# Machine Learning-Based Solutions for Handover Decisions in Non-Terrestrial Networks
## 1 Introduction
### Mobility Management  
In mobile communication systems, mobility management allows the control and coordination of the movement of mobile devices in a wireless network. It involves a set of protocols and mechanisms that enable mobile devices to connect and disconnect from the network, to move between different network cells or access points, and to maintain ongoing communication sessions while in motion. In addition to handling handovers, mobility management also entails controlling the identity and location of mobile devices. Mobility management seeks to maintain communication between mobile devices as they move in the network, while simultaneously making effective use of available resources and limiting the impact on overall network performance. According to the 3 GPP, for LEO NTN, mobility management procedures should be improved to take into account satellite movement-related factors such as measurement validity, user equipment (UE) velocity, movement direction, large and varying propagation delay, and dynamic neighbor cell set [13]. For GEO NTN, mobility management procedures need to be adjusted to accommodate large propagation delays. The mobility management in the non-terrestrial network is very different from the traditional network or terrestrial; In NTN, based on the architecture, we have many types of handover such as inter-satellite handover, intra-satellite handover, and inter-access network handover. However, in our study, we focus on an intra-satellite  
Figure 1: Non-terrestrial network typical scenario based on regenerative payload. handover; in our proposed architecture, both cells/beams are served by the same satellite, and no other satellite is involved in the handover process. In LEO satellite systems, intra-satellite handovers are the most typical kind of handovers encountered because of the small area covered by beams and the rapid satellite speed. Thus, we can consider the user mobility negligible compared to high satellite speed. Mobility management of LEO satellites is therefore much more challenging than GEO or MEO systems. With a few exceptions, terrestrial network systems and LEO satellite systems have somewhat comparable mobility. In both systems, the relative position between the cells and the UE changes continuously, requiring the handover of the UE between adjacent cells. In terrestrial network systems, the UE moves through the cells, while in LEO systems the cells move through the UE. The cell size of LEO satellite systems is larger compared to terrestrial network systems. Moreover, the speed of the UE can be ignored in LEO satellite systems, since that speed is negligible compared to the rotational speed of the LEO satellite. Conditional handover (also known as \"soft handover\") is a type of handover process that occurs in cellular networks. In traditional (or \"hard\") handover, a mobile device is disconnected from one cell and connected to another cell before the call is resumed. In conditional handover, the mobile device simultaneously maintains connections to both the current and the target cells 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. Conditional handover is typically used in cellular systems, but the studies on solutions for adapting NR to support NTN documented in 3 GPP TR 38.821 [13] defined 5 types of conditional Handover triggering methods such as (1) measurement-based triggering, (2) location (UE and Satellite) triggering, (3) time(r)-based triggering, (4) timing advance value-based triggering, and (5) elevation angles of source and target cells-based triggering. In non-terrestrial networks (NTNs), conditional handover is a technique used to manage the handover of mobile devices between different network nodes while maintaining a seamless communication session.