LTE 移動管理及其對通話控制影響之研究

Abstract

為了提供行動寬頻服務，第三代行動通訊規格組織（3GPP）制訂了長期演進技術（LTE），其網路架構演進自全球行動通訊系統（UMTS）。LTE的移動管理（Mobility Management）機制追蹤使用者設備（User Equipment）的位置，以便於通知使用者來電。在通話建立之後，如果使用者在通話過程中移動，LTE通話控制（Call Control）機制負責維持通話的穩定性，避免通話在使用者移動過程中無故斷線。本研究探討LTE系統的移動管理效能以及其對通話控制之影響。

在LTE的網路架構中，細胞（Cells，基地台的無線電訊號覆蓋範圍）被分類成不同的追蹤區域（Tracking Areas），這些追蹤區域再被進一步被分類成追蹤區域清單（Tracking Area Lists）。移動管理的機制包含位置更新（Location Update）與呼叫（Paging）。當使用者設備離開目前的追蹤區域清單時，它會透過位置更新機制向網路端回報自己的新位置。當網路端嘗試對使用者設備建立連線時，網路端會透過呼叫機制要求追蹤區域清單中相對細胞的基地台去尋找使用者設備。本論文先分析 LTE 的移動管理效能。接著比較LTE移動管理效能與以移動次數為基礎（Movement-based）和以移動距離為基礎（Distance-based）的移動管理效能。研究結果顯示，在某些移動模式下，LTE的移動管理會有較好的表現。我們也提出一個LTE的動態呼叫方法，此方法根據使用者設備移動和呼叫的行為即時決定呼叫細胞的順序。研究結果顯示，動態呼叫方法可以有效降低呼叫訊息流量。

在通話控制方面，3GPP提出增強單一無線語音呼叫連續性（eSRVCC）機制，允許使用者設備在通話過程中，在LTE與UMTS領域中轉換。如果使用者設備在通話過程中頻繁地轉換，可能會產生大量的轉換訊息流量。針對此議題，本研究限制一通電話能轉換的最大次數，來降低轉換的訊息流量。我們研究eSRVCC在轉換次數限制下的效能。研究結果顯示，適當的轉換次數限制可以有效降低轉換訊息流量，並且維持較高的LTE使用率。

針對以上的研究題目，我們分別發展數學模型與模擬實驗，以精確分析各種交通流量或移動模式對效能的影響。本論文的研究成果可提供電信業者各項移動管理與通話控制之參數建議，以期提升LTE系統的效能。

3rd Generation Partnership Project (3GPP) proposed Long Term Evolution (LTE) which evolved from Universal Mobile Telecommunications System (UMTS) to support mobile broadband services. The LTE mobility management tracks the locations of the User Equipments (UEs) so that incoming calls can be delivered to the UEs. After a call is established, if the UE moves around the service area of the LTE network during the call, the call control technique such as enhanced Single Radio Voice Call Continuity (eSRVCC) is responsible to maintain the connectivity of the ongoing call. In this dissertation, we investigate the LTE mobility management and its impact on call control.

In the LTE mobility management architecture, the cells (the radio coverages of base stations) are grouped into the Tracking Areas (TAs), and the TAs are further grouped into the TA List (TAL). Mobility management procedures include location update and paging. The location update is executed when the UE leaves the current TAL. When the network attempts to connect to the UE (e.g., when an incoming call arrives), the network executes the paging procedure by sending the paging messages to the cells in the UE's TAL. In the dissertation, we first study the performance of the LTE mobility management. Then we compare the LTE mobility management scheme with two well-known mobility management schemes: the movement-based scheme and the distance-based schemes. Our study shows that under some traffic/mobility patterns, the LTE mobility management scheme outperforms the previously proposed schemes. We also propose a dynamic paging scheme that determines the paging sequence of cells in real time according to the UE movement and call behavior. Our study indicates that the dynamic paging scheme can effectively reduce the paging traffic.

For the call control, 3GPP proposed eSRVCC to support the access transfer between LTE and UMTS during a call. If the UE frequently moves back and forth between LTE and UMTS during a call, it may incur large access transfer traffic. To resolve this issue, the number of access transfers should be limited in an eSRVCC call (referred to as the transfer limit) to reduce the transfer traffic. We investigate the performance of eSRVCC with the transfer limit. Our study indicates that an appropriate transfer limit effectively reduces the access transfer traffic with good LTE utilization.

We also develop analytic and simulation models to study the impacts of different traffic/mobility patterns. Our study provides guidelines for the telecommunications operators to achieve higher system performance by selecting appropriate parameter values in LTE.