## 3 Challenges in Terahertz-Based FSO Communication
### Line-of-Sight (LOS) Alignment  
Precise line-of-sight (LOS) alignment is fundamental, in THz-FSO communication systems, to maintain signal integrity and prevent beam divergence. Due to the short wavelengths of THz signals, even minor misalignments can result in significant power losses [50; 53]. The received power (\(P_{r}\)) is typically calculated using the Friis transmission equation:  
\[\mathrm{P_{r}}=\mathrm{P_{r}G_{t}G_{r}}\left(\frac{\lambda}{4\pi\mathrm{d}} \right)^{2} \tag{10}\]  
where \(\mathrm{P_{t}}\): the transmitted power (\(\mathrm{W}\)); \(\mathrm{G_{t}},\mathrm{G_{r}}\): the gains of the transmitting and receiving antennas; \(\lambda\): the wavelength (\(\mathrm{m}\)); \(\mathrm{d}\): the distance between the transmitter and receiver (\(\mathrm{m}\)). \begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Studies** & **Challenge** & **Cause** & **Proposed Solution** & **Limitation** \\ \hline \([16]\) & Signal attenuation & Water vapor absorption & Photonic crystal fibers & High fabrication cost \\ \hline \([54]\) & High absorption & Atmospheric composition & Low-loss materials & Limited bandwidth \\ \hline \([55]\) & Reduced range & Resonance of water vapor molecules & Frequency selection & Bandwidth constraints \\ \hline \([56]\) & Absorption peaks & Atmospheric pressure variations & Metamaterial-based designs & Manufacturing costs \\ \hline \([57]\) & Limited range & Scattering and absorption & Dynamic path optimization & Energy overhead \\ \hline \([58]\) & Variable losses & Environmental changes & Real-time sensing systems & Algorithm complexity \\ \hline \([57]\) & Bandwidth reduction & Selective frequency operation & Dynamic reconfigurations & System overhead \\ \hline \([59]\) & Environmental sensitivity & Temperature and pressure effects & Advanced atmospheric models & Limited precision in dynamic conditions \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Atmospheric absorption challenges in THz communication. The equation also indicates that the power received varies inversely with the square of the distance at a specific frequency; hence, high-frequency alignments require high accuracy. Additionally, the beam divergence (\(\Theta\)) is a critical factor that depends on the aperture size and wavelength. A slight divergence angle ensures a focused energy transfer but increases the sensitivity of the signal to environmental perturbations such as turbulence and vibrations. Some mitigation strategies for this sensitivity are as follows:  
* Adaptive beam steering: real-time adjustment of the beam direction to maintain alignment, even in dynamic scenarios;
* Vibration compensation systems: mechanisms that counteract environmental vibrations and platform instability;
* Advanced optics: the utilization of lens arrays and phased array antennas to improve the beam focusing and alignment precision. Table 6 presents the major problems related to LOS alignment for THz-FSO communication systems, considering their causes, expected solutions, and corresponding demerits. Techniques like the use of adaptive optics, phased array antennas, and vibration compensation systems make a system more robust in dynamic environments.