# Adaptive 3D Imaging for Moving Targets Based on a SIMO InISAR Imaging System in 0.2 THz Band
## 2 Architecture of the Terahertz SIMO InISAR Imaging System  
### The Architecture of the SIMO Antenna Array  
Figure 1 presents the block diagram of the SIMO InISAR system, which contains one transmitting channel and four receiving channels. To produce a wideband signal in the 0.2 THz band, the frequency multiply technique was adopted. The transmitting channel and receiving channels were connected to the corresponding transmitting chain and receiving chains, which are driven by the Ku-band radio frequency (RF) and local oscillator (LO) signals, respectively. To ensure an accurate synchronization between the RF and LO signals, based on the integration of a direct digital waveform synthesis (DDWS) module and phase locked loop (PLL) module, a fundamental frequency modulation continuous wave (FMCW) signal with a frequency range of 2.0-3.25 GHz was firstly produced and then split into two ways. One way is upconverted by a phase locked dielectric resonator oscillator (PLDRO) with a frequency of 14.042 GHz to get the RF FMCW signal, the other way is upconverted by another PLDRO with a frequency of 14.017 GHz to produce the LO FMCW signal. In the transmitting chain, the 0.2 THz signal was produced by upconverting the main branch of the RF signal with a \(\times\)12 multiplier. In the receiving chains, since heterodyne receivers are employed, the main branch of the LO signal was divided into four ways and then upconverted with four \(\times\)12 multipliers separately to form the THz LO signals for each channel. The measured intermediate frequency (IF) signals were acquired by processing the received echoes from the targets with de-chirping technology. In the IF module, the Ku band RF and LO FMCW signals were firstly mixed and then upconverted with a \(\times\)12 multiplier, resulting in the reference IF signal. Both the measured and reference IF signals were then sent to a multi-channel wideband quadrature detector to extract the amplitude and phase information of the received echoes, producing the I/Q signals. Finally, the signals were digitally sampled and transferred to the computer for data processing.  
As for the antennas, a rectangular horn antenna was selected due to its simple structure and excellent radiation performance. The transmitting antenna is a single horn while the receiving antenna is a horn array which places four rectangular horns symmetrically in a plane, corresponding to four receiving channels. The aperture size of the receiving horn is 4 mm \(\times\) 3 mm. The distances of the phase centers of two horns in the horizontal and vertical directions are both 5 mm, which equals the baseline length. Since no switchers are available in THz band, a high isolation is necessary between the transmitting and receiving channels. In this study, a quasi-optical isolator was designed by integrating a beam splitter and absorber, which not only isolate the transmitting and receiving channels effectively, but also lead to a compact antenna configuration. In the quasi-optical isolator, the beam splitter is deployed with a decline angle of 45\({}^{\circ}\), the receiving horn array lies along the horizontal line crossing the center of the beam splitter, while the transmitting horn is rotated to the vertical direction with the center of the beam splitter as origin. This way, the wave paths from the beam splitter to the phase centers of the transmitting and receiving horns will be the same, eliminating the phase difference caused by the bistatic configuration of transmitting and receiving antennas. The prototype of the SIMO antenna array is shown in Figure 2.