# Machine Learning-Based Solutions for Handover Decisions in Non-Terrestrial Networks
## 1 Introduction
### Mobility Management
In an NTN, the network architecture and infrastructure are different from traditional terrestrial networks, which can make handover more challenging. For example, in a satellite-based NTN, a mobile device may need to be handed over between different satellites or between a satellite and a ground station. Moreover, cells in terrestrial networks are stationary, although UE may be movable along various trajectories. As a result, to choose the appropriate target cell for each UE, the network needs the measurement report from the UE. The situation is very different in NTN, particularly for LEO satellites where the cells/beams are moving over time, and the high speed of LEO satellites will result in frequent handovers and high handover rates of a large number of UE. We considered the simulation in the earth-moving beams scenario; in NTN we have earth-fixed cells/beams and earth-moving cells; however, in our study, we did not consider earth-fixed beams. Therefore, we considered earth-moving cells where cells are moving on the ground; therefore, the NTN platforms and the beams are all moving but not at the same speed. One of the most important steps for the Handover is the measurement report from UE before the handover as shown in Figure 2. In NTN, the propagation delay is orders of magnitude more than in terrestrial systems, adding extra latency to mobility signaling activities including measurement reporting, HO command receiving, and HO request/ACK. As shown in Figure 2, the interruption time for the downlink can be calculated as the period of time between the network transmitting RRCReconfiguration with sync (Step 3) and the Target gNB receiving RRCReconfigurationComplete (Step 6). After step 3, the gNB is unable to communicate any further data; however, it can proceed once it has received the signal RRCReconfigurationComplete. The UE may continue sending data to the source gNB until RRCReconfiguration with sync is received (Step 6). Note that, the gNB is a 3 GPP 5G Next Generation base station that supports the 5G New Radio. As a kind of solution, 3 GPP defines several sets of the predefined set of measurement report mechanisms to be performed by UE. This predefined measurement report type is called Event, such as Event A3, which means the neighbor becomes offset better than serving, or A4, the neighbor becomes better than the threshold [13]. When certain conditions are met, the new handover procedure introduced in Release-16 by 3 GPP allows the user equipment (UE) to decide whether to perform the handover or not. The previous handover procedure, which was reactive and prone to handover failures, left it up to the network to decide whether to perform the handover. Therefore, the 3 GPP introduced the conditional handover (CHO) feature in 5G-NR Release-16, allowing UE to choose whether to conduct a handover when specific criteria are satisfied. Ideally, the fundamental handover process is still the same as it was in the basic handover procedure: UE transmits the measurement report to the source cell along with the neighbor cell PCI and signal strength (usually the reference signals received power), source cell decides to begin the handover procedure to best target cell, and target cell completes the handover operation. The conditional handover (CHO), as shown in Figure 2, is a handover that the UE performs when one or more handover execution conditions are satisfied. When receiving the CHO configuration, the UE begins evaluating the execution condition(s), and after a handover is carried out, the UE ceases analyzing the execution condition(s). In a conditional handover, the mobile device will maintain connections to both the current and the target network nodes for a short period of time, allowing the system to evaluate the quality of the new connection before committing to the handover. This can help to minimize the interruption of the call and increase the chances of a successful handover.