LTE切换优化专题-参数功能和优化思路

内容：参数功能及设置、切换原理、信令流程、优化案例等。

1 LTE切换原理

1.1 Intra-eNodeB切换

触发事件：A3事件（同频切换），A5事件（异频切换）

当UE从当前所处的服务小区切换到同一eNodeB下的另一小区时，会发生Intra-eNodeB切换。

基于X2接口的切换

触发事件：A3事件（同频切换），A5事件（异频切换）

当两个eNodeB之间存在X2接口时，UE从当前所驻留的服务小区切换到另一eNodeB时，可采用基于X2接口的切换。

基于S1接口的切换

触发事件：A3事件（同频切换），A5事件（异频切换）

当两个eNodeB之间不存在X2接口，或X2接口不可用时，UE从当前所驻留的服务小区切换到另一eNodeB时，可采用基于S1接口的切换。 1.1.1 LTE到3G的切换

实现LTE到3G的切换首先需要满足几个前提：

1. 网络侧，LTE系统和3G系统均支持 LTE到3G的PS切换 2. UE侧，UE需要支持LTE到3G的PS切换，UE的Feature Group Indicator bit位8 和bit位22数值必须为1。 LTE到3G切换的流程概述：

1. LTE基站如果收到UE上报的A2测量报告，发现LTE的覆盖较差。 2. LTE基站通过RRC重配置消息对UE配置B2事件的测量的相关参数。 3. LTE基站收到B2事件的测量报告后，通过MobilityFromEutranCommand通知UE发起到3G的切换。

4. LTE基站收到UE上发的MobilityToUtranComplete，切换成功。 主要的LTE RRC空口信令：

? UE上报B2测量报告：Measurement Report

? UE在LTE小区收到往3G切换命令：MobilityFromEutranCommand

? UE向LTE小区反馈到3G切换成功：MobilityToUtranComplete 具体的切换信令流程如下： 1．

UE 切换准备信令流程（3GPP 23.401）

Source eNodeB Target RNC Source MME Target Target SGSN Serving GW Serving GW PDN GW HSS Uplink and Downlink User Plane PDUs 1. Handover Initiation 2. H andover Required 3. Forward Relocation Request 4. Create Session Request 4a. Create Session Response 5. Relocation Request 5a. Relocation Request Acknowledge 6. Create Indirect Data Forwarding Tunnel Request 6a. Create Indirect Data Forwarding Tunnel Response 7. Forward Relocation Response reate Indirect Data Forwarding Tunnel Request 8. C reate Indirect Data Forwarding Tunnel Response 8a . C

2．

切换执行信令流程（3GPP 23.401）

UE Source eNodeB Target RNC Source MME Target Target SGSN Serving GW Serving GW PDN GW HSS Uplink and Downlink User Plane PDUs 1. Handover Command 2. HO from E-UTRAN Command - 4. UTRAN Iu Access Procedures4a. Handover to UTRAN Complete Sending of uplink data possible Downlink User Plane PDUs If Direct Forwarding applies Via Target SGSN in case Direct Tunnel is not used 5. Relocation Complete 6. Forward Relocation Complete 6a. Forward Relocation Complete Acknowledge 7. Modify Bearer Request If Indirect Forwarding applies. For Serving GW relocation Steps 7, 8 and 9, and the following User Plane path, will be handled by Target Serving GW (A) 8. Modify Bearer Request 8a. Modify Bearer Response 9. Modify Bearer Response Uplink and Downlink User Plane PDUs (Via Target SGSN if Direct Tunnel is not used) 10. Routeing Area Update procedure 11. Delete Session Request 11b. Release Resources 11a. Delete Session Response (B) 12. Delete Indirect Data Forwarding Tunnel

1.1.2 LTE到2G的切换

LTE到2G的具体切换信令流程： 1. 切换准备信令流程（3GPP 23.401）

UE Source eNodeB Target BSS Source MME Target SGSN Target Serving GW Serving GW PDN GW HSS Uplink and Downlink User Plane PDUs 1. Handover Initiation 2. Handover Required 3. Forward Relocation Request 4. Create Session Request 4a. Create Session Response 5. PS Handover Request 5a. PS Handover Request Acknowledge 6. Create Indirect Data Forwarding Tunnel Request 6a. Create Indirect Data Forwarding Tunnel Response 7. Forward Relocation Response 8. Create Indirect Data Forwarding Tunnel Request 8a. Create Indirect Data Forwarding Tunnel Response

2. 切换执行信令流程（3GPP 23.401）