Fading loss for Earth-to-space lasercom affected by scintillation and beam wander composite channel

When a link budget for Earth-to-space connectivity needs to be prepared for a scenario in which optical technology is used, in terms of atmospheric turbulence random degradation, the most important losses to account for are those due to scintillation and beam wander fading processes. Currently, the standard approach is to include the margins for scintillation and beam wander separately in the link budget, treating the statistical distribution of both processes independently. In essence, this approach is incorrect as the actual statistics for the received irradiance at the spacecraft are governed by the composite channel arising from the joint interaction between these two phenomena. A method to calculate a fading loss for the composite channel is developed. The calculation is performed by inverting the cumulative distribution function of the governing statistical model for a given outage probability p that the received irradiance is below a certain threshold value. A tractable approximation solution is found, and an accuracy analysis is conducted to give the limits of applicability of such an expression. Finally, the derived expression is used in a practical example framed in an Earth-to-space optical geosynchronous orbit feeder link scenario in which the fading loss approximation derived for the composite channel is found to be below 1 dB error from the exact solution for most of the range of interest, and well below 0.1 dB error near the optimum transmitter size. (C) 2020 Society of Photo-Optical Instrumentation Engineers (SPIE)