  The next phase of satellite technology is being characterized by a new
evolution in non-geostationary orbit (NGSO) satellites, which conveys exciting
new communication capabilities to provide non-terrestrial connectivity
solutions and to support a wide range of digital technologies from various
industries. NGSO communication systems are known for a number of key features
such as lower propagation delay, smaller size, and lower signal losses in
comparison to the conventional geostationary orbit (GSO) satellites, which can
potentially enable latency-critical applications to be provided through
satellites. NGSO promises a substantial boost in communication speed and energy
efficiency, and thus, tackling the main inhibiting factors of commercializing
GSO satellites for broader utilization. The promised improvements of NGSO
systems have motivated this paper to provide a comprehensive survey of the
state-of-the-art NGSO research focusing on the communication prospects,
including physical layer and radio access technologies along with the
networking aspects and the overall system features and architectures. Beyond
this, there are still many NGSO deployment challenges to be addressed to ensure
seamless integration not only with GSO systems but also with terrestrial
networks. These unprecedented challenges are also discussed in this paper,
including coexistence with GSO systems in terms of spectrum access and
regulatory issues, satellite constellation and architecture designs, resource
management problems, and user equipment requirements. Finally, we outline a set
of innovative research directions and new opportunities for future NGSO
research.
