# Cooperative Caching and Resource Allocation in Integrated Satellite-Terrestrial Networks
## 3 System Model
### Integrated Satellite-Terrestrial Network  
An integrated satellite-terrestrial network is considered to discuss the cooperative caching and resource allocation problem, where the integrated satellite-terrestrial network can consist of LEO satellites, a ground station, and a cloud center, as shown in Figure 1. Each LEO satellite has an edge server with certain storage resources and can provide edge caching services with particular contents for terminal users, different LEO satellites are connected by inter-satellite links (ISLs) to route the data in a cooperative way. A cloud center on the ground with sufficient storage resources can provide remote content services with all contents for terminal users. A ground station is responsible for resource management, strategy selection, and caching placement in the integrated satellite-terrestrial network, where the cloud center can communicate with the ground station by terrestrial networks and satellite satellites. Note that the detailed network implementation is beyond the scope of our work and will be neglected in this paper, such as channel design, network protocol, and information interaction.  
The integrated satellite-terrestrial network is composed of a satellite network \(G(V,E)\) and a cloud center \(dc\), where \(V\) and \(E\) are indicated as the set of satellites and the set of ISLs. Satellite network \(G(V,E)\) is built based on a central satellite and \(h\) hops, where the central satellite will be predetermined and unchanged over time when the network is established. For each satellite \(v\), \(v\in V\), there are two ISLs from the in-orbit adjacent satellites and two ISLs from the inter-orbit adjacent satellites [21], and the available storage resources are indicated as \(C_{v}\). For each link \(e\), \(e\in E\), the available bandwidth resources and the transmission delay can be indicated as \(B_{e}\) and \(t_{e}\), where the time of transmitting and receiving the content for each satellite can be neglected in order to simplify the system complexity. Considering the limited available resources of satellites, satellite \(v\) can just provide content services for the maximum number \(D_{v}^{max}\) of users. The sets of input degrees and output degrees for satellite \(v\) can be indicated as \(E_{v}^{in}\) and \(E_{v}^{out}\), respectively. Furthermore, it is assumed that the available storage and satellite-terrestrial bandwidth resources for cloud center \(dc\) are sufficient, and the cloud center is also in the coverage of access satellite \(v_{dc,a}\), where the distance between cloud center \(dc\) and access satellite \(v_{dc,a}\) is indicated as \(k_{dc}\).  
Considering that the operation costs will correspondingly increase as the number of satellites increases, a large LEO satellite network can be divided into multiple small satellite networks by a network partitioning approach. Several ground stations are distributed to manage the small network resources and provide content services in parallel. In that case, the centralized satellite-terrestrial network framework proposed in this paper can  
Figure 1: Satellite–terrestrial network framework for cooperative content service.  
be also extended to a distributed satellite-terrestrial network framework to decrease the operation costs.