【原】移动手机为啥读取WCDMA信号--LTE案例

和老康一起学5G 2022-05-26 发布于陕西

展开全文

1.背景介绍

4月26日中国移动VOLTE例行测试中终端在LTE网络先后发起业务和TAU请求；而在TAU拒绝回送BYE请求后，读取WCDMA网络消息显示联通3G图标；

最后终端选择驻留GSM网络，继续语音测试任务；

中国移动4G终端为什么会在业务测试中读联通3G网络信息呢？让我们对整个事件和信令消息，对照3GPP规范做一下回顾和分析；

图表1.终端由LTE到GSM流程消息

2 .业务请求和TAU拒绝

终端在(TAC:13643 CELLID:9951513)小区于16:10:58.824和16:10:58.825发起业务请求(Service Request）和TAU请求(图2）；

Log回放显示:终端发起业务请求后，MAC层的RACH接入失败（图3）；

而终端发起TAU请求时MAC层的RACH成功（图4）；网络TAU拒绝EMM Cause=111 Protocolerror,unspecified（图5）；

图表2.终端发起业务和TAU请求

图表3. 业务请求后RACH失败

图表4. TAU请求和RACH成功

图表5 .终端TAU被拒绝

3.TAU请求和拒绝定义（3GPP 24.301）

5.5.3 Tracking area updating procedure (S1 modeonly)

The tracking area updating procedure is always initiated by the UE and is used for the following purposes:

- normaltracking area updating to update the registration of the actual tracking areaof a UE in the network;

5.5.3.2.7 Abnormal cases on the network side

If the TRACKING AREA UPDATE REQUESTmessage has been received with a protocol error, the network shall return a TRACKINGAREA UPDATE REJECT message with one of the following EMM cause values:

#96:invalidmandatory information element error;

#99:informationelement non-existent or not implemented;

#100: conditionalIE error; or

#111: protocolerror, unspecified.

4.终端脱网后PLMN搜索

16:11:08.083 终端接收到服务小区（TAC 13643 eNodeB 9951513 Sector ID 3PCI 498）信号良好（RSRP -97.68 SINR 10.30，下图左）；而16:11:08.084 服务小区无信号 (下图右)，判断：底层链路中断，终端脱网。

图表6 .网络底层链路中断前后

16:11:08.084终端进入(EMM REGISTERED NO CELL AVAILABLE) 状态;NAS层启动(GMM_PLMN_ SEARCH)网络搜索流程;

图表7 . NAS层GMM触发PLMN搜索

TS 24.301clause5.5.3.2.5

If the tracking area updating cannot beaccepted by the network, the MME sends a TRACKING AREA UPDATE REJECT message tothe UE including an appropriate EMM cause value.

Uponreceiving the TRACKING AREA UPDATE REJECT message, the UE shall stop timerT3430, stop any transmission of user data, and take the following actionsdepending on the EMM cause value received.

5.PLMN搜索协议规定

TS23.122 3.5 No suitable cell (limited service state)

There are a number of situations in which the MS is unable to obtainnormal service from a PLMN. These include:

a) Failureto find a suitable cell of the selected PLMN;

b) No SIMin the MS;

c) A"PLMN not allowed" response when an LR is received;

d) An"illegal MS"or "illegal ME" response when an LR isreceived (Any SIM in the ME is then considered "invalid");

e) An"IMSI unknown in HLR" response when an LR is received (Any SIM in theME is then considered "invalid" for non-GPRSservices);

f) A"GPRS services not allowed" response when an LRof a GPRS MS attached to GPRS services only is received (The cell selectionstate of GPRS MSs attached to GPRS and non-GPRS depends on the outcome of thelocation updating), or an "EPSservices not allowed" response is received when an EPS attach, trackingarea update or service request is performed; or

g) Powersaving mode (PSM) is activated (see 3GPP TS 23.682 [27A]).

(In automatic PLMN selection mode, items a,c and f would normally cause a new PLMN selection, but even in this case, thesituation may arise when no PLMNs are available and allowable for use).

3GPP 36.304小区选择和重选

5.2 Cell selection and reselection

TheNAScan control the RAT(s)in which the cellselection should be performed, for instance by indicating RAT(s)associatedwith the selected PLMN, and by maintaining a list offorbidden registration area(s) and a list of equivalent PLMNs. The UE shallselect a suitable cell based on idle mode measurements and cell selectioncriteria.

In order to speed up the cell selectionprocess, stored information for several RATs may be available in the UE.

Whencamped on a cell, the UE shall regularly search for a better cell according tothe cell reselection criteria. If a better cell is found, that cell isselected.The change of cell may imply a change of RAT. Details onperformance requirements for cell reselection can be found in [10].

图表8 LTE终端网络搜索流程图

6.UMTS 网络搜索

根据3GPP36.304（5.2）16:11:16.069 和16:11:23.683终端分别搜到TDSCDMA和WCDMA网络：

图表9 终端对TD+WCDMA网络搜索

7.GSM重选驻留

16:11:39.139搜索到GSM网络，根据3GPP36.133重选到GSM网络；

图表10 终端搜索GSM网络

7.1小区重选（3GPP 36.133）

4.2 Cell Re-selection

4.2.1 Introduction

The cell reselection procedure allowsthe UE to select a more suitable cell and camp on it.

When the UE is in either Camped Normally state or Camped on Any Cell state on a cell, the UEshall attempt to detect, synchronise, and monitor intra-frequency,inter-frequency and inter-RAT cells indicated by the serving cell. For intra-frequency andinter-frequency cells the serving cell may not provide explicit neighbour listbut carrier frequency information and bandwidth information only. UEmeasurement activity is also controlled by measurement rules defined inTS36.304, allowing the UE to limit its measurement activity.

4.2.2 Requirements

The UE shallsearch every layer of higher priority at least every T_{higher_priority_search}= (60 * N_layers) seconds when the UE is not configured witheDRX_IDLE cycle, and at least every T_{higher_priority_search} = MAX(60* N_layers, one eDRX_IDLE cycle) when UE is configured with eDRX_IDLEcycle, where N_layers is the total number of configured higherpriority E-UTRA, UTRA FDD, UTRA TDD, CDMA2000 1x and HRPD carrier frequenciesand is additionally increased by one if one or more groups of GSM frequenciesis configured as a higher priority.

16:12:20.849 在GSM网络LUA完成和驻留（LAC13888 CELL ID17131）小区。