Guide to Optimizing LTE Service Drops

HUAWEI TECHNOLOGIES CO., LTD. Huawei Confidential

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www.huawei.com

Guide to Optimizing

LTE Service Drops



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Change History

Date Version Description Reviewer Author

2012.1.10 1.0 Completed

the draft.

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Abstract

This document

Defines the call drop rate.

Describes how to use the related counters to diagnose a call drop

and to analyze factors influencing the KPI.

Describes common diagnosis methods and standard actions to be

taken by front-line engineers to handle a call drop problem.

Describes the deliverables that the front-line engineers must

submit to R&D engineers if the front-line engineers fail to solve the

problem after taking the standard actions

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Content

• Definition of the Service Drop Rate

• Symptoms of a Service Drop

• Cause Analysis and Data Processing

• Checklist and Deliverables

• Case Study



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Calculation of the Call Drop Rate on the UE Side (1/3)

Call Drop Rate = eRAB AbnormRel / eRAB Setup Success x 100%

where eRAB AbnormRel is the number of e-RAB abnormal releases and

eRAB Setup Success is the number of successful e-RAB setup events.

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• eRAB AbnormRel is calculated by Huawei Genex PA as follows:

I. eRAB AbnormRel increments by 1 if the UE

Does not receive the DEACTIVATE EPS BEARER CONTEXT REQUEST message and,

Does not receive the DETACH REQUEST message from the MME and,

Does not send the DETACH REQUEST message and,

Receives the RRCConnectionReconfiguration message containing the IE drb-ToReleaseList.

In this case, if the ERAB num minus the eps-BearerIdentity contained in the ReleaseList is 0, the UE

transits to RRC_Idle mode.

II. eRAB AbnormRel increments by 1 if the UE

Does not receive the DEACTIVATE EPS BEARER CONTEXT REQUEST message and,

Does not receive the DETACH REQUEST message from the MME and,

Does not send the DETACH REQUEST message and,

Receives the RRCConnectionRelease message and the RLC layer performs data transmission in the

last 4s in any direction.

In this case, the UE directly transits to RRC_Idle mode.

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III. ERABAbnormalRel increments by 1 for each released e-RAB if the UE has

established e-RAB(s) and enters the RRC_Idle mode before receiving the

RRCConnectionRelease message.

IV. ERABAbnormalRel increments by 1 if the UE initiates the RRC connection setup

request without receiving the RRC Connection Reconfiguration, Deactivate EPS

Bearer Context Request, Detach Request, RRC State, or RRC Connection

Release message.

V. ERABAbnormalRel increments by 1 if the event RRCReestablishFail occurs.

The timestamp contained in these two events is the same.

Note: The acceptance criteria of some customers may require that all RRC

reestablishments initiated by the UE be counted as service drops.

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Calculation of the Call Drop Rate on the Network Side

Call Drop Rate = L.E-RAB.AbnormRel / (L.E-RAB.NormRel + L.E-

RAB.AbnormRel) x 100%

where L.E-RAB.AbnormRel is the number of e-RAB abnormal releases

and L.E-RAB.NormRel is the number of e-RAB normal releases.

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Counters Recorded by the Network

• As shown in point A of Fig1, if the eNodeB sends the E-RAB RELEASE INDICATION message containing a cause value that is not "Normal Release", "User Inactivity", "cs fallback triggered", or "Inter-RAT redirection", L.E-RAB.AbnormRel increments by 1. If the E-RAB RELEASE INDICATION message requests release of multiple e-RABs, L.E-RAB.AbnormRel increments by 1 for each e-RAB.

• As shown in point A of Fig2, when the eNodeB sends the UE CONTEXT RELEASE REQUEST message to the MME, the eNodeB releases all e-RABs of the UE. If the release cause is not "Normal Release", "User Inactivity", "cs fallback triggered", or "Inter-RAT redirection", L.E-RAB.AbnormRel increments by 1 for each release.

