724
19	32.67	21	0	0	−11.67	−127	115.33	0.0562
20	38.22	21	0	0	−17.22	−127	109.78	0.0295
21	38.74	21	0	0	−17.74	−127	109.26	0.0275
22	39.34	21	0	0	−18.34	−127	108.66	0.0257
23	40.04	21	0	0	−19.04	−127	107.96	0.0239
24	40.86	21	0	0	−19.86	−127	107.14	0.0218
25	41.88	21	0	0	−20.88	−127	106.12	0.0194

.

According to the analysis results of 5G terminal interference to the RDSS receiver in Table 5, when the frequency isolation of the two systems is greater than 25 MHz, the distance isolation is 19.4 m and no longer changes. As for the interference from 5G terminals, only 194 m of distance isolation is needed to realize coexistence in the worst case of frequency isolation of 0 MHz. Considering that the research scenario of this paper is an airport area, the interference from 5G user terminals has too small of an interference impact on the research environment of the airport, so only the impact of 5G base stations is considered in the subsequent analysis. In summary, the interference of 5G base stations and 5G terminals to the BeiDou RDSS system is analyzed from the viewpoint of the link budget, and the required distance isolation under different frequency isolation is given.

4.3. Interference Assessment during the Take-Off and Landing Phases of an Aircraft

In Section 4.2, it can be determined from the analysis that in the interference study of 5G systems on RDSS systems, the interference impact of the 5G terminal on the RDSS system is not significant, so this section focuses on the study of the interference of 5G base station, and the comprehensive consideration of the aggregate interference of the 5G base station on the ground at the airport is analyzed to determine the impact of interference on the airborne RDSS receivers during the takeoff and landing of the aircraft. In this paper, different routes are selected for comparison, and the data are obtained from the real trajectory files downloaded from the official FlightAware (https://www.flightaware.com/live/ 23 July 2021). In order to make the data accurate and reliable, the interpolation process is carried out to obtain the position information of the airplane per second. The Flight path of Berlin to Washington UAL235 (EDDB-KIAD), New York to Los Angeles DAL482 (KJFK-KLAX), and Tianjin to Chengdu CSC8861 (ZBTJ-ZUTF) are selected to be analyzed, and the changes of the aggregate interference in the takeoff phase under the simulation of different propagation environments are shown in Figure 12.

The simulation results show that the aggregate interference of the airborne RDSS receiver by the 5G signal from the ground is related to the 5G signal propagation environment and the flight altitude and horizontal projection distance. When the aircraft takes off, as the flight altitude increases, the projection distance of the aircraft to the ground increases from the horizontal distance of the center point of the aggregate interference, and the aggregate interference suffered by the RDSS receiver decreases continuously. The relationship between the airplane altitude and the 5G base station is shown in Figure 13.

In particular, the total aggregate interference value in the RMa NLOS scenario is slightly smaller than in the free space propagation due to the consideration of building shading equal to the factor. Since some routes in China lack ADS-B data information during the takeoff phase, the following analysis focuses on the aggregate interference data of EDDB-KIAD and KJFK-KLAX in

Table 6. Summary analysis of interference data from EDDB-KIAD and KJFK-KLAX during the takeoff phase.

Dissemination Environment	Flight Altitude (m)	Horizontal Distance (m)	Aggregate Interference (dBm)
	75.31	50.8	−121.85
EDDB-KIAD Free Space	89.62	158	−127.87
	103.93	251	−131.39
	27.5	84.6	−119.67
KJFK-KLAX Free Space	47	171.6	−125.69
	66.50	258.5	−129.21
	75.31	50.8	−126.60
EDDB-KIAD RMA	89.62	158	−141.03
	103.93	251	−149.47
	27.5	84.6	−126.17
KJFK-KLAX RMA	47	171.6	−140.60
	66.50	258.5	−149.04

.

The following simulation of the aggregate interference suffered by the aircraft in different environments during landing is shown in Figure 14.

The simulation results show that during the landing period, as the altitude of the aircraft decreases, the distance of the aircraft from the center point of the ground 5G base station’s aggregate interference is gradually close to the center point of the ground 5G base station, and therefore, the aggregate interference suffered by the aircraft gradually increases. The relationship between the aggregate interference and the flight altitude and horizontal isolation distance for different flight paths in the two propagation environments is shown in

Table 7. Summary analysis of interference data from EDDB-KIAD and KJFK-KLAX during the landing phase.

Dissemination Environment	Flight Altitude (m)	Horizontal Distance (m)	Aggregate Interference (dBm)
	113.4	1394	−143.60
EDDB-KIAD Free Space	73.7	658	−137.11
	41.9	60.5	−118.03
	115.3	747	−138.26
KJFK-KLAX Free Space	95.6	365	−132.24
	76	25	−118.26
	113.4	1394	−183.53
EDDB-KIAD RMA	73.7	658	−167.91
	41.9	60.5	−122.23
	115.3	747	−170.72
KJFK-KLAX RMA	95.6	365	−156.29
	76	25	−122.79

below. As can be seen from the data in Table 7, there is a slight difference in the total collective interference from ground-based 5G base stations for different flight paths. Similar to the takeoff phase of the airplane, the value of the total collective interference in the RMa propagation environment is slightly smaller than that of the free-space propagation case. When landing close to the ground, ground-based 5G base stations can cause harmful interference to RDSS receivers. The aggregate interference data of the two routes EDDB-KIAD and KJFK-KLAX in different 5G propagation environments during the landing phase are analyzed as shown in the following Table 7.

In summary, in practice, the focus should be on the process of the airplane taxiing, accelerating, and lifting from the ground to leaving the ground. The landing phase should focus on the process of the airplane descending from the air, approaching the landing area, decelerating, grounding, and taxiing to a stop. In these two stages, certain physical isolation measures should be taken in time to avoid affecting the normal operation of the RDSS receiver.

5. Conclusions

In this paper, the interference between 5G and BeiDou RDSS is analyzed from two perspectives: signal and system. The key conclusions are as follows.

In this paper, a compatibility assessment is first carried out from the perspective of the signal. The effects of interference on BeiDou capture and tracking performance are illustrated, and the effects of equivalent carrier-to-noise ratio on carrier tracking performance and code tracking performance under the influence of interference are analyzed. Finally, the safe coexistence distance of the two systems from the signal perspective is obtained from the perspective of capture probability.

Based on the link budget criterion, the ACLR model is introduced, and the interference of 5G base stations and 5G terminals to RDSS receivers under different frequency isolation, as well as the required distance isolation for safe coexistence, are analyzed. Combined with the subsequent research scenarios, the interference of 5G terminals can be ignored, and the worst-case scenario shows that the 5G base stations have an impact on the RDSS receivers in the range of 4.2 km.

Finally, the real flight data are combined with the simulation model to obtain the aggregate interference from ground 5G base stations during the takeoff and landing of an airplane on different routes and in different 5G propagation environments. The simulation results show that when the airplane is closer to the ground, the ground 5G base stations will cause harmful interference to the RDSS receiver. Attention should be focused on the process of the aircraft taxiing, accelerating, lifting up, and leaving the ground, as well as the process of the aircraft descending from the air, approaching the landing area, decelerating, grounding, and taxiing to a stop during the landing phase, and certain physical isolation measures should be taken to avoid affecting the normal operation of the RDSS receiver in a timely manner.