# Analysis of Electromagnetic Interference Effects of 5G Signals on Radio Altimeters
## 2 Analysis of Interference Mechanisms
### Analysis of RA Receiver Interference Mechanisms  
The RA determines altitude by calculating the frequency difference between the transmitted signal and the echo signal. Its working principle is illustrated in Figure 1. The RA transmits a signal that is reflected by the ground and received back, creating an echo signal. The echo signal of the RA is an FMCW signal, whose frequency varies continuously over time and can be expressed as follows:  
\[u_{s}(t)=U_{s}cos[2\pi(f_{s}t+F_{\text{B}}t^{2}/2T_{M})] \tag{3}\]  
Here, \(U_{s}\) is the amplitude of the echo signal; \(f_{s}\) is the carrier frequency of the echo signal; \(F_{\text{B}}\) is the modulation bandwidth; and \(T_{M}\) is the modulation period. The time delay between the transmitted and received signals, denoted as \(\Delta t\), corresponds to the round-trip time of the signal. This echo signal is captured by the receiving antenna and directed into the RA. After passing through a bandpass filter and a low-noise amplifier (LNA) in the RA front-end, the signal is mixed with the local oscillator signal in the mixer, producing an IF signal [18]. After undergoing further filtering and amplification, the IF signal is processed by an analog-to-digital converter and a counter to extract the altitude information. The frequency and time relationships of the transmitted and echo signals in the RA are depicted in Figure 2. The IF signal frequency obtained from the mixer processing is the difference between the frequencies of the transmitted and echo signals. The measured altitude can be calculated in relation to the IF signal frequency:  
\[H=\frac{f_{IF}T_{M}c}{4F_{B}} \tag{4}\]  
Here, \(H\) is the measured height; \(f_{IF}\) is the IF frequency obtained from the mixer processing; and \(c\) is the speed of light. Due to the limited frequency selectivity of antennas, adjacent frequency bands of 5G interference signals can couple into the RA receiver through the antenna [19]. Although the filter following the antenna is designed to provide frequency selectivity, the original design of some RA receivers did not anticipate the adjacent-band interference from emerging technologies such as 5G. As a result, the filtering performance of the RF front-end filters in certain RA receivers may not be sufficient to effectively mitigate out-of-band interference signals. When a strong 5G interference signal impacts the receiver, it can cause components  
Figure 1: Working principle of the RA. such as the LNA and mixer in the RF front-end to enter a state of saturation [20]. This saturation may lead to a reduction in the output power of the IF signal and an increase in the noise floor. In severe cases, the back-end circuit of the receiver might fail to detect the useful IF signal, resulting in an inability to calculate height information and causing a blocking phenomenon. The mechanisms underlying this nonlinear distortion and its blocking effect can be analyzed using the power-series method [21]. When the echo signal is superimposed with the 5G interference signal and enters the RF front-end via the antenna, the input signal can be expressed as follows:  
\[ \begin{split} ui(t)=& B_{s}u_{s}(t)+B_{5G}u_{5G}(t)\\ =& B_{s}U_{s}cos2\pi(f_{s}t+\mathrm{F_{B}}t^{2}/2 \mathrm{T_{M}})+\sum_{n=1}^{N}B_{n}U_{n}cos(2\pi f_{n}t+\theta_{n})\end{split}  \tag{5}\]  
Here, \(B_{s}\) and \(B_{n}\) are the amplitude-frequency coefficients of the bandpass filter with respect to the RA echo signal and the 5G subcarrier signal. \(U_{n}\) is the voltage amplitude of the 5G subcarrier signal. The input signal \(u_{i}(t)\) then enters the LNA. Due to the nonlinearity inherent in the LNA, the output signal can be represented as a power series:  
\[uo(t)=K_{0}+K_{1}u_{i}(t)+K_{2}{u_{i}}^{2}(t)+K_{3}{u_{i}}^{3}(t)+\cdots \tag{6}\]  
\(K_{i}(i=0,1,2,\ldots)\) represents a nonlinear coefficient, which is a constant associated with the transfer characteristics of the circuit.