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Note:

The eRAB Release procedure releases one or multiple e-RABs. After the procedure, at least the default bearer is maintained.

The UE Context Release procedure releases all connections. No bearer is maintained after this procedure.



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Counters That Count Abnormal

Releases by the Network (1/4)

• Currently, there are five counters that count e-RAB abnormal releases by the

network:

L.E-RAB.AbnormRel.Radio (Number of e-RAB abnormal releases caused by the

eNodeB)

L.E-RAB.AbnormRel.TNL (Number of e-RAB abnormal releases caused by the

transmission network)

L.E-RAB.AbnormRel.Cong (Number of e-RAB abnormal releases caused by network

congestion)

L.E-RAB.AbnormRel.HOFailure (Number of e-RAB abnormal releases caused by

handover failures)

L.E-RAB.AbnormRel.MME (Number of e-RAB abnormal releases caused by the EPC)

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• Abnormal releases caused by the EPC

As shown in point A of Fig1 and Fig2, if the eNodeB

receives the E-RAB RELEASE COMMAND or UE

CONTEXT RELEASE COMMAND message from the

MME containing a cause value that is not “Normal

Release”, “Detach”, “User Inactivity”, “cs fallback triggered”,

or “Inter-RAT redirection”, L.E-RAB.AbnormRel.MME

increments by 1.

Note: L.E-RAB.AbnormRel.MME does not include L.E-

RAB.AbnormRel. A release initiated by the EPC is not

counted as a call drop in eRAN2.1SPC400 and later

versions.

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• Abnormal release not caused by the EPC

As shown in point A of Fig3, if the eNodeB sends the E-

RAB RELEASE INDICATION message to the MME with a

cause value indicating a radio error, L.E-

RAB.AbnormRel.Radio increments by 1. If the cause value

indicates a transmission network error, L.E-

RAB.AbnormRel.TNL increments by 1. If the cause value

indicates network congestion, L.E-RAB.AbnormRel.Cong

increments by 1. If the E-RAB RELEASE INDICATION

message requires release of multiple e-RABs, the

concerned counter increments by 1 for each e-RAB.

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• Abnormal release not caused by EPC

As shown in point A of Fig4, the eNodeB sends the UE

CONTEXT RELEASE REQUEST message to the MME to

release all e-RABs of the UE. If the cause value indicates a

radio error, L.E-RAB.AbnormRel.Radio increments by 1. If

the cause value indicates a transmission network error, L.E-

RAB.AbnormRel.TNL increments by 1. If the cause value

indicates network congestion, L.E-RAB.AbnormRel.Cong

increments by 1. This counter measures the abnormal

releases caused by preemption and resource congestion. If

the cause value indicates a handover failure, L.E-

RAB.AbnormRel.HOFailure increments by 1. The concerned

counter increments by 1 for each e-RAB. The counters no

longer increment when the MME sends the UE CONTEXT

RELEASE COMMAND message.

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Content





• Case Study



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Symptoms of a Call Drop as Observed in a Drive Test

Huawei test UE and UE Probe, or other commercial UEs and their signaling trace

software are used in a drive test. Symptoms shown by the traffic monitoring software

installed on the drive test computer are:

The throughput suddenly falls to a low value or zero.

The UE begins to receive system information when a handover is not complete

or when the UE is not in a re-establishment scenario.

Low

throughput UE receives

system

information.



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Symptoms of a Call Drop as Observed from

the Traffic Statistics

The call drop problem of a commercial network is observed from the traffic statistics

and is reflected by the call drop rate and call drop count. The symptoms shown by the

traffic statistics exported from the M2000 are:

Global call drop rate, call drop count, and number of successful service setups

Call drop rate, call drop count, and time segment of top cells

High global call drop

rate

Top cells occupy a high

percentage of call drops

Time

segment of

call drops



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Content





• Case Study



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Steps in Analyzing a Call Drop Problem (1/2)

Step 1: Determine the scope of the call drop problem:

Analyze the traffic statistics and CHR to determine the scope of the call drop

problem, whether it is a top-cell or top-site problem, entire-network problem,

comprehensive problem, or top-terminal/top-UE problem.

