# Design and Simulation of Low-Orbit Satellite Broadcast Signal Receiving and Processing Terminal
## 3 Key Technologies
### Time-Frequency Domain Narrowband Anti-Interference Technology  
Currently, commonly used methods to combat narrowband interference include time domain interference suppression technology and frequency domain interference suppression technology [2]. Time domain interference suppression technology mainly detects and filters the received signal by utilizing the spectrum characteristics of useful signal information and interference signals. Although it can suppress interference more thoroughly, it requires a large amount of a priori information and calculations, and the algorithm converges slowly, only applicable to slowly changing interference [2; 5; 20; 21]. Frequency domain interference suppression technology does not require the use of a priori information, which greatly saves the amount of calculation and is more suitable for satellite measurement and control links [22; 23]. However, some other components will be introduced due to transformation in the frequency domain, making the interference suppression not thorough enough, and twice transformation includes the time-to-frequency domain and the frequency-to-time domain requires a lot of computing resources [24; 25; 21; 26; 27; 28].  
In view of the respective shortcomings of time domain interference suppression technology and frequency domain interference suppression technology, this design will  
Figure 4: Digital beamforming principal diagram.  
use a combination of time and frequency domain interference suppression technology, which combines the fact that frequency domain notch does not require a priori information, frequency point estimation is accurate, and time domain notch can completely eliminate interference, avoiding the shortcomings of spectrum leakage and data truncation, and the contradiction between stability and convergence in frequency domain notch, and use frequency domain transformation to estimate the interference frequency point, which can be used to configure the parameters of the notch filter [24; 29; 30; 31], then eliminate the narrow-band interference. Instead of using IFFT to restore the signal, this method only uses FFT transform to find the frequency and bandwidth of narrowband interference and directly performs time domain notch on the interference frequency. This can not only avoid signal distortion caused by the IFFT transform but also reduce the amount of calculation of overlapping FFT transform and save FPGA computing resources [32]. Based on this idea, this paper adopts an interference suppression technology that combines time and frequency domains. The principal block diagram of the time-frequency domain anti-narrowband interference technology is shown in Figure 5. The AD sampled data are divided into two channels, one channel is sent to the time domain notch filter for time domain notch processing, and the other channel is sent to the interference detecting module to accurately detect the frequency band location of the interference and then configures the notch parameters, thereby combining the advantages of the time domain and frequency domain to achieve the purpose of removing interference [25; 30; 33; 34; 35; 36].  
The principal block diagram of the time-frequency domain anti-narrowband interference technology is shown in Figure 5. The AD sampled data are divided into two channels, one channel is sent to the time domain notch filter for time domain notch processing, and the other channel is sent to the interference detecting module to accurately detect the frequency band location of the interference and then configures the notch parameters, thereby combining the advantages of the time domain and frequency domain to achieve the purpose of removing interference [32; 34; 35; 36; 22].