  Terrestrial communication networks mainly focus on users in urban areas but
have poor coverage performance in harsh environments, such as mountains,
deserts, and oceans. Satellites can be exploited to extend the coverage of
terrestrial fifth-generation (5G) networks. However, satellites are restricted
by their high latency and relatively low data rate. Consequently, the
integration of terrestrial and satellite components has been widely studied, to
take advantage of both sides and enable seamless broadband coverage. Due to the
significant differences between satellite communications (SatComs) and
terrestrial communications (TerComs) in terms of channel fading, transmission
delay, mobility, and coverage performance, the establishment of an efficient
hybrid satellite-terrestrial network (HSTN) still faces many challenges. In
general, it is difficult to decompose a HSTN into a sum of separate satellite
and terrestrial links due to the complicated coupling relationships therein. To
uncover the complete picture of HSTNs, we regard the HSTN as a combination of
basic cooperative models that contain the main traits of satellite-terrestrial
integration but are much simpler and thus more tractable than the large-scale
heterogeneous HSTNs. In particular, we present three basic cooperative models,
i.e., model X, model L, and model V, and provide a survey of the
state-of-the-art technologies for each of them. We discuss future research
directions towards establishing a cell-free, hierarchical, decoupled HSTN. We
also outline open issues to envision an agile, smart, and secure HSTN for the
sixth-generation (6G) ubiquitous Internet of Things (IoT).