Note: The analysis method varies for different scenarios. In a scenario of degraded performance after upgrade, you

need to compare the differences before and after the upgrade to determine the scope of the degradation. In a

scenario of inventory optimization where the call drop performance is below expectation or to be improved, you

need to determine the region of performance degradation.

Step 2: Classify the causes of a call drop problem:

Analyze the data sources to classify the causes of a call drop problem.

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Steps in Analyzing a Call Drop Problem (2/2)

Step 3: Do as required by the checklist:

Do as required by the checklist to determine the root cause and the closing action.

Note: The checklist is described in the next chapter.

Step 4: Close the problem:

Close the problem and evaluate the result. If the result is unsatisfactory, repeat the

preceding steps.

If the closing actions are reproducible, consider the merits of copying the closing

actions to the entire network.

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Determining the Scope of a Call Drop Problem –

Principles of Selecting Top Cells (1/2)

The principles of selecting top cells vary for different scenarios.

Scenario 1: Performance degradation in the time dimension:

The call drop performance degrades after an upgrade, or degrades

suddenly due to unknown reasons.

Principles of selecting top cells

Calculate the difference of the counters (call drop rate and

number of e-RAB abnormal releases) before and after the upgrade of

each cell. Sort the cells by the difference of the call drop rate and the

difference of the number of e-RAB abnormal releases to obtain the top

cells of degraded call drop rate and top cells of number of e-RAB

abnormal releases.

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Determining the Scope of a Call Drop Problem –

Principles of Selecting Top Cells (2/2)

Scenario 2: Performance degradation in an inventory

optimization:

The call drop performance of the live network is below expectation and

needs to be optimized to the target value.

Principles of selecting top cells

Sort the cells by the difference of the call drop rate and the

difference of the number of e-RAB abnormal releases to obtain the top

cells of degraded call drop rate and top cells of number of e-RAB

abnormal releases.

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Determining the Scope of a Call

Drop Problem – Criteria (1/2) Top-cell problem:

After one-fifth of the top cells of high call drop rate and large number of e-RAB

abnormal releases are removed from calculation of the entire-network call drop

performance, if the performance is significantly improved to the expected value,

the call drop problem is defined as a top-cell problem.

Entire-network problem



performance, if the performance is not significantly improved, the call drop

problem is defined as an entire-network problem.

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Determining the Scope of a Call

Drop Problem – Criteria (2/2)

Comprehensive problem



performance, if the call drop performance is improved a little to a value slightly

below the expected value, the problem is defined as a comprehensive (top-cell

plus entire-network) problem.

Top-UE problem

After one-fifth of the top UEs are removed from calculation of the entire-network

call drop performance, if the performance is significantly improved to the

expected value, the problem is defined as a top-UE problem.

Note

Currently, the CHR of the LTE system provides no information about the terminal type. The terminal type is

provided by complaining users or inferred from the symptoms.

Due to security concerns, the eNodeB does not provide IMSI information. Therefore, top UEs can be

inferred only from the TMSI, not from the IMSI.

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Classifying the Causes of Call Drop

Problems – Obtaining Data Source After determining the scope of the call drop problem, analyze the following data

sources to infer the causes of the problem:

Traffic statistics

Traffic statistics can be obtained from the M2000/PRS. For details, see

section 2.3.3 of LTE Service Drop Troubleshooting and Optimization

Guide.doc.

Signaling trace on the network side

Signaling trace can be performed on the M2000. For details, see section 2.2.2

of LTE Service Drop Troubleshooting and Optimization Guide.doc.

Drive test data

The drive test data can be obtained by performing a drive test. For details, see

section 2.1.3 of LTE Service Drop Troubleshooting and Optimization

Guide.doc.

