## 3 System Model  
### The Proposed Model  
In the NTN system, in order for the UE to receive the location information of fast-moving satellites at high altitudes such as LEO and MEO, the data transmitted by the reference signal from the satellite to the UE at the time the UE receives it is already old data and has an error as much as the propagation delay. Papers [13; 14] applying the ML algorithm presented a method to solve the LEO-based handover problem. In paper [13], a handover process prediction model for a terminal approaching a cell boundary was proposed through feature preprocessing using a K-means clustering algorithm. In paper [14], RSRP values and handover flags of several beams serviced by satellites were formulated and the values of the formulas were obtained for each 0.5 s time period. Paper [14] also proposed a model that predicts handover from the current time zone to the next beam using a CNN model. For satellites serving at high altitudes, the difference in signal strength between the center of a cell and the edge of a cell is not clear due to the long communication distance. In addition, MEO satellites as well as LEO satellites require service at higher altitudes. This means that the propagation delay between the satellite and the UE is different depending on the altitude. The previously reviewed papers presented a handover triggering model that did not consider propagation delay and the results [23; 24; 25; 26; 27].  
Figure 5: Flow of conditional handover.  
In order to improve these points, this paper proposes a handover triggering model that uses the distance and altitude angle between the center of the cell and the UE instead of using the difference in indistinct signal strength and considers the propagation delay according to the altitude. The proposed model considers the propagation delay in the suggested range when the satellite transmits data to the UE in order to consider it close to the actual situation when the NGSO satellite is deployed in the NTN. According to 3 GPP in Release 16, LEO and MEO satellites have different propagation delays according to their respective altitudes. The experiment is conducted considering the corresponding range and additionally the propagation delay at higher altitudes. As a feature for predicting handover triggering, handover triggering is predicted after propagation delay using the distance and elevation between the cell center and the UE. Among the NTN scenarios proposed by 3 GPP in Release 16, a GNSS system is supported by assuming a CHO-based transparent payload satellite, the UE can use the GNSS system, and the satellite used for NTN can broadcast cell information. The assumption is also made that the channel is not time-varying and is in a Line of Sight (LOS) condition. Figure 6 is the handover triggering flow chart of the proposed model.  
Figure 6: Flow chart of handover triggering.  
Upon receiving the location information of the satellite, the UE calculates the center distance and elevation angle between cells based on the location information. Then, the calculated value is obtained through a model that predicts the distance and altitude angle after the propagation delay. Based on that value, whether to generate a measurement report is predicted through a model that predicts handover triggering. Figure 7 shows the system model for learning.