# Link-State Aware Hybrid Routing in the Terrestrial-Satellite Integrated Network +
## 3 System Model
### Scenario Description  
Although some terrestrial networks have been proposed to deal with the urgent requirement for massive user connections, due to the increasingly scarce resources, limited coverage range, and severe damage caused by force majeure factors, the terrestrial networks will face local congestion and failure, which affects the overall network transmission performance. Thus, the proposed solution is mainly intended to address the mobile communications scenario, which would increase the communication capacity and guarantee network connectivity in natural disasters. In this way, the data from the terrestrial terminal can be transmitted to the satellite networks conveniently and seamlessly through the TSE; therefore, reducing the burden on terrestrial communication networks. A general data transmission architecture of TSIN is presented in Figure 1, which comprises two main segments: satellite and ground. These two segments can work independently or inter-operationally, by integrating heterogeneous networks among the two segments, it is easy to build a hierarchical broadband wireless network. The satellite network is composed  
of orbit satellites \(S=\{s_{1},s_{2},\ldots,s_{M}\}\) and constellations as well as their corresponding terrestrial infrastructures (e.g., TSEs \(G=\{g_{1},g_{2},\ldots,g_{N}\}\)). The ground network mainly consists of terrestrial communication systems, such as cellular networks, wireless local area networks, and so on. The ground network is able to provide high data rates to users, but the network coverage in rural and remote areas is limited. Satellite networks can provide global coverage on the earth but have long propagation latencies. Although ground networks have the lowest transmission delay, they are vulnerable to natural disasters or artificial infrastructure damage. Mobile user terminals \(UT=\{UT_{1},UT_{2},\ldots,UT_{L}\}\) are assumed to be randomly distributed, and the session data can be transmitted from ground to space only when they are covered by specific satellites. TSEs play an important role in the process of traffic delivery from the ground network to the satellite network. It is noticed that different selections of TSEs would produce varying transmission delay. Thus, selecting the appropriate TSEs can enhance the network performance. As shown in Figure 1, there are three kinds of transmission links in TSINs:  
* Inter-satellite link (ISL): the link between satellites, the distance of which would change rapidly with the highly moving constellations.
* TSE-satellite link (TSL): the link between a TSE and a satellite. Since a satellite can cover different TSEs for a different time, the TSL is time-varying.
* Inter-terrestrial link (ITL): the link between terrestrial nodes. As the terrestrial network service equipments are usually static, the ITL distance is invariant.  
During a hybrid network communication session, the data could be transmitted through ITLs, TSLs, and ISLs at the same time. Generally, the TSL can only be established by one satellite at one time, due to the rapid movement of satellites, the communication link between one satellite and one TSE would be handed over to other satellite. Thus, the communication link of satellite-TSE is time-varying. Although the space segment is designed to provide continuous coverage to TSEs, a TSE would be covered by one satellite at one time, and it is a dynamic spatial coverage relation, i.e., a TSE needs to be served by several satellites during a communication session. Due to the time-varying TSE-satellite link, one of the most critical issues in designing a hybrid routing mechanism is to schedule the transmitted data and link resources.