# Atmospheric Effects on Satellite-Ground Free Space Uplink and Downlink Optical Transmissions
## 1 Introduction
The scintillation index is determined by the refractive index structure parameter, denoted as \(\mathrm{C}^{2}\mathrm{n}\). The strength of turbulence in the atmosphere is determined by this parameter. \(\mathrm{C}^{2}\mathrm{n}\) varies depending on several factors, including geographic location, weather conditions, and time of day [7; 8; 9]. In satellite-ground FSO scenarios, there is a big difference between uplink and downlink, which is caused by completely different link geometry. Regarding uplink, turbulence is only present in the very short slice at the beginning of the path. No turbulence will interfere with the propagation of the signal beyond the top layers of the atmosphere. Because the beam is distorted early in its route through the atmosphere before propagating in free space, the effect of scintillation on uplink is more severe than on downlink. On downlink, the laser beam covers the first 99% of the total distance without interruption. The transmitter's wavefront properties alone determine its divergence, and the turbulence is concentrated at the end of the propagation route [10]. The scintillation index, the Fried parameter, and the Greenwood frequency (the reciprocal of coherence time) are the three key atmospheric-turbulence characteristics for any future satellite-ground link design because they significantly affect the link budget [11]. Previous FSO communication studies widely explored FSO channel characteristics. [PERSON] and [PERSON] [12] addressed many FSO link concerns from the perspective of communication theory. They discussed various losses experienced in terrestrial FSO communications, information about FSO transceivers, channel coding, and modulation, as well as strategies to lessen the effects of air turbulence on fading. However, terrestrial FSO communications are the focus of most of their work. [PERSON] et al. [13] evaluated appropriate laser wavelength selection for FSO transmission considering atmospheric attenuation, including absorption and scattering by molecules and aerosols. [PERSON] [14] examined the impact of adverse weather on an FSO link and used cases for three optical transmission windows (850 nm, 1310 nm, and 1550 nm) of an FSO connection in the presence of attenuation caused by fog, snow, rain, and scattering. Furthermore, research was conducted on link budget analyses of free-space optical satellite networks showing appropriate system models for the links and to study link transmission power at different distances and at various elevation angles on uplink/downlink [15]. [PERSON] [16] analyzed measurement data and the selection of theoretical models for LEO downlink scenarios. However, these conventional works still did not give readers a thorough analysis of the correlation between channel parameters such as coherence length, zenith angle, and the refractive index structure parameter for both uplink and downlink scenarios. Therefore, in this paper, we examine the impacts of the atmosphere on FSO uplink and downlink communications, considering the correlation between channel parameters. Various atmospheric influences are considered by presenting mathematical equations for atmospheric factors such as fog attenuation coefficients, refractive index parameters, interference lengths, turbulence models, and variations in the angle of arrival. Unlike conventional FSO channel research, which was focused mostly on variations in atmospheric turbulence and changing the wavelength, we analyzed variations in atmospheric turbulence according to coherence length, zenith angle, and refractive index structure parameter. FSO uplink and downlink channels were investigated by simulating various channel conditions, and our experimental results show that using a 1550 nm wavelength is preferred to mitigate the effects of turbulence, while a larger receiver aperture reduces the angle of arrival fluctuation.