  Free-space quantum links have clear practical advantages which are
unaccessible with fiber-based optical channels --- establishing
satellite-mediated quantum links, communications through hardly accessible
regions, and communications with moving objects. We consider the effect of the
atmospheric turbulence on properties such as quadrature squeezing,
entanglement, Bell nonlocality, and nonclassical statistics of photocounts,
which are resources for quantum communications. Depending on the
characteristics of the given channels, we study the efficiency of different
techniques, which enable to preserve these quantum features---post-,
pre-selection, and adaptive methods. Furthermore, we show that copropagation of
nonclassically-correlated modes, which is used in some communication scenarios,
has clear advantages in free-space links.
