# Research on Tbps and Kilometer-Range Transmission of Terahertz Signals
## 1 Introduction  
THz communication stands as a core candidate technology for next-generation wireless communication. Its necessity stems from the fundamental contradiction between the exponential growth in wireless data rates and the limitations of existing spectrum resources [1, 2, 3, 4, 5]. As 5G mmWave bands gradually become saturated, traditional microwave bands (\(<\)6 GHz) face inherent drawbacks including excessively wide beams, high power consumption, and insufficient bandwidth, pushing their spectral efficiency near the theoretical limit [6, 7, 8, 9, 10]. Furthermore, their practical deployment is hindered by large device size and low integration density. While laser communication offers high bandwidth potential, its practical implementation is severely constrained by difficulties in aligning its extremely narrow beams and its high sensitivity to adverse weather conditions [11, 12]. Occupying the frequency gap between microwaves and optical bands, the THz band bridges this technological void with its unique physical properties, becoming crucial for future 6 Gdevelopments and beyond [13; 14]. The exponential increase in wireless data rates, coupled with the inability of mega-bit-per-second microwave speeds to meet the demands of massive data transmission, has driven exploration and research into the THz band with its rich spectrum resources [15; 16; 17]. Its enormous contiguous bandwidth lays the physical layer foundation for building 6G networks with high data rates, low latency, and high-density ubiquitous connectivity, enabling potential speeds reaching gigabits per second (Gbps) or even terabits per second (Tbps) [18; 19; 20]. The sub-terahertz band (typically 0.1-0.3 THz), as standardized in ITU-R V.431 [21] and IEEE 802.15.3d [22], utilizes solid-state electronics such as Complementary Metal Oxide Semiconductors (CMOSs) and SiGe Heterojunction Bipolar Transistors (HBTs) for integrated circuit implementation to support moderate-bandwidth communications. In contrast, the multi-terahertz band (3-10 THz) falls within the 'optically-oriented spectrum' per IRMMW-THz Conference definitions, where quantum cascade lasers (QCLs) and photoconductive antennas dominate, enabling high-resolution spectroscopy despite severe molecular absorption losses. Bridging these extremes, the 0.3-3 THz 'gap' predominantly employs hybrid approaches combining Schottky diode multipliers with photo-mixers [23; 24; 25; 26; 27; 28; 29; 30]. THz communication emerges as a technology capable of breaking the performance limits of next-generation wireless networks due to its distinct advantages. Spectrum resources: the THz band offers hundreds of GHz of contiguous bandwidth, enabling single-link Tbps-level transmission without complex spectral efficiency enhancement techniques, bridging the technological gap between mmWave and Free-Space Optical (FSO) communication. Physical properties: its short wavelength not only suppresses multi-path interference and eavesdropping risks but also supports ultra-highly directional beams, exhibiting superior stability compared to FSO in complex environments [31; 32; 33; 34; 35; 36; 37]. However, achieving long-distance THz signal transmission faces significant challenges. As indicated by Figure 1, FSPL increases exponentially with frequency; THz signals experience path loss exceeding 120 dB per kilometer, far surpassing attenuation levels in mmWave bands. Simultaneously, atmospheric attenuation effects are particularly severe in specific frequency regions: molecular resonance absorption by water vapor (H\({}_{2}\)O) and oxygen (O\({}_{2}\)) creates multiple attenuation peaks, where attenuation at sea level can exceed 10 dB/km. Under rain or fog conditions, this loss can surge to tens of dB/km. The combined effect of these two physical limitations creates a \"dual attenuation dilemma\" for long-distance THz transmission: signals must overcome both energy dissipation due to free-space spreading and avoid strong absorption bands outside atmospheric windows [33; 38; 39; 40; 41; 42].