# Research on Tbps and Kilometer-Range Transmission of Terahertz Signals
## 3 Enabling Technologies for THz Communication Systems
### High-Gain Antennas and Lenses  
THz waves combine the advantages of millimeter waves and optical waves. Compared to millimeter waves, they offer broader usable frequency bands and superior beam directivity; they also provide enhanced security and superior anti-interference performance. Relative to optical waves, they are more efficient, exhibit stronger penetration capabilities, and demonstrate greater adaptability to harsh environments [92,93]. The main types of THz waveguides include conventional metallic rectangular waveguides, dielectric fiber waveguides, novel THz photonic crystal waveguides, surface plasmon polariton waveguides, and emerging two-dimensional material-based waveguides. The metallic rectangular waveguide, with its advantages of low dispersion and high power capacity, is employed in our experimental system for connecting with Cassegrain antennas [94, 95, 96]. THz antennas are indispensable devices in terahertz wireless communication systems for radiating and detecting terahertz waves. Driven by the unique properties of terahertz waves, key performance metrics of these antennas--particularly their broad operating bandwidth and gain--critically impact the overall system quality [97,98]. Antenna gain typically refers to the gain in the direction of maximum radiation. It can be expressed as  
\[G=\frac{4\pi A_{e}}{\lambda^{2}}. \tag{13}\]  
where \(A_{e}\) is the antenna effective aperture area and \(\lambda=c\!f\) is the signal wavelength. As the frequency \(f\) increases to 0.1-10 THz, \(\lambda\) becomes significantly shorter than millimeter wavelengths. For a fixed physical aperture, THz antenna gain improves by \(>\)10\(\times\) over mmWave systems, making aperture efficiency critical. Enhancing effective radiation area requires metasurfaces or diffractive optics at THz frequencies. Path loss in long-distance THz communication systems is a dominant attenuation factor modeled as follows [38,99]:  
\[P_{L}=20\log_{10}\left(\frac{4\pi df}{c}\right)+\alpha_{gas}d. \tag{14}\]  
where \(d\) is the wireless transmission distance and \(\alpha_{gas}\) is the molecular absorption loss. In the THz frequency band, absorption peaks of water vapor and oxygen cause additional attenuation, increasing \(\alpha_{gas}\). Thus, THz base station antennas require extremely high gain to overcome this enhanced path loss and improve system performance. While HAs serve as a low-cost, high-performance reference prototype--achieving up to 25 dBi gain with a simple structure--they prove insufficient for long-range THz wireless systems operating in complex environments [100]. Consequently, dual-reflector Cassegrain antennas (CAs) are widely adopted in THz systems. Figure 11 illustrates schematic diagrams of a rectangular HA, CA and lens antenna. The horn antenna features an intuitive and classic design--essentially a flared waveguide opening that gradually expands to form a trumpet-shaped radiating aperture. Signals feed into the narrow end of the waveguide; as electromagnetic waves propagate through  
Figure 10: MIMO terahertz transmission system using PDM and MRC; VOA: variable optical attenuator; PBC: polarization beam combiner; Pol.Mux. : polarization multiplexer; CLA: cylindrical lens antenna [90]. the expanding horn and reach the broad aperture, the beam widens and shapes into a directional pattern. Here, a and b denote the width and height of the rectangular waveguide, while a\({}_{\text{E}}\) and b\({}_{\text{H}}\) represent the aperture side lengths in the E-plane and H-plane, respectively; R is the distance between the upper and lower horn apertures [101]. In contrast, the Cassegrain antenna employs a sophisticated dual-reflector system. Signals radiate from the feed source, first illuminating the sub-reflector. The sub-reflector redirects the signals to the primary reflector, which then collimates the beam for long-distance transmission.