## 2 Related Work  
### Literature Review  
Experiments were conducted in several papers to evaluate 5G-based handover performance in NTN. The performance of terrestrial and non-terrestrial networks was compared using conditional handover. Most of the studies were conducted based on CHO [23; 24; 25]. Handover performance was measured in the FR2 band frequency band. The performance measurement comparison target was compared with the terrestrial network, and the KPIs used are shown in Table 1[23]. HO Success means whether the UE succeeded in handover to the target cell, All Mobility Failures means whether HOF occurred from the source cell to the RLF or target cell, and Ping Pong (PP) means whether or not the UE returned to the source cell within 1 s from the last handover. Table 2 shows the parameters used in the experiment [23]. In paper [23], \(o_{prp}\) means execution when the source cell differs from the target cell by the corresponding value, and \(o_{exec}\) means execution when the target cell differs from the source cell by this value. As a result of the experiment, the CHO HO Success value was superior to BHO when the UE's moving speed was 30 km/h or higher. It was confirmed that the higher the \(o_{prep}\) value, the better the HO Success. However, when a large \(o_{exec}\) value was given, the HO Success value was low due to the late handover execution and the PP significantly decreased. If the \(o_{exec}\) offset is large, performing handover execution in a situation where the target cell signal strength is worse than the currently serving cell will lead to a handover, which lowers the handover success rate. Conversely, when the \(o_{exec}\) value is relatively small, the signal strength of the target cell is good, which increases the handover success rate. However, the closer the handover is to the target cell, the better the signal strength of several nearby cells, which increases the probability of PP. \begin{table}
\begin{tabular}{c c} \hline \hline
**KPIs** \\ \hline HO Success \\ All Mobility Failures \\ Ping Pong \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Key Performance Indicators (KPIs) of Conditional Handover. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Experiment** & **Handover** & **Preparation** & **Execution** & **UE Speed** & **Cell Count** \\
**Parameter** & **Type** & **Offset (db)** & **Offset (db)** & **(km/h)** & **Cell Count** \\ \hline Variable & BHO, CHO & 3,7 & 3,6 & 3, 30, 60, & 1, 4 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: FR2 band handover experiment parameter. In addition, a CHO-based UE-oriented handover scheme has been proposed. There will be too many resource requests to handle the handover of all UEs in the satellite, and a technique for handling the process in the UE has been proposed to distribute it. In addition, the handover between several candidate satellites rather than one satellite was dealt with, and the experiment was conducted by dividing the number of candidate satellites, elevation angle, signal strength, and service time-based handover [24]. Among the proposed methods for mobility enhancement in 3 GPP Rel in 16, research has also been conducted using the location-based handover triggering technique. The cell radius is defined as \(R_{c}\) and \(R_{c}-\) and \(R_{c}+\) are defined as conditions for location-based handover triggering (LHT) to occur, limiting handover triggering in the range not included in the corresponding section and, at the same time, satellite service according to time. A method using the rate of change of the distance from the center of each cell was also proposed. Table 3 illustrates the parameters used in the experiment [25]. The paper assumed HOM as 0 db and TTT as 0 ms. When comparing the performance of Measurement-Based Handover Triggering (MHT) and Location-Based Handover Triggering (LHT), it was confirmed that LHT performed better. In order to solve the problems existing in NTN with ML, methods applied in 3 GPP Rel in 18 and several papers are proposed [9; 10; 11; 12; 13; 14].