隨著低軌衛星(LEO: Low Earth Orbit)在非地面網路(NTN: Non-Terrestrial Network)中扮演愈加關鍵的角色，其高速移動與短暫覆蓋特性，使得用戶設備(UE)在連線過程中需頻繁進行衛星基站之間的換手(Handover)。傳統地面網路常以參考訊號接收功率(RSRP: Reference Signal Received Power)做為換手依據；但在 LEO 環境中，RSRP 變化緩慢且反應延遲，難以即時反映鏈路品質，容易導致換手失準、通訊中斷、或來回換手(Ping-Pong)乒乓效應而失敗作收。本研究提出一項以都卜勒視頻(非真頻、由觀察者與波源之相對速度決定)之差異(DFD: Doppler Frequency Difference、由觀察者與波源之角度決定－亦即由接近地表之接收者與衛星發射者之相對角度決定)效應為核心的換手機制；亦即透過觀測即將換手前後之兩顆衛星對地面使用者所產生的DFD效應，可進行更即時且明確的換手。換言之，相較於上述兩顆衛星的RSRP之微弱差異換手須延時以明確認定有足夠差異能發動換手而言，DFD 則無需延時便能即時明確換手，因此具備更高的即時換手靈敏度與預測性換手潛力。本研究以MATLAB自建了完整的模擬實驗架構，針對不同的衛星軌道角度與地空波束最大重疊程度進行模擬，並比較基於 RSRP差異換手與 DFD換手兩者之機制差異。模擬結果顯示：DFD 換手確比RSRP 換手更具即時性，並有效減少乒乓效應，且提升整體換手穩定性。;As Low Earth Orbit (LEO) satellites play an increasingly critical role in Non-Terrestrial Networks (NTNs), their high-speed movement and brief coverage periods demand User Equipment (UE) frequently perform Handovers between satellite base stations. Traditional terrestrial networks typically rely on Reference Signal Received Power (RSRP) as the basis for Handover decisions. However, in LEO environments, RSRP changes slowly and responds with delay, making it difficult to reflect link quality in real time. This often results in inaccurate Handovers, communication interruptions, or failures caused by the Ping-Pong effect. This study proposes a Doppler Frequency Difference (DFD)-based Handover mechanism, where the ‘Doppler Frequency’ refers to the ‘Observed Frequency’ (not the True Frequency), caused by the relative velocity between the observer and the wave source; whereas the ‘Difference’ refers to that the satellite Observed Frequency is determined by the relative angle between the near-ground UE (the observer) and the satellite (the wave source). By observing the DFD effect generated by two satellites before and after a handover, ground users can achieve more immediate and precise handover decisions. By observing the DFD effect generated by two satellites before and after a handover, ground users can achieve more immediate and precise handover decisions. In other words, unlike RSRP-based handovers that require delay to confirm sufficient signal difference, DFD enables real-time and clear handover execution without delay. This results in higher handover sensitivity and predictive potential. The study constructs a complete simulation framework in MATLAB, modeling various satellite orbital angles and maximum beam overlap scenarios. It compares the mechanisms of RSRP-based handover and DFD-based handover. Simulation results show that DFD-based handovers are significantly more responsive, effectively reduce the Ping-Pong effect, and improve overall handover stability.
