# Deployment Protection for Interference of 5G Base Stations with Aeronautical Radio Altimeters
## 2 Methodology
### Interference Thresholds  
From an aviation safety perspective, when analyzing interference with radio altimeters, it is common to consider the worst-case scenario by using the lowest interference threshold. The interference threshold of a radio altimeter can be influenced by multiple factors, including the type of altimeter, altitude AGL, the operational scenario, the safety threshold margin, and more. By considering the worst conditions and using the lowest interference threshold, the analysis ensures that the radio altimeter is adequately protected against interference. In this paper, we primarily focus on the interference caused by fundamental 5G emissions and do not consider the interference threshold caused by spurious 5G emissions. The interference thresholds are shown in Table 2[2]. First, radio altimeters are divided into the following three categories: category 1, commercial air transport aircraft; category 2, regional, business aviation, and general aviation aircraft; and category 3, helicopters. In this study, we focus on category 2 radio altimeters to establish interference thresholds. This is because category 2 has the lowest interference threshold among the three categories, as indicated in Table 2, resulting in the largest prohibited zone[31]. Second, because the distance measurement echo power changes with the flight altitude, the sensitivity of modern radio altimeters is generally designed to be adjustable according to the altitude. Therefore, the interference threshold of the radio altimeter also changes with the flight altitude. When considering the prohibited zone, we should discuss the size of the prohibited zone at different altitudes AGL in detail. Third, the ICAO _Handbook on Radio Frequency Spectrum Requirements for Civil Aviation_ recommends that an aviation safety margin of 6-10 dB be included in the interference analysis concerning aeronautical safety systems in order to address the risk that some factors cannot be foreseen[32]. A safety margin of not less than 6 dB should be applied until it is established on a case-by-case basis. For the radar altimeter's integrity, availability, and/or continuity, a safety margin of not less than 6 dB must be included in the interference analysis. The occurrence rate of harmful interference is expected to be evaluated on the \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Altimeter**} & \multirow{2}{*}{
\begin{tabular}{c} **Frequency** \\ **(GHz)** \\ \end{tabular} } & \multicolumn{5}{c}{**Altitude AGL (ft)**} \\ \cline{3-6}  & & **200** & **1000** & **2000** & **5000** \\ \hline \multirow{3}{*}{Category 1} & \multirow{3}{*}{3.93} & \(T\) & \(-16\) & \(-24\) & - & \(-30\) \\ \cline{3-6}  & & \(T_{\text{6 dB}}\) & \(-22\) & \(-30\) & - & \(-36\) \\ \cline{3-6}  & & \(T\) & \(-42\) & \(-50\) & \(-63\) & - \\ \cline{3-6}  & & \(T_{\text{6 dB}}\) & \(-48\) & \(-56\) & \(-69\) & - \\ \cline{3-6}  & & \(T\) & \(-37\) & - & - & - \\ \cline{3-6}  & & \(T_{\text{6 dB}}\) & \(-43\) & - & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Interference thresholds for categories 1, 2, and 3 at 3.93 GHz. \(T\): threshold (dBm); \(T_{\text{6 dB}}\): threshold with ICAO 6 dB margin (dBm). order of \(1\times 10^{-5}\) per flight hour [3]. This occurrence rate is associated with operational scenarios with major failure conditions. Finally, all thresholds are provided based on the worst-case landing scenario (WCLS). Takeoff and approach are the most susceptible phases to interference. For the radio altimeter, it is essential to consider the RA signals generated by other aircraft on the taxiway and apron during these phases. As shown in Figure 8, it is assumed that five aircraft are taxiing on the taxiway parallel to and at a distance of \(d_{3}\) from the runway, with a spacing of \(d_{1}\). Additionally, eleven aircraft with a spacing of \(d_{2}\) are parked on the apron parallel to and at a distance of \(d_{4}\) from the runway.