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Problems – Acquiring Tools The following table lists available tools, usages, and acquisition method.

Tool Name Usage Acquisition Method

TraceViewer Plays back signaling messages traced on the

LMT.

Released together with the product version and integrated in

OfflineTool file package.

Probe Installed on Huawei UE and traces signaling,

scheduling, and signal quality information.

http://support.huawei.com/support/pages/editionctrl/catalog/Sh

owVersionDetail.do?actionFlag=clickNode&node=000001099

409&colID=ROOTENWEB|CO0000000174

Assistant

Installed on Huawei UE, counts and analyzes

signaling, scheduling, and signal quality

information.




NIC Batch data collection tool




PRS Parses and analyzes traffic statistics of the

eNodeB.



110&colID=ROOTWEB|CO0000000065

OMstar Parses and analyzes original traffic statistics

and CHR. Compares parameters.






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Huawei UE

Probe

Signaling Trace

Management

interface of the

M2000

Classifying the Causes of Call Drop Problems –

Interfaces of the Tracing Tools



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Huawei

UE

Probe

eNodeB TrafficReview


Problems – Interfaces of the Analysis Tools



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Identifying the RRC CONNECTION RECONFIGURATION message Start the Message Browser to view the details of the message.

If the message contains the IE measConfig, the message is a measurement control message.

If the message contains the IE cqi-ReportConfig, the message is a CQI reconfiguration message.

If the message

contains the IE

targetPhysCellId, the

message is a

handover command.

Classifying the Causes of Call Drop Problems –

Identifying Reconfiguration Messages



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• Trend analysis

Obtain the call drop KPI of the global network for at least one to two

weeks, or two weeks before and one week after the upgrade in case

an upgrade has been performed. An example is shown in the upper

right figure.

• Cause analysis

The counters indicate whether an abnormal release is caused by the

Uu interface or cell resource congestion, as shown in the lower left

figure.

• Top analysis

Analysis of the traffic statistics can show the top cells and top

time segments that have the highest RRC connection setup

failure and e-RAB setup failure, as shown in the lower right

figure.

Page 29

Analyzing Traffic Statistics to Obtain

Causes of Call Drop Problems



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Analyzing Signaling Trace to Obtain

Causes of a Call Drop The signaling trace clearly shows the signaling procedure that causes the call

drop and is effective for diagnosing problems found during a drive test or

reproducible problems. The disadvantage is that the trace must be performed

before the problem is triggered and that manual analysis is required. The signaling

trace cannot be used for irreproducible or small-probability problems.

Standard interface trace (a major means): Obtain top cells and top time segments by

analyzing the traffic statistics, start the standard interface trace on the top cells and at top

time segments, check which signaling procedure causes the call drop.

Single-UE global-network trace (a minor means): Query the IMSI of a TMSI from the

EPC, start the global-network trace of this IMSI. This method is effective for ensuring VIP

service.

Page 30



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Analyzing Drive Test Data to Obtain

Causes of a Call Drop The advantage of a drive test is that the downlink signal strength, uplink

transmit power, bit error rate, and scheduling information can be obtained,

depending on the drive test software and UE capability. The disadvantage is

that in terms of signaling trace, only the signaling (including the RRC and

NAS messages) of the Uu interface is traced. Therefore, it is desirable to

combine a drive test with the signaling trace on the eNodeB.

Determine whether a call drop is caused by uplink or downlink problem.

The drive test can show whether the UE or eNodeB fails to receive the signaling

message; the downlink RSRP/SINR obtained from the drive test indicates the

downlink channel quality; the uplink transmit power indicates whether the uplink is

insufficient.

Determine whether a call drop is caused by UE.

The UE log shows whether the UE correctly processes the received signaling

messages and whether the UE suddenly does not send any data.

