# Free Space Optical Communication: An Enabling Backhaul Technology for 6G Non-Terrestrial Networks
## 2 Fundamentals of FSO Signal Transmission
### Pointing Error
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **HAPS-Ground Links** & \multicolumn{2}{c}{**UAV-Ground Links**} \\ \hline \hline \multirow{7}{*}{**Factors /Reasons**} & \multirow{7}{*}{HAPS platforms may experience altitude variations, or the effect of winds at high altitudes, leading to platform movements that can cause misalignment between the transmitter and receiver.} & \multirow{7}{*}{Rotary wing UAVs generate lift by pushing air down, resulting in inherent platform movements, vibrations, and fluctuations in position, altitude, and orientation, introducing significant pointing errors.} & \multirow{7}{*}{HAPS only sensors to measure their motion and attitude, but these sensors can have inherent inaccuracies.} & \multirow{7}{*}{Rotary wing UAVs generate lift by pushing air down, resulting in inherent platform movements, vibrations, and fluctuations in position, altitude, and orientation, introducing significant pointing errors.} & \multirow{7}{*}{HAPS only sensors to measure their motion and attitude, but these sensors can have inherent inaccuracies.} & \multirow{7}{*}{Rotary wing UAVs generate lift by pushing air down, resulting in inherent platform movements, vibrations, and fluctuations in position, altitude, and orientation, introducing significant pointing errors.} & \multirow{7}{*}{HAPS only sensors to measure their motion and attitude, but these sensors can have inherent inaccuracies.} & \multirow{7}{*}{Rotary wing UAVs generate lift by pushing air down, resulting in inherent platform movements, vibrations, and fluctuations in position, altitude, and orientation, introducing significant pointing errors.} & \multirow{7}{*}{HAPS only sensors to measure their motion and attitude, but these sensors can have inherent inaccuracies.} & \multirow{7}{*}{Rotary wing UAVs generate lift by pushing air down, resulting in inherent platform movements, vibrations, and fluctuations in position, altitude, and orientation, introducing significant pointing errors.} & \multirow{7}{*}{HAPS only sensors to measure their motion and attitude, but these sensors can have inherent inaccuracies ( yaw, pitch, roll) can lead to pointing errors.} & \multirow{7}{*}{HSTO beams have a finite divergence angle, meaning that the spot of the received beam for such a relatively short link may not have a uniform distribution and may not be comparably large with the detector’s aperture.} & \multirow{7}{*}{Adaptive algorithms can dynamically adjust the beam pointing direction in response to changing platform dynamics and pointing errors. These algorithms can help compensate for uncertainties and improve the robustness of the link.} & \multirow{7}{*}{Beam steering or scanning techniques widen the field of view and increase the chances of maintaining a link even with pointing errors. By dynamically adjusting the beam direction within a wider range, the system can adapt to the UAV’s movements and maintain connectivity.} & \multirow{7}{*}{Deploying multiple ground stations or using multiple UAVs can provide redundancy and diversity, allowing for alternative communication paths and improving link reliability in the presence of pointing errors.} & \multirow{7}{*}{Deploying multiple ground stations or using multiple UAVs can provide redundancy and diversity, allowing for alternative communication paths and improving link reliability in the presence of pointing errors.} \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Pointing errors in HAPS-to-ground and rotary-wing UAV-to-ground FSO Links. in another. This is known as single-sided pointing error [101]. In contrast, in airborne FSO links, both \(x\) and \(y\) axes are likely to experience considerable displacement, due to the dynamic movements of the UAV. This is known as double-sided pointing error [101]. Assuming Independent and Identical Distributed (i.i.d) Gaussian displacements along the \(x\) and \(y\) axes with zero mean and a variance of \(\sigma_{s}^{2}\), the PDF of the pointing error coefficient \(I_{P}\) is given by [102].