# Beam-Hopping-Based Resource Allocation in Integrated Satellite-Terrestrial Networks
## 1 Introduction  
In order to support emerging applications at any time and any place, sixth-generation (6G) mobile communication networks are expected to provide ubiquitous wireless access services with high data rates [1]. Terrestrial communication systems can meet most of the traffic demands in densely populated areas, but they are unable to provide full wireless coverage in remote and isolated areas because of the lack of infrastructure. Thanks to the advantages of wide coverage and being unlimited by geographical conditions, the satellite communication system can enhance the capacity and support seamless connectivity, especially in areas with adverse environments, such as ocean, desert, and polar regions [2]. Therefore, the integrated satellite-terrestrial network (ISTN) is considered a promising network architecture to satisfy the 6G requirements [3]. With the rapid expansion of both satellite and terrestrial systems, the problem of spectrum scarcity is becoming increasingly severe [4, 5]. Spectrum sharing between satellite and terrestrial systems is a feasible way to alleviate the spectrum scarcity issue and enhance the integrated network capacity [6]. However, inter-system resource coordination and management should be carried out to avoid intensive co-channel interference (CCI) in a spectrum sharing scenario. At present, existing spectrum sharing schemes for ISTNs include database-assisted schemes [7, 8], exclusion zone (EZ)-based schemes [9, 10], cognitive radio (CR)-basedschemes [5; 11; 12; 13; 14; 15; 16; 17], and so on. The database in [7; 8] collected the frequency, channel bandwidth, and coverage of each satellite system, and the terrestrial base stations (BSs) could obtain the available frequency band by querying the database. In EZ-based schemes in [9; 10], a protection area for the satellite system was constructed to avoid CCI, where terrestrial systems could not transmit on the same frequency band. Considering the highly dynamic topology of the ISTN, database-assisted and EZ-based schemes may lack flexibility, so CR-based schemes have attracted more attention in recent years. To avoid CCI to the primary system, a secondary system can access the frequency band based on the spectrum sensing results or control the interference power below a certain threshold [5; 11; 12; 13; 14; 15]. Moreover, a terrestrial BS can act as a cognitive relay to assist the primary satellite system in exchange for spectrum access [16; 17]. However, CR-based schemes still face the problems of inaccurate channel state information and spectrum sensing results, as well as a lack of information exchange between the primary and secondary systems, which bring great challenges to satisfy the differentiated and unevenly distributed traffic demands of satellite and terrestrial systems. With the deepening of satellite-terrestrial integration, it is possible to realize centralized network control and resource management in the ISTN. A software-defined network-based framework for ISTNs was presented in [18], which constructed a unified resource pool for integrated resource management to match the demands of all terminals. The centralized spectrum management server in [19] collected cell load information and set spectrum resource ranges for both terrestrial and non-terrestrial networks to realize coordinated spectrum sharing between them. Compared with isolated spectrum management schemes, integrated resource management is expected to further improve spectrum efficiency and quality of service, as well as suppress CCI between different systems. Considering the uneven distribution of traffic demands in the ISTN, limited resources should be allocated to each system according to its practical demand. In multi-beam satellite systems, beam-hopping (BH) technology has been used to flexibly provide services to the terminals by changing beam directions in different time slots, which has proved to be more adaptive to unevenly distributed and time-variant traffic demands [20].