# Caching-Aware Intelligent Handover Strategy for LEO Satellite Networks
## 3 Caching-Aware Intelligent Handover Strategy  
### Handover Flow  
This paper proposes an inter-satellite handover strategy, where the handover decisions are made by the serving satellite, and handover flow is shown in Figure 6. Firstly, the user periodically reports the user's location to the serving satellite, and the satellite calculates the remaining service time according to the user's location. At the same time, each satellite will periodically send its resource information to adjacent satellites, mainly including the number of idle channels and the remaining caching capacity. For example, at a certain time slot, the serving satellite will receive resource information from adjacent satellites and make handover decisions according to the user location information and the satellites resource information. In detail, the trained DRL network is adopted to decide when will the handover be executed and the candidate satellite. If the handover procedure is activated, the serving satellite will send the handover request to the optimal candidate satellite. Then, the optimal candidate satellite applies for the resources. If the resources are sufficient, it will send the handover response to the serving satellite, who is caching the unsent user's data and sends the handover notification to the user. From then on, the user and the candidate satellite can establish the communication link, and the handover can be completed. If the candidate satellite has no remaining resources, or the serving satellite's caching is overflow or the link is interrupted, handover failure occurs.  
Based on the handover signaling flow chart, we will analyze the handover latency. The latency of this process is composed of propagation latency and data transmission latency, which is ignored here as the signaling transmission latency is much smaller than the propagation latency. Firstly, the signaling interaction from user to satellite for reporting location information and information exchange among adjacent satellites occurs periodically, so this part of the signaling overhead is not counted as part of the handover delay. When the serving satellite decides to activate handover based on the state of the environment, it interacts with the optimal candidate satellite for the handover request signaling. The request and response signaling is carried out on the inter-satellite link, and the maximum signaling time between two satellites at a time is approximately 17 ms, as can be calculated from the STK simulation environment. When the serving satellite finishes caching the unsent data, it sends a handover command to the ground user, and the propagation delay from satellite to ground user is about 3.3 ms. The user receives the handover command and establishes a connection with the optimal candidate satellite. Then, the handover process is finished. This process involves two Earth-satellite link transmission and three inter-satellite link transmissions. The total propagation delay of the process is 57.6 ms.  
Here, we should note that there will be several reasons that will cause satellite handover. Besides the satellite movement, we also focus on the following two situations in this paper:  
* When the new coming user asks for access, the communication links of the connected users may be reset from the serving satellite that has no idle channels to another candidate satellite. Thus, the channels can be released for the new coming users.
* If the remaining caching capacity is less than the amount of data that will be sent by the users, the handover cannot be carried out. Otherwise, the handover will fail, and it will result in packet loss and a sharp decline in user experience.  
Figure 6: Handover flow chart based on serving satellite decision.  
Moreover, the handover decisions will be made by considering the joint-effects of several attributes, such as remaining service time, remaining idle channels, and remaining caching capacity, to obtain the best overall system performance.