# 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  
In long-distance THz wireless communication systems, single-antenna architectures face severe challenges. Limitations in optoelectronic conversion efficiency cause significant energy loss during signal transmission [77]. Concurrently, the inherently poor penetration characteristics of high-frequency electromagnetic waves and severe atmospheric attenuation collectively result in extremely weak signal strength at the receiver, critically degraded SNR, and fundamental constraints on communication quality and transmission capacity [78; 79; 80]. To overcome these bottlenecks and dramatically enhance system performance, MIMO technology emerges as a critical solution. As shown in Figure 7, by deploying antenna arrays at both the transmitter and receiver ends, MIMO technology enables deep spatial exploitation, effectively addressing insufficient received power and low SNR issues [81; 82]. Our primary enabling pathways include: MIMO combined with PDM (MIMO-PDM) and MIMO with MRC (MIMO-MRC) [83; 84; 85; 86; 87]. Figure 6: QPSK, 16 QAM, and 64 QAM constellation diagrams under normalized power: (**a**) QPSK; (**b**) 16 QAM; (**c**) 64 QAM. For MIMO-PDM technology, THz signals are first generated using the photonic-assisted approach introduced in Section 2, employing two optical THz signals sharing the same optical frequency but possessing orthogonal polarization states. These two signals can be generated from either (a) a common CW laser source, subsequently combined via an OC, or (b) a PDM1/Q modulator, which directly produces orthogonally polarized optical THz signals. Prior to the photodetection (optical-to-electrical conversion) of the polarization-multiplexed optical signal, its orthogonal polarization components must be separated using optical components. Separation options include schemes based on optical couplers combined with polarization beam splitters (PBSs) or, alternatively, using a polarization-diverse phase-diversity 90-degree optical hybrid. So first, the data is modulated onto one optical carrier wave. After passing through the PBS, it beats with another LO light source (also passed through a PBS) to generate a terahertz signal. Subsequently, in the detection stage, the dual-diode balanced structure within balanced photodetectors (BPDs) effectively suppresses common-mode noise while enhancing tolerance to input signal power and improving SNR. After passing through two high-speed balanced photodetectors, two electrical THz signals are produced, collectively forming the polarization-division-multiplexed electrical THz signal. Wireless transmission of this signal is realized via a 2 \(\times\) 2 MIMO link. At the receiver, the received THz signal must first undergo analog down-conversion. This is achieved using balanced mixers driven by an electrical LO operating in the THz frequency band, converting the THz carrier signal down to a lower intermediate frequency (IF) signal. These IF signals are then captured by a real-time oscilloscope equipped with two input ports. Finally, the data recorded by the real-time oscilloscope is fed into an advanced DSP module for analysis and processing to recover the original information. The optical baseband transmitter generates the original optical baseband signal, whose X- and Y-polarization components can be represented as the following column vector:  
\[\mathbf{E}_{in}=\left( \begin{array}{c}E_{in,x}\\ E_{in,y}\end{array} \right). \tag{1}\]  
The received signal after fiber transmission can be represented as follows:  
\[\mathbf{E}_{out1}=\mathbf{JE}_{in}= \begin{pmatrix}J_{xx}&J_{xy}\\ J_{yx}&J_{yy}\end{pmatrix} \left( \begin{array}{c}E_{in,x}\\ E_{in,y}\end{array} \right). \tag{2}\]  
where \(\mathbf{J}\) is the 2 \(\times\) 2 Jones matrix; the diagonal elements (\(J_{xx}\), \(J_{yy}\)) are principal polarization channels; and off-diagonal elements (\(J_{xy}\), \(J_{yx}\)) are the polarization crosstalk from fiber impairments.