Subcarrier M-PAM using vortex Gaussian beams for aeronautical FSO links in weak non-Kolmogorov turbulence.

By employing the Rytov method, the off-axis scintillation index for vortex Gaussian beams is analytically derived for atmospheric vertical links in weak non-Kolmogorov turbulence channels. Using the off-axis scintillation index together with the log-normal distribution, the performance of this beam, both in terms of the off-axis scintillation index and the average bit error rate (⟨BER⟩) of subcarrier M-pulse amplitude modulation (M-PAM), is investigated and evaluated for aeronautical vehicle laser communication uplinks and downlinks. The ⟨BER⟩ performance of the subcarrier M-PAM, defined as the symbol error probability, is expressed through the performance metrics of laser communication systems. The off-axis scintillation index obtained from the plots as a function of source size and propagation distance is used to illustrate the ⟨BER⟩ performance of the subcarrier. The results show that, when comparing vortex Gaussian beams with non-vortex Gaussian beams, the vortex beam is more affected during upward propagation and less affected during downward propagation. Furthermore, the vortex Gaussian beam exhibits significant advantages in vertical links as the topological charge increases, particularly in its ability to tolerate differences between Kolmogorov and non-Kolmogorov turbulence conditions affecting the beam models.