# Atmospheric Effects on Satellite-Ground Free Space Uplink and Downlink Optical Transmissions
## 1 Introduction  
Recently, information and communication technologies have seen tremendous growth, and with the increased use of high-speed internet, video conferencing, live streaming, and other services, bandwidth and capacity requirements are increasing [1]. This ever-increasing demand for data and multimedia services has caused congestion in the conventionally used radio frequency (RF) spectrum, requiring a shift from RF carriers to optical carriers [1]. While the use of RF spectrum is restricted, optical carriers do not require spectrum licensing, making them an attractive prospect for high-bandwidth, high-capacity applications [2]. Free-space optical (FSO) communications make use of laser beam propagation as the signal carrier, allowing wireless communications between transmit-and-receive optical terminals. By modulating data in low-divergence laser beams, the optical carrier's beam spread is much narrower than that of an RF carrier. This causes an increase in signal intensity at the receiver for a given to transmit power [3]. Other benefits of FSO communications include easy expandability and reduced sizes of network segments, being lightweight and compact, and being useful where fiber optic cables cannot be used. FSO technology is being developed for short- and long-distance terrestrial links, satellite uplink/downlink, inter-satellite links, deep-space probes to ground, and ground-air/air-ground applications. Among them, satellite-ground communication research is the most relevant these days for both industry and academia. With the advent of space-borne scientific equipment, numerous spacecraft can gather considerable volumes of data to be downloaded to a ground station [4]. However, due to limitations on the number and duration of periods in which a spacecraft is above the local horizon and available for line-of-sight (LOS) communications with a given ground station, as well as onboardmemory storage on the spacecraft, RF systems are falling short of meeting the increasing demand from spacecraft to transmit information to a ground station at high speed. To this end, laser communications can be a viable option because it provides a higher data rate than traditional RF systems. In addition, satellite-ground laser communications play a major role in 5G and beyond-5G technologies because they can extend a coverage area providing data services not coverable by terrestrial cellular networks and enabling data delivery to the edge of a network. Project Loon and Project Aquila are good examples of FSO communications used in real-world settings. Project Loon, a subsidiary of Alphabet's X, floats a balloon up to 20 km (12 miles) above the ground to deliver Internet service to rural areas [5]. Project Aquila is another emerging FSO project--a solar-powered drone developed by Facebook for use as an atmospheric satellite, serving as a relay station to provide internet access to remote areas [5]. In satellite-ground communications, to establish an accurate LOS link between the ground station and the satellite, an uplink beacon is utilized for precise acquisition and tracking. Once the signal is received, the controller logic on the satellite turns the onboard laser on for the downlink. When the downlink beam is picked up by the ground station, transmission from the onboard laser can take place. For both uplink and downlink applications, the laser beam travels through the atmosphere in which significant factors such as cloud, snow, fog, and rain can severely impact (and even terminate) the link. These factors have variable properties and cause attenuation and degradation of the received signal due to photon absorption and scattering [6]. Moreover, the beam suffers from atmospheric turbulence, which is a random phenomenon caused by temperature and pressure variations in the atmosphere along the propagation path. The turbulence in the atmosphere causes intensity fluctuations in the received signal, which is known as scintillation.