# Design and Simulation of Low-Orbit Satellite Broadcast Signal Receiving and Processing Terminal
## 4 Simulation Results
### Digital Beamforming Simulation  
This scheme uses HFSS 15.0 software to simulate the digital beamforming module. The Digital Beamforming system (hereinafter referred to as \"DBF system\") generates variable beams by controlling the phase and amplitude of each element signal of the array antenna through data technology. Therefore, the beamforming problem is mainly to determine the amplitude and phase weighting coefficients of each element signal required in different directions. In engineering implementation, there are two main ways to generate weighting coefficients for beamforming: real-time calculation of weighting coefficients and look-up table calculation. The real-time calculation method is to use an adaptive algorithm to adjust the weighting coefficients based on certain decision criteria (such as detecting the signal power after beamforming). The advantage of this method is that it adjusts the weighting coefficients in real-time according to the characteristics of the synthesized signal, suppresses the sidelobes, and optimizes the received signal. It can adapt to different application environments in real-time. The disadvantage is that the calculation amount is large, and the hardware platform and algorithm optimization requirements are high. The look-up table method calculates the weighting coefficients of different directions through simulation analysis and solidifies the coefficients in the memory. The weighting coefficients of the corresponding direction are extracted according to the specific pointing parameters for beam synthesis. The advantages of this method are small calculation amounts, fast running speed, and simple algorithm design. The disadvantage is that the weighting coefficients cannot be adjusted in real time and the adaptability to interference signals is poor. Considering that the communication time of low-orbit satellites is short each time and the beam changes quickly, this scheme adopts the look-up table method. In this scheme, Taylor distribution or Chebyshev distribution is used to optimize the amplitude weighting of each unit of the beam to achieve the low sidelobe characteristics of the array beam and at the same time reduce the beam gain, taking both low sidelobe and constant gain performance into account. Table 2 shows the gains and sidelobe levels of several typical directional beams after optimization. In these angles, the sidelobe level decreases by 1 to 8 dB. In the entire three-dimensional beam space, the gain is within 14 to 15 dBic, and the sidelobe levels are lower than \(-11\) dB. Figure 16: Simulation link of RF channel. Figure 17: RF link budget results. Figures 18 and 19 show the beam characteristic diagrams of (\(\theta\), \(\varphi\)) = (0\({}^{\circ}\), 0\({}^{\circ}\)) and (\(\theta\), \(\varphi\)) = (0\({}^{\circ}\), 45\({}^{\circ}\)) before and after optimization, respectively. Due to space limitations, the beam characteristic diagrams of other angles before and after optimization are not listed here. As can be seen from Figure 18, before optimization, at the desired signal direction \(\varphi=0^{\circ}\), the mainboard signal strength is 15.3 dBic, and there is a sidelobe with a strength \begin{table}
\begin{tabular}{c c c} \hline \hline
**Beam Pointing Angle (\(\theta\), \(\varphi\))** & **Sidelobe Level Reduction (dB)** & **Gain (dBic)** \\ \hline (0\({}^{\circ}\), 0\({}^{\circ}\)) & 5 & 15.00 \\ (20\({}^{\circ}\), 0\({}^{\circ}\)) & 8 & 14.60 \\ (45\({}^{\circ}\), 0\({}^{\circ}\)) & 3 & 14.89 \\ (65\({}^{\circ}\), 0\({}^{\circ}\)) & 3 & 14.97 \\ (75\({}^{\circ}\), 0\({}^{\circ}\)) & 7 & 14.63 \\ (75\({}^{\circ}\), 30\({}^{\circ}\)) & 4.5 & 14.30 \\ (75\({}^{\circ}\), 60\({}^{\circ}\)) & 1 & 14.57 \\ (75\({}^{\circ}\), 90\({}^{\circ}\)) & 2.5 & 14.73 \\ (75\({}^{\circ}\), 120\({}^{\circ}\)) & 2 & 14.71 \\ (75\({}^{\circ}\), 150\({}^{\circ}\)) & 2 & 14.94 \\ (75\({}^{\circ}\), 180\({}^{\circ}\)) & 1 & 14.92 \\ \hline \hline \end{tabular}
\end{table}