Channel Modeling for Mobile Airborne FSO Backhauling

With its high capacity and immunity to electro-magnetic interference, free space optical (FSO) communication is positioned to be a key connectivity solution for airborne backhauling. In this paper, we consider a scenario where a fleet of high-altitude platform stations (HAPSs) follow a predefined circular trajectory and provide airborne backhauling to ground base stations. Unlike terrestrial FSO links that are primarily limited by atmospheric turbulence-induced fading, aerial links between a HAPS and a ground station are subject to fading effects induced by mobility. The instantaneous transmission distance continuously changes due to movement, leading to a time-varying atmospheric attenuation loss and geometric loss. These mobility-induced variations in the average received power effectively introduce a fading effect. In this paper, we present a statistical model for the aggregate airborne channel coefficient. First, we derive a probability density function (PDF) to describe the instantaneous changes in the propagation distances as a result of mobility. Then, we derive the PDF for the aggregate channel coefficient that includes both geometric losses and atmospheric attenuation. We present numerical results to corroborate our analytical findings and discuss the effects of several system and channel parameters on the severity of fading.