## 3 System Model  
### Design of Inter-Satellite Terahertz Communication System  
For the inter-satellite terahertz communication scenario, a spaceborne terahertz communication system with an operating frequency of 310 GHz is designed. The system is consistent with the design principle of the terahertz communication system applied to the ground. At the same time, it also has the advantages of having a simple and compact structure, being easy to carry by satellites, and obvious potential for later power improvement, which is convenient for carrying out inter-satellite terahertz communication experiments. The system adopts an all-electronic method. The transmitter adopts the zero-intermediate frequency (zero-IF) transmitter of American VDI harmonic mixer, and the receiver adopts the harmonic-mixing zero-IF receiver. Assuming the same terminal configuration on each satellite, the 310 GHz satellite communication system based on VDI harmonic mixing superheterodyne transceiver is shown in Figure 4, and the system parameters are shown in Table 2.  
The microwave signal source generates a sinusoidal signal with a frequency of 19.375 GHz, which goes through a quadruple, a filter, and a doubl  
\begin{table}
\begin{tabular}{c c c c} \hline \hline Parameter & Symbol & Value & Unit \\ \hline Carrier frequency & f & 310 & GHz \\ Transmit antenna gain & G\({}_{\rm t}\) & 60 & dB \\ Receive antenna gain & G\({}_{\rm r}\) & 60 & dB \\ Transmit antenna diameter & D\({}_{\rm t}\) & 50 & cm \\ Receive antenna diameter & D\({}_{\rm r}\) & 50 & cm \\ Average noise temperature & T & 2700 & K \\ Bandwidth & B & 5 & GHz \\ Transmit power & P & 23 & dBm \\ Boltzmann constant & K\({}_{\rm B}\) & 1.381 \(\times\) 10\({}^{-23}\) & / \\ Modulation & / & OOK & / \\ Antenna feeder loss & L\({}_{\rm k}\) & 2 & dB \\ Polarization loss & L\({}_{\rm i}\) & 1 & dB \\ \hline \hline \end{tabular}
\end{table}
Table 2: Related parameters of satellite terahertz communication system.  
Figure 4: Block diagram of satellite terahertz communication system.  
then performs sub-harmonic mixing with the input signal IF2 when it reaches 155 GHz. A modulated signal with a center frequency of 310 GHz and a bandwidth of 5 GHz is obtained through the zero-IF up-mixing output. After passing through the power amplifier (PA), the signal is transmitted through the terahertz antenna and received by the receiver antenna. The receiving end adopts a superheterodyne mixing receiver. In order to make the received signal cover the frequency band of 307.5-312.5 GHz, the center frequency of the local oscillator (LO) of the receiver is selected as 310 GHz, and the intermediate frequency bandwidth is selected as 0-2.5 GHz. Considering the noise of the receiving IF low noise amplifier (LNA), the overall noise temperature at the receiving end is estimated to be 2700 K. In order to make the LO of the zero-IF receiver in phase with the LO of the received signal, an analog phase shifter is used to adjust the phase of the LO. Since the terahertz antenna and the terahertz front-end device are hard-connected by a waveguide, and the distance between the two is generally negligible relative to the transmission distance of thousands of kilometers between satellites, the terahertz antenna is usually not considered in the research. Instead, the transmitter, receiver and antenna of the terahertz communication system are regarded as a whole.