## 2 Related Work
### Non-Terrestrial Network Overview  
Existing terrestrial 5G networks provide communication services around installed base stations. Telecommunications companies want to provide services with wide coverage at low cost. However, in order to cover all domestic terrestrial networks using terrestrial networks, terrestrial cell coverage must be installed without any vacancy. NTN provides a wide range of coverage at high altitudes and can provide communication services in mountainous areas where ground networks are not installed, in the middle of the sea, and in airplanes. In order to apply NTN with these advantages to the communication system, 3 GPP listed and suggested solutions to problems for providing NTN services from an overall overview of NTN through Releases 15, 16, and 17. In order to actually service and commercialize using NTN, several papers have been proposed to solve the problems presented by 3 GPP. In this section, representative contents of NTN are summarized. In 3 GPP Release 16, scenarios defined by dividing GEO and LEO into transparent payload and regenerative payload were described. Each scenario provided by NTN is shown in Figures 1 and 2 [31]. There are several definitions of satellites according to altitude. Table 4 is a representative satellite. \begin{table}
\begin{tabular}{c c c} \hline
**Experiment Parameter** & **Handover Type** & **Triggering Type** \\ \hline Variable & BHO, CHO & MHT, LHT(R), LHT(P), LHT(R,P) \\ \hline \end{tabular}
\end{table} 
Table 3: Handover experiment parameter. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Platforms** & **Altitude Range (km)** & **Orbit** & **Beam Foot Print Size (km)** \\ \hline Low-Earth Orbit (LEO) satellite & 300–1500 & Circular around the earth & 100–1000 \\ Medium-Earth Orbit (MEO) satellite & 7000–25,000 & Circular around the earth & 100–1000 \\ Geostationary-Earth Orbit (GEO) satellite & 35,786 & Notional station keeping & 200–3500 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Definition of satellite in 3 GPP Rel 16. Figure 1: Transparent payload in 3 GPP Release 16. \begin{table}
\begin{tabular}{c c} \hline \hline
**Network Elements** & **Interface Type** \\ \hline UE—Satellite & NR Uu \\ Satellite—Satellite & Inter Satellite Link (ISL) \\ Satellite—sNB & NR Uu \\ gNB—SG Core(5 GC) & NG \\ Satellite(gNB on board)—Gateway & Satellite Radio Interface (SRI) \\ Satellite(gNB on board)—5 GC & NG \\ Satellite(gNB-DU on board)—Gateway & Satellite Radio Interface (SRI) \\ Satellite(gnB-DU on board)—sNB-CU & F1 \\ gNB-CU—5 GC & NG \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Definition of interface type between network elements. Figure 2: Regenerative payload in 3 GPP Release 16. Table 6 is an NR-RAN-based NTN architecture proposed based on the interface of Table 5.  
The scenario presented in TR38.821 of 3 GPP Rel 16 provides services at high altitudes. Thus, there is a delay issue according to distance. Table 7 is a table showing overall values corresponding to the scenarios defined in Table 4.  
Unlike the terrestrial network, NTN propagation delays for each scenario in Table 7 cause delays in the measurement report, handover command reception, and handover request/ack in the handover process. In order to trigger handover, a measurement report is transmitted to the receiving cell or gNB when various measured values satisfy the criteria. However, the NTN environment that communicates at high altitudes cannot trust the validity of the measured values. Figure 3 shows the difference in signal attenuation received at the cell center and cell edge in terrestrial and non-terrestrial networks. In the left image depicting the terrestrial network, there is a noticeable difference in signal strength between User Equipments (UEs) located close to the gNB (ground-based station) and those located farther away. However, in the right image representing the