## 3 Challenges in Terahertz-Based FSO Communication
### Hardware Limitations  
THz communication systems have strong hardware restrictions, which is one of the main barriers to THz development and realization. Present THz gadgets, including QCLs and photodetectors, have low efficiency, low power, and a high fabrication cost. These challenges also make it difficult to realize large-scale and low-cost THz communication systems [59].  
THz communication depends on the technology of quantum cascade lasers, which produce less than 10% efficiency on the best of occasions, to provide adequate power for long-range transmission [12; 13; 60]. QCL systems face added challenges due to the need for cryogenic cooling to drive current and laser operations, which considerably raise their operational costs and complexity. To overcome these problems, new high-efficiency semiconductor materials and design configurations like distributed feedback structures are in the pipeline [61].  
Photodetectors, another critical component of every system, are also characterized by sensitivity and responsivity problems at THz frequencies. Recent advancements in new concepts such as black phosphorus-based materials are expected to improve the performance of detectors and reduce their production costs. The challenge of scalability and cost is a major issue in the development of these materials. In the same vein, unitraveling-carrier photodiodes (UTC-PDs) [10], which are designed to generate high-power THz signals, are only adequate in terms of their power conversion efficiency.  
In addition to the above, there are design challenges that arise with the antenna, thanks to the short wavelengths exhibited by THz signals [62; 63]. Miniature antennas require critical assembly procedures which contribute to the cost and development of THz instruments. Currently, there appear to be ongoing attempts to combine metamaterials with phased array technology to improve antenna system performance, but this solution has not come of age for volume production yet. A comparative summary of key THz device characteristics, including their operating temperature, detector sensitivity, output power, and modulation bandwidth, is presented in Table 7.  
\begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline
**Parameter** & **Device Type** & **Value/Range** & **Challenge** & **Cause** & **Solution** & **Citation** \\ \hline Operating Temperature & Quantum Cascade Lasers (QCLs) & \(\sim\)77 K (Cryogenic Cooling) & Cooling requirements & High thermal dissipation in QCLs & Cryogen-free designs and better thermal management & [57] \\ \hline  & Difference-Frequency Generation & Room Temperature & Limited operational efficiency & Higher energy loss at room temp & Material advancements in active regions for better efficiency & [57] \\ \hline Detector Sensitivity & Schottky Diode Mixers & \(\sim\)10–100 pW/Hz\({}^{0.5}\) & Limited responsivity & High noise levels in mixers & Noise-reduction techniques and improved materials & [59] \\ \hline  & Bolometers & Sub-pW Sensitivity & High cost & Expensive cryogenic cooling needs & Development of room-temperature bolometers & [60] \\ \hline  & Photoconductive Antennas & \(\sim\)100 fW/Hz\({}^{0.5}\) & Low efficiency & Carrier lifetime limitations & Advanced semiconductor material integration & [60] \\ \hline \hline \end{tabular}
\end{table}
Table 7: Hardware challenges in THz communication.  
The above table presents the major hardware issues in THz communication: low efficiency, high cost, and scalability. There are limitations to existing LiDAR systems, and proposed solutions, like enhanced materials and optimized photodiode designs, are offered to overcome these limitations, albeit at the cost of increased prices and manufacturing challenges.