  The upcoming sixth generation (6G) wireless communication network is
envisioned to cover space, air, and maritime areas, in addition to
urban-centered terrestrial coverage by the fifth generation (5G) network, to
support intelligent Internet of Things (IoT). Towards this end, we investigate
structured integration of satellites, unmanned aerial vehicles (UAVs), and
terrestrial networks, aiming to serve future universal IoT possibly with a
massive number of devices in the coverage holes of current 5G. The hybrid
satellite-UAV-terrestrial network usually leads to high system complexity, due
to the heterogeneity and dynamics of space/air/ground links. With a systematic
thinking, we propose to create and exploit hierarchies for the integrated
network. Four basic structures are discussed by learning from the synergies in
our human body. To orchestrate multiple heterogeneous basic structures, we
further propose a process-oriented on-demand coverage method, which
characterizes the system behavior as a series of events over time and is able
to tackle the system complexity elaborately. We also outline open issues for
promoting the agility and intelligence of structured satellite-UAV-terrestrial
networks in the making.
