長期演進技術之加強單一無線語音通話連續性的限制通話轉移次數研究

Abstract

長期演進技術(Long Term Evolution; LTE)是由通用移動通訊系統(Universal Mobile Telecommunications System; UMTS)演進而來，其利用IP多媒體子系統(IP Multimedia Core Network Subsystem; IMS)來提供語音服務。在現今大多數的商用行動網路中，UMTS網路已提供全域的服務覆蓋範圍，LTE網路僅提供局部的覆蓋範圍。使用者設備(User Equipment; UE)可以經由LTE或UMTS網路發起或接聽IMS語音電話。UE只要可取得LTE網路便會使用LTE網路進行通話；反之，當離開LTE網路覆蓋範圍時，UE便會切換到UMTS網路。為了支援IMS語音通話在LTE與UMTS之間的轉移，第三代合作夥伴計畫(3rd Generation Partnership Project; 3GPP)提出了加強單一無線語音通話連續性(Enhanced Single Radio Voice Call Continuity; eSRVCC)。當UE在IMS語音通話過程中，頻繁地在LTE與UMTS網路中轉移，可能會造成龐大的轉移流量。為了解決這個問題，本論文提出了限制轉移次數的演算法，限制一通語音通話最多可以轉移的次數，以降低轉移流量，並且提出一個數學分析模型來探討此演算法的效能。本論文亦探討以軟體定義網路(Software-Defined Networking; SDN)來實作eSRVCC供後續研究。研究結果顯示，選擇一個合適的轉移次數限制並不是一件簡單而直截了當的事，適當的轉移次數限制可以有效地降低因轉移所造成的流量，並且增進LTE語音控制的效能。

Long Term Evolution (LTE) evolved from Universal Mobile Telecommunications System (UMTS), which utilizes IP Multimedia Core Network Subsystem (IMS) to provide voice service. In most existing commercial operations, the LTE networks provide zonal coverage as compared with UMTS networks which provide full service coverage. A User Equipment (UE) can initiate or receive an IMS call in either LTE or UMTS. The UE uses LTE whenever it is available. If LTE is out of service, then the UE is transferred to UMTS. To support access transfer between LTE and UMTS during an IMS call, 3rd Generation Partnership Project (3GPP) proposed Enhanced Single Radio Voice Call Continuity (eSRVCC). If the UE frequently moves back and forth between LTE and UMTS during an IMS call, it may incur large access transfer traffic. To resolve this issue, we propose the limited access transfer algorithm that limits the number of access transfers in an eSRVCC call (referred to as the transfer limit) to reduce the transfer traffic. An analytic model is proposed to investigate the performance of the limited access transfer algorithm. We also present the Software-Defined Networking (SDN) implementation for eSRVCC for further studies. Our study indicates that the selection of the transfer limit is not a trivial issue, and an appropriate transfer limit effectively reduces the access transfer traffic to enhance the LTE call control performance.