# Cooperative Caching and Resource Allocation in Integrated Satellite-Terrestrial Networks
## 3 System Model
### Cooperative Caching and Resource Allocation  
Different from terrestrial networks, similar to the existing related work in [9; 44], the cooperative caching and resource allocation problem is discussed to reduce the problem complexity in a quasi-static scenario. Specifically, a time-varying satellite network is divided into several segments by a division approach based on time slots, where the network topology remains unchanged within each time slot but can vary over different time slots. Furthermore, it is assumed that the caching service strategies are unchanged, and the service requirements can be also guaranteed during the service periods.  
The procedure of cooperative caching and resource allocation in dynamic environment can be illustrated as follows. Initially, the ground station can abstract, manage and orchestrate all the available physical resources of an integrated satellite-terrestrial network by introducing software-defined network and network virtualization technology [24; 42]. Then, in order to describe the proposed system model more clearly, it is assumed that all of the information concerning the content services of new users can be accessed through the integrated satellite-terrestrial network via their access satellites at the beginning of each time slot, and the strategies can be made and deployed during the current time slot. However, considering the dynamic movement of satellites, the caching content on satellites will be replaced over different time slots. In that case, a maximum threshold time for the onboard cache contents not being accessed can be pre-set. The cache contents on satellites will be released, and the occupied storage resources will be reallocated for new contents after the time is more than the maximum threshold.  
However, different strategies of cooperative caching and resource allocation can have a significant impact on network bandwidth consumption and storage resource usage. Therefore, from the perspective of network operators, the problem is formulated to minimize the weighted sum of storage resource usage and network bandwidth consumption while content services should be provided for users as much as possible.  
An example of a cooperative-content service in an integrated satellite-terrestrial network is shown for different users in Figure 2, where there are 7 users \(\{u_{1},u_{2},\cdots,u_{7}\}\), 6 satellites \(\{v_{1},v_{2},\cdots,v_{6}\}\), 6 contents \(\{w_{1},w_{2},\cdots,w_{6}\}\), and one cloud center \(dc\), the users are randomly located in the coverage of the satellites, and the access satellite for cloud center \(dc\) is viewed as \(v_{5}\). In the beginning, the ground station can sense the information concerning user service requirements in real-time and then make the joint strategies of cooperative caching and resource allocation based on network running states and user service requirements. All of the current contents are deployed on the cloud center, and just parts of the contents can be placed on satellites due to the limited onboard storage resources. In that case, users can retrieve the required contents from the integrated satellite-terrestrialnetwork by the obtained joint strategies. Specifically, user \(u_{1}\) can retrieve the caching content \(w_{2}\) from the access satellite \(v_{1}\). As the access satellite \(v_{4}\) for user \(u_{2}\) can not deploy the required content \(w_{1}\), user \(u_{2}\) will retrieve the content \(w_{1}\) from satellite \(v_{1}\) by the path \(v_{1}\to v_{4}\). Similarly, user \(u_{3}\) retrieves the required content \(w_{4}\) from satellite \(v_{3}\) by the path \(v_{3}\to v_{2}\), and user \(u_{7}\) retrieves the required content \(w_{5}\) from cloud center \(dc\) by the path \(dc\to v_{5}\to v_{6}\).