Page 31



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Content





• Case Study



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Checklist for the Entire-Network Problem (1/2)

Page 33

Standard Action Analysis Action Deliverables Closing Action

Preliminary analysis of traffic statistics

1. Analyze the traffic statistics to determine the range and cause of the call drop. 2. Analyze the trend of the call drop rate to determine change of the call drop rate.

1. Distribution of the causes and top causes 2. Actions that affect the call drop rate

1. Optimize the network according to the top causes of the call drop problem. 2. Describe the actions that affect the call drop rate and the impact.

Version check 1. Check whether the eNodeB version is upgraded or a new patch is installed. 2. Check whether the EPC version is upgraded or a new patch is installed.

New and old version numbers

Describe the changes that may affect the call drop rate based on the Release Notes.

Equipment and transmission alarms

1. Global alarm check Critical and major alarms

1. Analyze the impact of alarms on the call drop rate.

2. Clear the alarms and check whether the call drop KPI is restored.

Parameter configuration check

1. Global parameter configuration check 2. Inspection of EPC parameter change

1. Difference of parameters before and after the upgrade 2. Difference of parameters compared with the baseline 3. Purpose and impact of the change of EPC parameters

1. Determine whether the parameter change affects the call-drop KPI. 2. Roll back the parameters and check whether the call-drop KPI is restored.



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Checklist for the Entire-Network Problem (2/2)

Page 34

Note

The standard actions of a comprehensive problem (entire-network plus top-cell problem) are a

combination of the checklist for the entire-network problem and the checklist for the top-cell

problem.


Operation record check

Check whether batch operations affecting the global network are recorded and whether neighboring cells and PCI are re-planned.

Records of batch operations affecting the global network

Analyze the impact of batch operations on the call drop rate. Determine whether the batch operations can be rolled back.

Neighbor relationship check

Check for missed configuration of neighbor relationship. Deployment of scattered sites causes incorrect neighbor relationship.

Missed configuration of neighbor relationship

Add neighboring cells that are not configured in the neighbor relationship. Check whether the call drop KPI is restored.

Major event check

Check for allocation of a large quantity of phone numbers and major activity (such as ceremony, holidays, and games)

1. Check the terminal type involved in the number allocation, quantity of number allocation, and subscription policy. 2. Determine the range and time segment of the major event.

Check whether the major event is coupled to the deterioration of the call drop rate in the time dimension.



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Checklist for the Top-cell Problem (1/2)

Page 35

Standard Action

Analysis Action Deliverables Closing Action

Preliminary analysis of traffic statistics of top sites

1. Analyze the traffic statistics to determine the range and cause of the call drop. 2. Analyze the trend of the call drop rate to determine change of the call drop rate.

1. Distribution of the causes and top causes

2. Actions that affect the call drop rate

1. Optimize the network according to the top causes of the call drop problem. 2. Describe the actions that affect the call drop rate and the impact.

Version check of top sites

Check whether the eNodeB version is upgraded or a new patch is installed.

New and old version numbers

Describe the changes that may affect the call drop rate based on the Release Notes.

Equipment and transmission alarms of top sites

Alarm check of top sites Critical and major alarms Analyze the impact of alarms on the call drop rate. Clear the alarms and check whether the call drop KPI is restored.

Parameter configuration check of top sites

Parameter configuration check of top sites

1. Difference of parameters before and after the upgrade 2. Difference of parameters compared with the baseline

1. Determine whether the parameter change affects the call-drop KPI.

2. Roll back the parameters and check whether the call-drop KPI is restored.



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Checklist for the Top-cell Problem (2/2)

Page 36


Operation record check of top sites

Check whether batch operations affecting the global network are recorded and whether neighboring cells and PCI are re-planned.

Records of batch operations affecting the global network

Analyze the impact of batch operations on the call drop rate. Determine whether the batch operations can be rolled back.

Neighbor relationship check of top cells

Check for missed configuration of neighbor relationship. Scattered site deployment or network optimization leads to incorrect neighbor relationship.

