# Research on Tbps and Kilometer-Range Transmission of Terahertz Signals
## 3 Enabling Technologies for THz Communication Systems
### Adopting MIMO Architecture Combining Polarization Multiplexing and MRC
Figure 7: MIMO architecture diagram and MIMO diversity reception architecture. At the Rx, the received THz signal is  
\[\mathbf{E}_{out2}=\mathbf{GE}_{out1}\cos(\omega t)= \begin{pmatrix}G_{xx}&G_{xy}\\ G_{yx}&G_{yy}\end{pmatrix} \mathbf{E}_{out1}\cos(\omega t). \tag{3}\]  
where \(\mathbf{G}\) is the 2 \(\times\) 2 MIMO gain matrix; \(\omega\) is THz carrier frequency; and off-diagonal elements (\(W_{xy}\), \(W_{yx}\)) are wireless crosstalk. Combining fiber and wireless stages,  
\[\mathbf{E}_{out2}=\mathbf{GJE}_{in}\cos(\omega t)=\mathbf{HE}_{in}\cos(\omega t). \tag{4}\]  
where the composite channel matrix \(\mathbf{H}\) is  
\[ \begin{split}\mathbf{H}=\mathbf{GJ}=\begin{pmatrix}H_{xx}&H_{ xy}\\ H_{yx}&H_{yy}\end{pmatrix},\\ H_{ij}=\sum\limits_{k=1}^{2}G_{ik}I_{kj}\quad(i,j\in\{x,y\}).\end{split}  \tag{5}\]  
The received signals are demultiplexed via a butterfly equalizer. In reference [88], we successfully achieved a major breakthrough in high-speed long-distance wireless transmission in the D-band using MIMO-PDM, innovatively constructing a photonics-assisted 2 \(\times\) 2 MIMO wireless transmission system. The experimental setup is shown in Figure 8. At the transmitter, dual external cavity lasers (ECL-1 @1550 nm, ECL-2 @1551 nm) provide the optical carrier and local oscillator, respectively; a 25-Gbaud 16-QAM baseband signal (generated by an arbitrary waveform generator (AWG) and amplified by an EA) is loaded via an I/Q modulator; and PDM is used to generate two independent signals. Critical opto-electronic conversion is performed by a high-performance UTC-PD, up-converting the optical signal to a 125 GHz D-band millimeter wave. Subsequently, the signal is amplified by cascaded LNAs and PAs (total gain \(\sim\)31 dB, output power 13 dBm), transmitted through a pair of orthogonally polarized (horizontal/vertical) horn antennas (gain 25 dBi), and innovatively collimated and focused using large-aperture (60 cm) and small-aperture (10 cm) plano-convex lens assemblies to overcome high free-space transmission losses in the D-band. The receiver similarly employs orthogonally polarized horn antennas and lens assemblies to capture the signal, which is amplified by LNAs, down-converted to a 13 GHz intermediate frequency, and acquired by a high-speed oscilloscope. The receiver-side DSP employs a sophisticated algorithm chain including MIMO-constant modulus algorithm (CMA) equalization, frequency offset estimation, carrier phase recovery, advanced 51-tap MIMO-CMA equalization, a 183-core second-order MIMO Volterra nonlinear equalizer (MIMO-VNLE), and 155-tap decision-directed least mean square (DD-LMS) equalization, effectively mitigating impairments induced by long-distance transmission. Ultimately, over a 4.6 km real-world urban wireless link (between campuses of Fudan University), a total transmission rate of 200 Gbps (100 Gbps per channel) was successfully achieved, with a measured minimum BER of 2.05 \(\times\) 10\({}^{-2}\) meeting the 20% soft-decision forward error correction (SD-FEC) threshold requirement. This accomplishment established a record-breaking capacity-distance product of 920 Gbps\(\cdot\)km. In single-polarization MIMO systems, multiple transmitter antennas simultaneously transmit identical signals. The predominant challenge in this configuration is maximizing gain while simultaneously improving receiver accuracy (i.e., signal fidelity). MIMO-MRC addresses this by applying beamforming (BF) principles. It determines the weights at the receiver side based on the received power level of each distinct channel. Subsequently, it combines and sums the signals from all channels using these weighted coefficients to achieve diversity gain.