  Free-space ground-to-ground links will be an integral part of future quantum
communication networks. The implementation of free-space and fiber links in
daylight inter-modal configurations are however still hard to achieve, due to
the impact of atmospheric turbulence, which strongly decreases the coupling
efficiency into the fiber. In this work, we present a comprehensive model of
the performance of a free-space ground-to-ground quantum key distribution (QKD)
system based on the efficient-BB84 protocol with active decoy states. Our model
takes into account the atmospheric channel contribution, the transmitter and
receiver telescope design constraints, the parameters of the quantum source and
detectors, and the finite-key analysis to produce a set of requirements and
optimal design choices for a QKD system operating under specific channel
conditions. The channel attenuation is calculated considering all effects
deriving from the atmospheric propagation (absorption, beam broadening, beam
wandering, scintillation, and wavefront distortions), as well as the effect of
fiber-coupling in the presence of a partial adaptive correction with finite
control bandwidth. We find that the channel fluctuation statistics must be
considered to correctly estimate the effect of the saturation rate of the
single-photon detectors, which may otherwise lead to an overestimation of the
secret key rate. We further present strategies to minimize the impact of
diffuse atmospheric background in daylight operation by means of spectral and
temporal filtering.
