# Link-State Aware Hybrid Routing in the Terrestrial-Satellite Integrated Network +
###### Abstract  
In this paper, we study data transmission in the Terrestrial-Satellite Integrated Network (TSIN), where terrestrial networks and satellites are combined together to provide seamless global network services for ground users. However, efficiency of the data transmission is limited by the time-varying inter-satellite link connection and intermittent terrestrial-satellite link connection. Therefore, we propose a link-state aware hybrid routing algorithm, which selects the integrated data transmission path adaptively in this paper. First of all, a space-time topology model is constructed to represent the dynamic link connections in TSIN. Thus, the transmission delay can be analyzed accordingly, and the data transmission problem can then be formulated. To balance the effectiveness and accuracy of searching a hybrid path, we carefully discuss the optimization of space-time topology updating, and propose an inter-satellite link selection algorithm. For the terrestrial-satellite link in hybrid routing, the data transmission problem is transformed into a weighted bipartite graph matching problem and solved with a Kuhn-Munkres-based link selection algorithm. To verify the effectiveness of our proposed routing algorithm, extensive simulations are conducted based on a realistic Hongyun constellation project. Results show that the network performance is improved with respect to data transmission delay, packet loss rate, and throughput. t 2022202  
networks and terrestrial networks, TSIN has become a promising solution for providing seamless global network services, and mitigating network congestion, especially when the traffic exceeds capacity of the terrestrial links [10]. In most planned TSINs, satellite systems are operated with dedicated ground stations for data transmission, which means that with the limited number of ground infrastructures, the satellite communication system would work in a store-and-forward mode for data exchange [11]. Hence, the increasing data volume could cause high transmission delays or serious traffic congestion. With the development of satellite communication techniques, Terrestrial-Satellite gateway Equipments (TSEs) can communicate with satellites directly and be regarded as satellite access points in data transmission [12]. Given a wide distribution of smart TSEs in the near future, the hybrid routing in TSIN becomes a promising solution to mitigate the traffic burden of satellite or terrestrial networks, which however, is quite challenging due to the following reasons. First, the TSE-satellite link connections are intermittent and dynamic; thus, the throughput of the data transmission will be reduced. Second, due to the rapid satellite movement, network topology needs to be updated frequently. If the time interval of topology updating is too large, the link delay will be inaccurate, which results in suboptimal path planning, while the short value of the time interval will lead to an increase of network overhead. Finally, the unbalanced transmission resources of TSIN could cause local communication congestion, and finally degrade the transmission efficiency. With the increasing demand of seamless global communication services for UTs, the TSIN routing strategy has attracted more and more attention in recent years. Basically, the routing methods can be classified into relay routing [13; 14] and extended terrestrial routing [15; 16; 17]. In the relay routing method, satellites act as relay forwarding nodes, which are interconnected through ground stations. Therefore, short delay can hardly be guaranteed because of the data forwarding dependence on the limited number of ground stations. For the extended terrestrial routing mode, terrestrial routing algorithms are extended to the satellite networks. However, due to the rapid movement of satellites, the communication link connection should be updated frequently.