## 3 Challenges in Terahertz-Based FSO Communication
### Atmospheric Dispersion and Its Impact on THz-FSO Communication  
One of the significant challenges in THz-FSO communication that has often been overlooked is atmospheric dispersion, which further limits the usable instantaneous bandwidth and degrades high-data-rate transmissions. Dispersion arises due to the frequency-dependent propagation delay, which distorts signal waveforms and leads to pulse broadening and inter-symbol interference (ISI) [66]. The primary cause of dispersion in the THz band is the variation in the refractive index of the atmosphere due to molecular interactions with humidity, temperature fluctuations, and other environmental factors. This effect is particularly severe at higher THz frequencies, where the phase velocity varies across different spectral components, causing severe signal distortion. As the THz signal propagates through the free-space optical channel, it undergoes group velocity dispersion (GVD), which causes different frequency components to travel at different speeds, leading to the temporal spreading of the signal pulse [67]. This spreading effect reduces spectral efficiency, increases bit error rates (BERs), and severely impacts high-speed data transmission in THz-FSO systems. The combined impact of dispersion and atmospheric absorption further exacerbates the transmission loss, limiting the feasibility of long-range THz-FSO communication, particularly beyond 500 GHz, where the attenuation surpasses 100 dB/km [68]. However, research suggests that a relatively lower-loss transmission window exists at 680 GHz, which may provide a practical frequency band for optimized high-data-rate THz-FSO links [55]. \begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Studies** & **Challenge** & **Cause** & **Solution** & **Limitation** \\ \hline [28] & High distortion & Atmospheric turbulence & Adaptive equalization & Computational cost \\ \hline [12] & Scattering effects & Environmental factors & Beam-shaping techniques & Implementation complexity \\ \hline [64] & High error rates & Signal distortion & Error-correction codes & Latency overhead \\ \hline [65] & Beam divergence & Uneven wavefront distortion & Real-time sensing systems & Hardware costs \\ \hline \multirow{2}{*}{[66]} & Environmental & \multirow{2}{*}{Rapid atmospheric changes} & \multirow{2}{*}{Polarization techniques} & \multirow{2}{*}{Limited scalability} \\  & variability & & & \\ \hline [67] & Turbulence impact & High \(C_{n}^{2}\) regions & Kolmogorov turbulence & Precision challenges \\ \hline [68] & Range limitations & Attenuation and absorption & Enhanced receiver designs & Increased energy demand \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Signal distortion challenges in THz communication. Mitigation Strategies for Atmospheric Dispersion  
Several techniques have been proposed to counteract the effects of dispersion in THz-FSO communication. Adaptive modulation and equalization techniques, including machine learning-based adaptive equalizers, can be deployed to compensate for phase and frequency dispersion, ensuring real-time signal restoration [69]. Additionally, pre-compensation techniques at the transmitter can mitigate dispersion effects by shaping signals to counteract the anticipated spectral broadening along the THz-FSO propagation path [70]. Frequency selection also plays a crucial role in mitigating dispersion; for instance, choosing 680 GHz transmission windows can significantly reduce the impact of frequency-dependent propagation delays. Moreover, optical filtering techniques, particularly photonic-assisted dispersion compensation, can be employed to dynamically correct spectral broadening effects, improving the system's performance [71]. The integration of AI-driven network adaptation can further enhance THz-FSO communication by dynamically adjusting transmission parameters to optimize the spectral efficiency under varying atmospheric conditions [1].