## 3 Experimental Setup  
Figure 4a illustrates a 4.6 km 2 \(\times\) 2 MIMO photonic-assisted terahertz communication system in the D-band. At the transmitter, offline baseband signals are first generated via MATLAB and loaded into a 120 GSa/s arbitrary waveform generator (AWG) to produce 16 QAM signals with varying symbol rates. These signals are then amplified by two parallel electrical amplifiers (EAs) with a gain of 25 dB and modulated using an I/Q modulator with a 3 dB bandwidth of 30 GHz. An external cavity laser-1 (ECL1) at a wavelength of 1550 nm serves as the optical carrier and has 12 dBm optical power. The modulated signal is amplified by a polarization-maintaining erbium-doped fiber amplifier (PM-EDFA) and coupled with a 14.5 dBm LO optical source emitted by laser 2 (ECL2) at a wavelength of 1551.1 nm. Both the external cavity lasers have a linewidth of 100 kHz. The signal is transmitted in two polarization directions. After transmission through 100 m of SMF-28 fiber, the signal's optical power is adjusted by a variable optical attenuator (VOA) and then split into two paths by PM-OC2. To eliminate correlations between the signal paths effectively, a 2 m long polarization-maintaining optical delay line (PM-ODL) is added to one path. The optical signals of the two paths, after transmission through a 1 m polarization-maintaining fiber, are input into UTC-PD1 and UTC-PD2, respectively. These devices operate within a frequency range of 110 to 170 GHz and generate a 124.8 GHz sub-terahertz signal through optical mixing technology. Because of the rapid attenuation of sub-terahertz band signals during free-space transmission, the signal is amplified by a cascade of LNA1/LNA3 with a gain of 18 dB and PA1/PA2 with an output power of 13 dBm to support a wireless transmission distance of 4600 m. Subsequently, the amplified terahertz signal is transmitted via a pair of high-gain antennas (HAs) with horizontal/vertical polarization (H-/V-pol. ), each with a gain of 25 dB. Given the small aperture of high-gain antennas in the sub-terahertz band, a pair of plano-convex lenses (Lens1/Lens3 and Lens2/Lens4) is used to focus the terahertz beam to maximize the reception power at the receiving-end HAs. Specifically, the diameters of Lens1/Lens2 and Lens3/Lens4 are 10 cm and 60 cm, respectively. After 4.6 km wireless transmission, the sub-terahertz signal is focused by Lens3/Lens4 and received by H-/V-pol. HAs. The received H-/V-pol. signals are amplified by LNA2/LNA4 with a gain of 33 dB and down-converted by Mixer1/Mixer2. The local oscillator (LO) operates at 112 GHz, generating a 12.8 GHz intermediate frequency (IF). After down-conversion, the IF signals are amplified by EA3/EA4 with a gain of 26 dB and captured by an 80 GSa/s digital storage oscilloscope (DSO). During the experiment, a high-power telescope was used to assist alignment, which helped improve the accuracy and efficiency of alignment. The transmitter and receiver are located in two campuses of  
Figure 3: (**a**) Schematic diagram of the iterative pruning process. (**b**) Weight matrix diagram of pruning in the fully connected layer. Fudan University, with the transmitter on the rooftop of Guanghua Tower at an altitude of 142 m, and the receiver on the rooftop of the Physics Building. They are 4.6 km apart. Figure 4b shows the channel equalization process at the receiving end of the dual-polarization system. The DSP flow at the receiving end includes down-conversion, resampling, matched filtering, T/2 constant modulus algorithm (CMA), frequency offset estimation (FOE), carrier phase recovery (CPR), direct detection least mean square (DD-LMS) equalization, and DNN-based neural network equalization. First, the IF signal is down-converted and resampled to the baseband. Next, CMA linear equalization and DD-LMS are used to compensate for inter-symbol interference (ISI) caused by channel propagation characteristics. FOE compensates for frequency drift effects due to the laser frequency offset, while CPR reduces phase noise introduced by the laser linewidth.