## 2 Handover Issues in TN and NTN Networks  
Handover is a crucial mechanism in wireless communication systems that ensures seamless connectivity as UE moves between different network cells. In TN, handovers are primarily triggered based on signal strength measurements, while in NTN, additional challenges arise due to satellite mobility, long propagation delays, and frequent cell transitions. The process of handover has been well standardized by the 3 GPP to maintain service continuity across network boundaries. However, the suitability of these standardized handover mechanisms varies depending on the environment in which they are applied. In conventional cellular networks, 3 GPP specifies a structured handover procedure to facilitate smooth transitions between cells. As shown in Figure 1, the process begins when the serving cell continuously monitors the UE's measurements of neighboring cell signals. The UE periodically reports signal quality indicators, including the \(RSRP\), to the serving cell. When the signal strength of a neighboring cell surpasses that of the serving cell by a predefined margin, and this condition persists beyond a \(TTT\) threshold, the network initiates a handover decision. The serving cell then sends a handover request to the target cell, which evaluates the request and responds with a handover acknowledgment if resources are available. Upon approval, the serving cell issues a handover command to the UE, instructing it to disconnect from the current cell and establish a new connection with the target cell. Following a successful handover, the target cell confirms completion, and the serving cell releases the context of the UE's previous session. This sequence ensures a controlled and efficient transition, minimizing service disruptions. Despite its effectiveness in terrestrial networks, the 3 GPP handover procedure faces several challenges in NTN scenarios. Unlike stationary terrestrial base stations, NTN employs moving satellites that introduce dynamic signal variations, resulting in frequent handovers. The reliance on RBH mechanisms in such an environment can lead to inefficient mobility management due to the fluctuating nature of signal strength in satellite communications. To address these limitations, LBH has been explored as an alternative approach, leveraging satellite trajectory information to predict handover events rather than relying solely on signal strength measurements. \(RSRP\)-based handover is a widely adopted approach in terrestrial networks, where handover decisions are made based on the UE's received signal strength. As illustrated in Figure 1, the UE continuously measures \(RSRP\) from the serving and neighboring cells, reporting this information to the network. When the \(RSRP\) of a neighboring cell becomes stronger than that of the serving cell for a sustained period, the network triggers a handover. This approach is effective in TN environments, where signal propagation is relatively stable, and handover decisions are predominantly influenced by physical obstructions and distance-based path loss. \(RSRP\) is a key metric used in long-term evolution (LTE) and 5G NR networks to assess signal quality and determine the optimal cell selection and handover timing. It represents the received power of reference signals transmitted by the base station (or satellite in NTN). Mathematically, \(RSRP\) is defined as the linear average of the received power from  
Figure 1: The handover procedure. reference signal resource elements (RSREs) within the same subcarrier and is expressed as follows:  
\[RSRP=\frac{1}{N}\sum_{i=1}^{N}P_{i} \tag{1}\]  
where the following apply:  
\(P_{i}\) represents the power of the \(i\)-th reference signal resource element. \(N\) represents the total number of resource elements used for transmitting the reference signal. \(RSRP\) is typically measured in dBm and is used as a fundamental input for mobility decisions. The higher the \(RSRP\) value, the better the received signal strength from a particular cell.