  Integrating high altitude platforms (HAPs) and free-space optical (FSO)
communications is a promising solution to establish high data rate aerial links
for the next-generation wireless networks. However, practical limitations such
as pointing errors and angle-of-arrival (AOA) fluctuations of the optical beam
due to the orientation deviations of hovering HAPs make it challenging to
implement HAP-based FSO links. For a ground-to-HAP FSO link, tractable,
closed-form statistical channel models are derived in this paper to simplify
the optimal design of such systems. The proposed models include the combined
effects of atmospheric turbulence regimes (i.e., log-normal and gamma-gamma),
pointing error induced geometrical loss, pointing jitter variance caused by
beam wander, detector aperture size, beam-width, and AOA fluctuations of the
received optical beam. The analytical expressions are corroborated by
performing Monte-Carlo simulations. Furthermore, closed-form expressions for
the outage probability of the considered link under different turbulence
regimes are derived. Detailed analysis is carried out to optimize the
transmitted laser beam and the field-of-view of the receiver for minimizing
outage probability under different channel conditions. The obtained analytical
results can be applied to finding the optimal parameter values and designing
ground-to-HAP FSO links without resorting to time-consuming simulations.