Missed configuration of neighbor relationship

Add neighboring cells that are not configured in the neighbor relationship. Check whether the call drop KPI is restored.

Coverage check of top cells

Analyze the MCS and CQI contained in the traffic statistics, CHR, and drive test data to check for coverage overlap or weak coverage of the top cells.

Coverage evaluation report of top cells

Perform network optimization to optimize the coverage.

Interference check of top cells

Analyze the real-time trace data of the top cells to check for inter-modulation interference and external interference.

Interference evaluation report of top cells

Find out and remove the interference.

Major event check

Check for allocation of a large quantity of phone numbers and major activity (such as ceremony, holidays, and games) in the vicinity of top cells.

1. Check the terminal type involved in the number allocation, quantity of number allocation, and subscription policy. 2. Determine the range and time segment of the major event.

Check whether the major event is coupled to the deterioration of the call drop rate in the time dimension.



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Diagnosing Radio Problems • Fault Description

If the abnormal release is recorded in the counter L.E-RAB.AbnormRel.Radio, the abnormal release is

caused by Uu interface and occurs in a non-handover scenario.

• Possible Cause

The abnormal release is caused by weak coverage, uplink interference, or abnormal UE that lead to

maximum number of RLC retransmissions, out-of-sync, or failure of signaling interactions. For details about

diagnosing the interference problem, see LTE RF Channel Check and Troubleshooting Guide.

• Fault Handling Procedure

Analyze the CHR to check whether some top UEs have the highest count.

Analyze the cause values recorded in the CHR.

If the call drop is caused by a factor other than the signaling procedures, analyze the DRB scheduling at layer 2 to

determine whether the call drop is caused by weak coverage or interference.

If the call drop is caused by signaling procedures, observe the last ten signaling messages to determine the faulty

signaling procedure. Determine whether the fault of the signaling procedure is due to failure to receive or process the

signaling messages by either the UE or eNodeB.

The cause values recorded in the CHR are UEM_UECNT_REL_UE_RLC_UNRESTORE_IND,

UEM_UECNT_REL_UE_RESYNC_TIMEROUT_REL_CAUSE,

UEM_UECNT_REL_UE_RESYNC_DATA_IND_REL_CAUSE,

UEM_UECNT_REL_UE_RLF_RECOVER_FAIL_REL_CAUSE, UEM_UECNT_REL_RRC_REEST_SRB1_FAIL, and

UEM_UECNT_REL_RB_RECFG_FAIL_RRC_CONN_RECFG_CMP_FAIL.

Page 37



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Diagnosing Handover Failures

• Fault Description

If the abnormal release is recorded in the counter L.E-RAB.AbnormRel.HOFailure, the

abnormal release is caused by outgoing handover failure.


Obtain the top cells that have the highest counter L.E-RAB.AbnormRel.HOFailure,

analyze the pairs of source and target cells to obtain the top target cells that have the

highest failure rate.

Analyze the CHR of the source and target cells to determine whether the handover

failure is caused by failure to receive the handover command or random access failure.

Examples of the cause values are UEM_UECNT_REL_HO_OUT_X2_REL_BACK_FAIL

and UEM_UECNT_REL_HO_OUT_S1_REL_BACK_FAIL.

Optimize the handover parameters and neighbor relationship and check whether the call

drop KPI is improved.

Page 38



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Diagnosing the Transmission Network

Problem


If the abnormal release is recorded in the counter L.E-RAB.AbnormRel.TNL, the

abnormal release is caused by the transmission network.

• Possible Cause

This call drop is caused by the abnormal transmission between the eNodeB and MME,

such as S1 interface break.


Check for alarms about the transmission network. Clear the alarms and check whether

the problem of abnormal release is solved.

Observe the M2000 and check whether alarms about the transmission network are

recorded in the M2000.

Clear the alarms.

