長期演進技術之電路交換回退效能研究

Abstract

當電信營運商將原有的第三代行動通訊網路(3rd Generation；3G)升級至長期演進技術(Long Term Evolution； LTE)時，LTE網路和3G網路會共存一段時間。因為3G的語音通話服務比LTE的網路電話語音服務(Voice over IP； VoIP)還要成熟而且覆蓋範圍較大，在LTE佈建初期，營運商可能考慮使用電路交換回退(Circuit-Switched fallback； CS fallback)的技術，來提供LTE系統的語音通話服務。根據第三代合作夥伴計劃(3rd Generation Partnership Project；3GPP)的電路交換回退流程，當使用者設備(User Equipment)在LTE網路需要使用語音通話服務時，使用者設備會從LTE網路回退到3G網路。當通話結束後，使用者設備會立刻回到LTE網路。如果接下來要進行另一通語音電話，使用者設備與網路端必須再執行一次電路交換回退，造成通話建立效率不佳。為了解決這個問題，我們提出延遲返回(Delayed-Return)的方法，此方法延遲使用者設備從3G返回到LTE以避免不必要的電路交換回退流程。我們提出了數學分析來探討延遲返回的效能。本論文後續研究亦探討CS fallback以導入軟體定義網路(Software-Defined Networking; SDN)實做的3G、LTE架構和CS fallback流程供後續研究。 研究結果指出，根據實際3G和LTE網路的測量數據，延遲返回的方法可以減少高達60%的電路交換回退成本。

When a mobile operator migrates its network from the 3rd Generation (3G) system to Long Term Evolution (LTE), both 3G and LTE will co-exist for a period of time. Since the 3G Circuit-Switched (CS) voice mechanism is more mature and available than that for LTE Voice over Internet Protocol (VoIP), the operator may consider CS fallback as a solution to provide reliable voice calls. According to the 3rd Generation Partnership Project (3GPP) CS fallback procedure, when a mobile user in the LTE network has an incoming or an outgoing call, the User Equipment (UE) falls back from LTE to Universal Mobile Telecommunications System (UMTS). When the call is complete and released, the UE immediately returns to LTE. If the next activity for the UE is another voice call, immediately switching from UMTS to LTE may not be efficient. In this case, the UE has to perform another CS fallback. To resolve this issue, we suggest delaying the returns to avoid unnecessary CS fallbacks, which is called delayed-return (DR). Based on the measurements from the real UMTS and LTE networks, we develop analytic model to investigate the performance of the CS fallback with DR. We also present the Software-Defined Networking (SDN) implementation for CS fallback for further studying. The study indicates that the DR scheme can effectively reduce the CS fallback costs up to 60%.