  The next frontier towards truly ubiquitous connectivity is the use of Low
Earth Orbit (LEO) small-satellite constellations to support 5G and Beyond-5G
(B5G) networks. Besides enhanced mobile broadband (eMBB) and massive
machine-type communications (mMTC), LEO constellations can support
ultra-reliable communications (URC) with relaxed latency requirements of a few
tens of milliseconds. Small-satellite impairments and the use of low orbits
pose major challenges to the design and performance of these networks, but also
open new innovation opportunities. This paper provides a comprehensive overview
of the physical and logical links, along with the essential architectural and
technological components that enable the full integration of LEO constellations
into 5G and B5G systems. Furthermore, we characterize and compare each physical
link category and explore novel techniques to maximize the achievable data
rates.
