## 2 Related Work
### Handover  
The terminal node is connected to the base station that has expanded the cell site at the current location and uses the communication service. Connected terminal nodes may stay in one location. However, there are cases where it continues to move. In this case, the location may be moved from a source cell to an adjacent cell, or the adjacent cell may be a cell of a base station other than a cell expanded from one base station. Handover occurs when moving from one cell to an adjacent cell. Handover is a process to ensure continuity of service. Figure 4 shows a case in which handover occurs as the user moves to an adjacent cell as the user moves. There are several criteria for handover triggering. Representative A1-A6 events among them are shown in Table 10. \begin{table}
\begin{tabular}{c} \hline
**Triggering Method** \\ Measurement-based triggering \\ Location-based triggering \\ Time/Timer-based triggering \\ Timing advance value-based triggering \\ Elevation angles of source and target cell-based triggering \\ \hline \end{tabular}
\end{table} 
Table 8: Method of handover triggering enhancement. \begin{table}
\begin{tabular}{c c} \hline
**Scenario** & **MEO-Based Non-Terrestrial Access Network** \\ \hline Orbit type & Circular orbiting at medium altitude around \\  & the earth \\ Altitude (km) & 10,000 \\ Payload & Transparent \\ Max beam footprint size (km) & 4018 \\ Max Round Trip delay (ms) & 186.9 (service and feeder links) \\ \hline \end{tabular}
\end{table} 
Table 9: Characteristic of MEO satellite. Based on the events in the table above, handover is performed from the receiving cell to the adjacent cell. Efficient handover minimizes dropped calls or loss of data transmission, and mobility management allows users to move seamlessly while maintaining a reliable connection. Handover works differently in different situations. There are several cases such as handover between cells within one base station, handover between base stations, and handover from 5 GC to EPC. An inter-cell handover process within one of the base stations will be described. Conditional Handover (CHO) is a handover that improves the handover process performance of Baseline Handover (BHO). Handover process triggering can be determined by using additional criteria. In addition, configuration information of an adjacent cell list capable of handover other than RACH to the target cell after the RRC reconfiguration step is stored in the UE. Then, if the conditions for handover triggering are satisfied, the handover process is performed on the target cell. Therefore, CHO can ensure more robust connectivity and continuity of service. Figure 5 shows the flow of CHO. The UE measures the criterion of the measurement index. If the standard of the measurement index is met, a measurement report is transmitted to the source cell. CHO is requested for target cells that can handover the source cell. The candidate target cell list transmits a handover request ack message to the source cell. Then, the source cell transmits an RRC Reconfiguration message including the configuration of CHO target cells to the UE. The UE transmits an RRC Reconfiguration Complete message to the source cell. After receiving the CHO configuration, the UE maintains a \begin{table}
\begin{tabular}{c c} \hline
**Event Type** & **Description** \\ \hline A1 & Serving becomes better than threshold \\ A2 & Serving becomes worse than threshold \\ A3 & Neighbor becomes amount of offset better \\ than PCell/PSCell \\ A4 & Neighbor becomes better than threshold \\ PCell/PSCell becomes worse than absolute \\ A5 & threshold1 AND Neighbor/SCell becomes \\ better than another absolute threshold2 \\ A6 & Neighbor becomes amount of offset better \\ than SCell \\ \hline \end{tabular}
\end{table} 
Table 10: Definition of handover triggering A events. Figure 4: Handover scenario. cell and determines CHO execution conditions for the target cell list.