# Link-State Aware Hybrid Routing in the Terrestrial-Satellite Integrated Network +
## 6 Conclusions  
In this paper, to provide seamless integration of the terrestrial network and satellite network, a link-state aware terrestrial-satellite integrated network hybrid routing algorithm is proposed for data transmission in the dynamic TSIN. To represent the data transmission process, a space-time topology model is proposed. The link resources in both the spatial and temporal dimensions are quantified by dividing them into discrete time intervals. Then, based on the space-time topology, with the objective of minimizing the end-to-end delay, the routing problem of link selection is formulated as an integer linear programming problem. To obtain the data transmission path with the minimum delay, we propose the relationship function between the time interval and the satellite constellation parameters, and further propose a hybrid routing algorithm based on the bipartite graph model. Simulation results show that the proposed hybrid routing algorithm can achieve the best performance in terms of end-to-end delay, packet loss rate, and throughput.  
It is worthwhile to mention that the introduced concept of hybrid routing and the space-time topology graph is possible to be extended into other relevant satellite link connection design problems, e.g., in satellite-enabled wireless collaborative transmitting, mission scheduling of mobile edge computing, and sensing. In addition, for future 6G heterogeneous networks, UAVs/HAPs would communicate with ground and satellite stations, forming a space-air-ground network facing frequent satellite-UAV/HAPs-ground link switching. The dynamic 6G heterogeneous networks can be represented by the proposed space-time topology graph, in which the collaborative communication with each other can be described via the edges between them. During the design of the topology model from the transmission period, we derive an optimal time interval approach for the topology relationship, which has a high potential in addressing other satellite link selection problems. Moreover, the methodology of this work in handling the time-varying and limited inter-satellite link capacity can be extended to carry out more designs for TSINs. For our future work, we will explore the hybrid routing for serving the communication for massive Internet-of-Thing equipment. In order to provide ubiquitous and saleable networks, we also further explore UAV/HAPs mobility to construct the graph-based routing approach in the 6G heterogeneous network.  
Figure 13: End-to-end delay in congestion situation.  
**Author Contributions:** Conceptualization, H.X. and Z.S.; data curation, H.X. and Z.S.; formal analysis, G.H. and M.L.; funding acquisition, N.Z.; investigation, Y.Y.; methodology, G.H. and H.X.; project administration, H.X.; resources, Z.S.; validation, M.L. and H.X.; writing--original draft, Y.Y. and H.X.; writing--review and editing, Z.S., N.Z. and M.L. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by China Postdoctoral Science Foundation grant number 2020M672412.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Conflicts of Interest:** The authors declare no conflict of interest.