# Research on Tbps and Kilometer-Range Transmission of Terahertz Signals
## 2 Terahertz Signal
### Electronic Approaches  
Electronic approaches primarily rely on silicon-based devices [48]. In recent years, significant progress has been made in technologies based on advanced semiconductor materials such as III-V compound semiconductors with Indium Phosphide (InP) and Gallium Nitride (GaN), including High Electron Mobility Transistors (HEMTs), Monolithic Millimeter-Wave Integrated Circuits (MMICs), and HBTs. For instance, HEMTs exhibit ultra-high electron saturation velocity and cut-off frequencies reaching 1.5 THz, laying the hardware foundation for THz communication systems. The core implementation path for all-solid-state-electronic solutions is the cascaded frequency multiplication technique [49; 50; 51]. An oscillator first generates a microwave/millimeter-wave fundamental frequency signal. This signal is then progressively up-converted into the THz band through frequency multiplier chains based on Schottky diodes or HBTs. Finally, an HEMT power amplifier (PA) outputs the effective radiated power [52; 53; 54; 55; 56].  
In paper [57], researchers successfully demonstrated real-time transmission of 34-GBd polarization division multiplexing (PDM) quadrature phase shift keying (QPSK) signals using a purely electronic THz system integrated with a commercial Digital Coherent Optical (DCO) real-time mod. The experimental setup is shown in Figure 4. THz signal generation was achieved via electronic frequency multiplication and direct conversion mixing techniques: First, a signal generator produced an 8.311 GHz local oscillator (LO) signal, which was multiplied to ~300 GHz using a 36-fold frequency multiplier. Subsequently, the four analog baseband signals (corresponding to the \(\mathrm{X_{I}}/\mathrm{X_{Q}}/\mathrm{Y_{I}}/\mathrm{Y_{Q}}\) components of the 34-GBd PDM-QPSK optical signal) demodulated from the first fiber span were directly fed into two MMIC-based direct-conversion in-phase and quadrature (I/Q) mixer transmitter frontends. Here, the baseband signals were up-converted to the THz band by mixing with  
Figure 3: Photonic-assisted terahertz signal; ECL: external cavity laser; EA: electrical amplifier; UTC-PD: Uni-Traveling-Carrier Photodiode; PM-EDFA: polarization-maintaining erbium-doped fiber amplifier; PM-OC: polarization-maintaining optical coupler; ATT: attenuator.  
Figure 2: Multiplier-based terahertz signal; FFS: Fundamental Frequency Source; DA: Drive Amplifier; BPF: Band Pass Filter; MS: Multiplier Stage.  
the 300 GHz LO. Spatial multiplexing technology (using two independent antenna pairs) transmitted the dual-polarization data synchronously over a 50 cm wireless link. This method achieved the first real-time hybrid transmission with a net data rate of 100 Gb/s: Within a system comprising two fiber spans (total length 103 km) and the 300 GHz THz link, the Bit Error Rate (BER) remained stably below the forward error correction (FEC) threshold (\(3.4\times 10^{-2}\)) by employing single-carrier real-time Digital Signal Processing (DSP) to jointly compensate for accumulated dispersion, polarization rotation, and phase noise. This validates the transparency of the THz wireless extender to linear impairments inherent in optical fibers and provides a multi-hundred-gigabit wireless fiber extension solution for 5G/6G backhaul.