Modeling Atmospheric Attenuation in Vertical FSO Links: An Altitude-Aware Piecewise Framework

Free-space optical (FSO) communication is emerging as a key backhaul technology for next-generation vertical heterogeneous networks (VHetNets), whose architecture spans satellites, high-altitude platform stations (HAPSs), autonomous aerial vehicles (AAVs), and terrestrial nodes. Along these vertical and slant paths, optical beams traverse successive atmospheric layers that may contain clouds, fog, rain, and aerosols, conditions that conventional single-coefficient Beer–Lambert (BL) models typically handle only in isolation. Instead of such simplified formulas, we present a unified attenuation model that incorporates aerosols, fog, rain, cloud layers, and drizzle, accounts for the zenith angle, and provides a holistic estimate of the cumulative power loss across atmospheric layers. Numerical results show several-decibel attenuation variations across representative weather scenarios, while the difference between the proposed model predictions and the layer-resolved Moderate Resolution Atmospheric Transmission (MODTRAN) simulations remains within 1 dB, thereby validating the accuracy of the proposed model and its practical relevance for VHetNet link budget studies.