## 2 Antenna Simulation for LEO Downlink Analysis  
Figure 1 shows the conceptual figure of the downlink scenario when the LEO satellite transmits high-rate data in interference situations, where the side lobe of the ground station antenna is exposed to a strong interference signal source. The elevation angle of the satellite is \(\theta_{gs}\), and the elevation angle of the interference source is \(\theta_{j}\). When the LEO satellite transmits data through the downlink to the ground station, the free-space loss \(L_{f}\) can be obtained by Formula (1)  
\[L_{f}(\mathrm{dB})=20\log_{10}(d)+20\log_{10}(f)+20\log_{10}(\frac{4\pi}{c_{0}}) \tag{1}\]  
where \(d\) is the distance between the LEO satellite and the ground station and \(f\) is the carrier frequency. The LEO satellite and the ground station antenna can be slightly misaligned with off-axis situations in the space environment, and a bore-sight error loss \(L_{b}\) can be calculated using Equation (2)  
\[L_{b}(\mathrm{dB})=12\bigg{(}\frac{\theta_{b}}{\theta_{h}}\bigg{)}^{2} \tag{2}\]  
where \(\theta_{h}\) is the half-power beam width (HPBW) of the ground station antenna, and \(\theta_{b}\) is the bore-sight error angle. It is assumed that there is no manufacturing error of the receiving antenna, and the antenna is ideally well matched to the RF system. It is also assumed that the performance of the receiver system does not change even when the side lobe of the receiving antenna is exposed to strong interference [12]. To calculate the link budget between the LEO satellite and the ground station, the received power \(P_{r}\) can be expressed as (3)  
\[P_{r}(\mathrm{dBm})=P_{t}+G_{t}+G_{r}-L_{f}-L_{b}-L_{at} \tag{3}\]  
where \(P_{t}\) is the transmitting power, \(G_{t}\) is the gain of the transmitting antenna in the LEO satellite, and \(G_{r}\) is the receiving antenna gain of the ground station. We also consider the atmospheric loss \(L_{at}\) due to significant refraction and attenuation in the atmosphere. This loss is significantly observed, especially when the LEO satellite is at the low elevation angle. The atmospheric loss in the space environment will be discussed in more detail in the next section. Figure 2a,b show the transmitting antenna of the satellite and the far-zone radiation pattern. Patch arrays [13; 14; 15], helical wires [16; 17], and corrugated or choke ring horns [18; 19; 20; 21; 22; 23] are often used as data transmission antennas for LEO satellites. Among them, the choke ring horn antenna is used as the transmitting antenna in this study, because it has a relatively simple shape and can easily have the required radiation pattern with wide beam coverage. In particular, an isoflux pattern is required to maintain uniform received power at the Earth's  
surface during downlink data transmission through the X-band. To obtain the isoflux pattern, the antenna is designed based on a requirement mask for the radiation pattern, which is announced in the CNES [24; 25; 26]. The designed antenna has a diameter \(d_{t}\) of 77.4 mm, and its characteristics are obtained using the CST studio suite full EM simulation tool. It has a beam coverage of \(\pm 51.6^{\circ}\) with a bore-sight gain of 4.4 dBi at 8 GHz. Figure 1: Conceptual figure of the LEO downlink scenario in interference situations. Figure 2: Transmitting antenna of the satellite and the far-zone radiation pattern: (**a**) transmitting antenna of the LEO satellite; (**b**) radiation pattern and requirement masks. Figure 3a,b show the receiving antenna of the ground station and the far-zone radiation pattern. In general, the ground station antenna is designed considering the regulation of ITU-R S.508-6 for efficient communication by minimizing interference [27]. The receiving antenna is a parabolic type with a diameter \(d_{r}\) of 11.3 m, of which the radiation pattern is obtained using the GO and PO methods. This antenna is fed by a rectangular horn antenna, and has an HPBW of 0.2\({}^{\circ}\) with a maximum gain of 59 dBi at 8 GHz.