利用 Mobile Relay 改善 LTE-A 網路環境中高速移動的換手效率

Abstract

在交通發達的世代中，高速鐵路是許多人經常搭乘使用的交通工具之一，然而在高速行駛的車廂內使用手機因屏蔽效應，導致訊號不佳，3GPP的LTE-A網路標準中即使用移動中繼站來解決這個問題，使用者可以透過移動中繼站達到較好的通訊品質。在高速移動的情況下，行動網路勢必會遇到不斷換手的問題，頻繁的換手便會造成整體網路表現下降，而為了解決這個問題，我們提出一個透過移動中繼站改善換手效率的機制。由於高速鐵路是沿著固定且簡單的路線，下一個換手的基地台是很好預測的。LTE-A標準換手流程包含了Handover decision and admission control、Synchronization、Tracking area update、Path switch等四個階段。我們分析LTE-A標準換手流，發現同步至下一個基地台的階段 (Sychronization) 與切換路徑的階段 (Path switch) 可以合併，原本無法同時執行的理由是在一般非高速的情況，如果同步至下一個基地台的流程失敗，便會連回原本的基地台。但高速的情況，當嘗試連回原本基地台時會因為遠離該基地台，而導致連回的機率微乎其微。基於以上的理由，我們採用預測的機制和合併的流程來縮短換手流程的時間並降低換手失敗的機率。透過分析，提出的換手機制可以節省大約30.6%的換手時間，即使預測結論錯誤還是可以減少17.6%的換手時間。我們透過MATLAB去模擬在時速高達360公里的情況下，標準和本論文所提之換手流程的換手失敗機率。模擬結果顯示我們所提出的機制相較標準的換手機制，基地台間的距離可以較遠，但卻能維持相同的換手效率，因此電信業者可較為稀疏地佈建基地台，並能同時維持相同的服務品質。換言之，採用本文所提之換手機制，每個基地台的有效服務範圍增加2.74%。除此之外，模擬結果也顯示，我們所提出之換手機制有效降低對於車速的敏感度，能因應未來移動速度更快的交通工具。

In recent years, high-speed railway systems have been developed rapidly. In a high movement environment, handover occurs frequently in a mobile network and degrades the quality of mobile communication and service. The shorter the handover delay is, the better the communication quality can be obtained. To shorten the handover delay for a bunch of UEs moving in the same direction, a mobile relay (MR), an intermediate role between a UE and an eNodeB (eNB), has been introduced to support better mobile communication and service in a Long-Tern Evolution Advanced (LTE-A) network for high-speed rail systems. A MR acts as an eNB for the UE and a UE for the eNB. The standard LTE-A handover scheme has four phases, Handover decision and admission control, Synchronization, Tracking area update, and Path switch. Since a train travels along a fixed trajectory, the target eNB is predictable when handover is required. Based on such an assumption, we design an enhanced handover scheme to improve the handover performance in a high-speed moving environment. In this scheme, a serving eNB requests handover resource for a MR from a target eNB in advance. The MR installed on the carriage can determine when and whether the handover is required according to measurement results. By predicting a target eNB and simultaneously performing Synchronization and Path switch phases among the serving eNB and the target eNB, the proposed scheme can reduce the handover delay. By a numerical analysis, we prove that the proposed scheme can shorten 30.6 % of handover delay if the prediction is correct. In spite of an incorrect prediction, the handover delay can be reduced by 17.6 %. The results of simulations show that a eNB in the proposed scheme can provide a 2.74% larger service area than the standard LTE-A handover scheme and the proposed scheme can lower the sensitivity to the velocity of a high speed rail by 2.6 times.