If abnormal releases are still recorded in the counter L.E-RAB.AbnormRel.TNL, collect

the logs and submit them to R&D engineers for further analysis.

Page 39



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Diagnosing the Congestion Problem


If the abnormal release is recorded in the counter L.E-RAB.AbnormRel.Cong, the

call drop is caused by resource congestion.

• Possible Cause

This call drop is caused by radio resource congestion, such as exceeding the

maximum number of users.


If the long-term congestion of a top cell leads to call drops, a short-term solution is

to enable the MLB algorithm or inter-operation to alleviate the load of the local cell.

The long-term solution is to expand the capacity.

Enable the MLB algorithm and check whether the congestion problem is alleviated.

Page 40



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Diagnosing MME Faults


If an abnormal release is recorded in the counter L.E-RAB.AbnormRel.MME, the

abnormal release is initiated by the EPC. However, this abnormal release is not

recorded in the counter L.E-RAB.AbnormRel.


Analyze the information of the EPC.

The cause value recorded in the CHR is UEM_UECNT_REL_MME_CMD. Analyze the

last ten signaling messages recorded in the CHR. If these messages show that the

problem is not caused by the eNodeB, focus on analysis of the EPC.

Analyze the S1 interface trace of the top cells to obtain the distribution of the cause value.

Discuss with the EPC engineers about the analysis result and signaling messages.

Page 41



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Deliverables

• Output of the activities in the checklist

• If the front-line engineers fail to solve a difficult problem, collect the following

information and submit them to R&D engineers for further analysis:

One-click log (Mandatory)

Logs of the LMPT and LBBP of the top cells

Standard interface signaling (Mandatory)

Signaling trace of the S1, X2, and Uu interfaces

Network configuration (Mandatory)

Topology information, engineering parameters, and configuration files of the top sites

TTI trace (Optional)

IFTS trace and cell trace. These traces generate large amount of data. Only the data of the top cells and

top time segments is collected.

Single-UE trace (Optional)

The single-UE trace is used for in-depth diagnosis of top UEs. The entire-network single-UE trace can be

performed by using the IMSI queried from the EPC using the TMSI.

Page 42



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Content





• Case Study



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Case 1: RRC Reestablishment Failure of a UE

As shown in the upper right figure, the cause value of the abnormal

release is RRC_REEST_SRB1_FAIL.

As shown in the middle right figure, this problem occurs repeatedly

from 11:51 o'clock to 18:49 o'clock in cell 0.

As shown in the lower right figure, the TMSI column shows that this

problem is contributed by a single UE whose TMSI is C2 B0 B0 40

and the cause value is "Reconfiguration Failure".

As shown in the lower left figure, the message type indicates that

this reconfiguration message is not a handover command or

measurement control. This message is probably for reconfiguration

of the CQI, SRS, or transmission mode (TM). Upon reception of the

RRC CONN REESTAB message, the UE does not respond.

Therefore, the eNodeB releases the UE in 5s.

Page 44



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Case 2: UE Exception

Analysis of the CHR shows that the cause value of the abnormal release is

RLC_UNRESTORE_IND. This cause value indicates that the maximum number

of DRB RLC retransmissions is exceeded.

This problem occurs repeatedly from 10:51 to 13:49 in cell 2.

The TMSI column indicates that this problem is contributed by a single UE whose

TMSI is C2 7F 20 56.

The last 16 DRB scheduling procedures at a period of 64ms indicate that the

symptoms are similar. The symptoms are that the UE encounters suddenly

terminated data transmission shortly after the access. The duration from access

to release is tens of seconds to 2 minutes, indicating that the problem is not

caused by script test. The access type is MO-DATA, indicating that the user is

performing a service.

Page 45



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Case 3: Poor Uplink Quality

• As shown in the right figure, the uplink

RSRP and SINR received by the

eNodeB are poor from the last four 512

ms to the last sixteen 64 ms: The uplink

RSRP is below –135 dBm and the SINR

of the SRS and DMRS is below –3 dB,

indicating that the service drop is caused

by uplink weak coverage.

