# Time Series Irradiance Synthesizer for Optical GEO Satellite Downlinks in 5G Networks
## 1 Introduction  
Next generation networks such as Long Term Evolution (LTE) Advanced Pro, 5G are expected to have rigorous specifications concerning the connectivity so that every user globally can connect to high bandwidth mobile internet for accessing a variety of services including live video streaming, remote healthcare and tutorship, Internet of Things (IoT), Vehicle-to-Vehicle (V2V) and Device-to-Device (D2D) communications [1; 2; 3; 4]. Satellite communications in the upcoming 5G networks are planned to alleviate multimedia data traffic congestion by providing edge delivery services, effectively establishing fixed 5G backhaul links between terrestrial nodes to infrastructures and to optimize the backhaul links for high mobility means of transport, such as trains, airplanes and ships, by heavily improving the handover and user experience [1; 2; 3; 4]. Future 5G satellite backhaul links operating along with terrestrial networks will ensure provision of broadband and highly reliable connectivity, full coverage as well as minimal delay by offering efficient data caching capabilities as proposed by the 3 rd Generation Partnership Project (3 GPP) standards [3; 4]. Therefore, in order to comply with the 5G standards and key performance indicators, satellite communication systems are evolving to satisfy the needs of the increasing demands in terms of bandwidth and capacity of the growing number of connected users and devices. Optical communication technology constitutes a flexible and beneficial solution for satellite systems as it manifests significant properties: relatively easy and quick deployment, tremendous amounts of bandwidth that can reach hundreds of terabits/s, no spectrum usage permissions needed, point-to-point technology due to the very narrow transmitted laser beam, thus making free space optics (FSO) suitable for 5G backhauling and gateway connections [5; 6]. Unguided light beam propagation however suffers from numerous adverse atmospheric phenomena and most importantly by the occurrence of fog and clouds [2; 5; 6]. While rain is the dominant attenuation factor for a mmWave link inducing losses up to 50 dB/km for a 150 mm/h rain rate [2], in the case of an optical link fog losses can reach even 350 dB/km due to the comparable fog droplet size to the optical wavelength [5]. Moreover, dense clouds and mostly cirrus clouds may completely block Line of Sight (LOS) communication [7; 8; 9; 10]. Therefore, mitigation techniques are employed, such as hybrid Free Space Optics/Radio Frequency (FSO/RF) systems, site diversity through an interconnection of optical ground stations (OGSs) or high-altitude platforms (HAPs) and wavelength diversity where larger wavelengths in the order of 10 um are used, for example, 1550 nm [7; 8; 9; 10]. The Earth's atmosphere still remains a challenging environment for optical carriers, even for large values of atmospheric transmittance due to atmospheric or clear air turbulence [5; 6; 11]. Turbulence in the atmosphere is caused by variations in wind speed, pressure and temperature that subsequently cause spatial and temporal variations in the refractive index [5; 6; 11]. These turbulence effects are exhibited as small-scale fluctuations of the received irradiance known as scintillation and as beam effects, such as beam spreading and beam wander. Scintillation is the primary signal degradation factor in the downlink case while the beam aberrations are considered negligible. Physical layer methods like aperture averaging and adaptive optics are among the most applied techniques for mitigating scintillation. All in all, the optical system suffers from pointing and tracking errors caused by weak earthquakes, equipment vibrations and optical misalignments [1; 5; 6; 11]. Many theoretical models are used to accurately analyze the downlink scintillation effects, but the most common are the log-normal distribution for weak turbulence conditions, gamma-gamma distribution for strong turbulence and double-Weibull for moderate to strong intensity [1; 5; 6; 12].