  With a large number of deep space (DS) missions anticipated by the end of
this decade, reliable and high-capacity DS communications are needed more than
ever. Nevertheless, existing technologies are far from meeting such a goal.
Improving current systems does not only require engineering leadership, but
also, very crucially, investigating potential technologies that overcome the
unique challenges of ultra-long DS links. To the best of our knowledge, there
has not been any comprehenive surveys of DS communications technologies over
the last decade. Free space optical (FSO) is an emerging DS technology, proven
to acquire lower communications systems size weight and power (SWaP) and
achieve a very high capacity compared to its counterpart radio frequency (RF),
the currently used DS technology. In this survey, we discuss the pros and cons
of deep space optical communications (DSOC) and review their physical and
networking characteristics. Furthermore, we provide, for the first time,
thoughtful discussions about implementing orbital angular momentum (OAM) and
quantum communications (QC) for DS. We elaborate on how these technologies
among other field advances including interplanetary network (IPN) and RF/FSO
systems improve reliability, capacity, and security. This paper provides a
holistic survey of DSOC technologies gathering 247 fragmented pieces of
literature and including novel perspectives aiming to set the stage for more
developments in the field.