Page 46

• As shown in the left figure, from the last four 512

ms to the last sixteen 64 ms, the uplink RSRP is

about –130 dBm but the SINR of the uplink SRS

and DMRS is below –3 dB, indicating that the

service drop is due to weak coverage caused by

weak uplink interference.



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Case 4: Target Cell Reconfiguration Failure • Release cause

TGT_ENB_RB_RECFG_FAIL is the cause value contained in the RB

reconfiguration failure message during a handover.

The symptom is that after the UE is successfully handed over to the target

cell, the target eNodeB sends the PATH SWITCH REQ ACK message to

the MME and, in 100 ms, sends the UE CONTEXT REL REQ message

containing the cause value "unspecified". The lower left figure shows the

last ten signaling messages.

• Fault diagnosis

During the handover procedure, the EPC delivers the PATH_SWITCH_ACK message

containing the downlink AMBR value that is inconsistent with the downlink AMBR

contained in the S1/X2 handover request. Analysis shows that this is a defect of the

RR module. The upper-layer control module of the RR module sends the AMBR

Update message to the RB module who thinks that there is no need to deliver a

reconfiguration message to the UE. Therefore, the RB module returns a null value to

the upper-layer control module. However, the upper-layer control module regards this

return value as an exception and releases the UE. This problem is solved in

eRAN2.1SPC430.

Page 47



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Case 5: Service Drop Caused by

Inter-RAT Redirection

• Release cause: Inter-RAT redirection

IRHO_REDIRECTION_TRIGER is the

release caused by inter-RAT redirection. In

eRAN2.1SPC400/SPH401, this cause value

is counted as a call drop, as shown in the

following figure.

This problem is solved in eRAN2.1 SPC420,

as shown in the right figure.

Page 48



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Abnormal Transmission

• On December 11, the service drop rate of the entire network deteriorates for the Tele2 900M,

Telenor 900M, and Tele2 2.6G bands, as shown in the following figure.

• Huawei field engineers discussed with the customer and suspected the EPC. However, they got

no positive answer.

Page 49



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Abnormal Uu Interface • Release cause

UE_RESYNC_TIMEROUT_REL_CAUSE indicates that the abnormal release is caused by resynchronization upon timeout of the

resynchronization timer. The same problem is recorded by the standard interface trace as "Radio Connection With UE Lost".

UE_RLC_UNRESTORE_IND indicates that the abnormal release is caused by restoration failure after exceeding the maximum number of RLC

retransmissions. The same problem is recorded by the standard interface as "Radio resources not available".

UE_RESYNC_DATA_IND_REL_CAUSE indicates that the abnormal release is caused by resynchronization triggered L2 report data. The same

problem is recorded by the standard interface trace as "Unspecified".

• Cause analysis

The DRB scheduling information at the last 4 512ms and 16 64ms periods shows that most abnormal releases are caused by suddenly

terminated data transmission, possibly caused by unplugging the data card or UE fault. The following figure shows the CHR information.

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Case 8: RRC Connection

Reestablishment Failure • Release cause (“Radio Connection With UE Lost” recorded in the standard interface trace)

RRC_REEST_SRB1_FAIL indicates failure to restore SRB1 during RRC reestablishment.

The last 10 signaling messages as shown in the following figure indicates that after sending the

RRC_CONN_REESTAB message, the eNodeB fails to receive the RRC_CONN_REESTAB_CMP

message from the UE before the 5s timer on the Uu interface expires.

The L2 scheduling information shows that the UE sends the ACK message upon reception of the

RRC_CONN_REESTAB message.

We suspect that the problem is caused by failure of some UEs to send the

RRC_CONN_REESTAB_CMP message. Some Samsung UEs have such a problem.

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Guide to Optimizing LTE Service Drops

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