Bit error rate and average capacity of vortex gamma beams propagating in anisotropic non-Kolmogorov atmospheric turbulence with anisotropic tilt angle

In recent years, there has been significant interest in free space optical (FSO) communications, particularly through the use of various vortex beams, due to their ability to offer low bit error rate (BER) and high channel capacity. Current anisotropic non-Kolmogorov (ANK) turbulence models mostly assume that the long axis of turbulence cells is parallel to the ground. Nevertheless, recent experimental findings by Beason et al. and Wang et al. have demonstrated that these turbulence cells can exhibit an anisotropic tilt angle, which means that the long axis of turbulence cells is not always parallel to the ground but instead forms a specific angle with it. This study investigates a new type of vortex beam known as the gamma beam and its application in FSO communications. For the first time, it provides a comprehensive analysis of the average capacity and BER of the orbital angular momentum (OAM) channels for a gamma beam transmission through weak-to-strong ANK atmospheric turbulence with an anisotropic tilt angle. The turbulence cell ellipsoid model is employed, and it is demonstrated that the average capacity of OAM channels initially increases and then declines as the anisotropic tilt angle $\gamma$ increases, and the variation in average capacity is symmetric around $\gamma =90^{\circ }$. Further numerical results indicate that the FSO communication link utilizing the gamma beam can achieve higher channel capacity by selecting a lower OAM topological charge, using a narrower receiver aperture, and optimizing the initial beam waist. These findings are promising for enhancing FSO communication systems that rely on OAM.