guide to td-lte handover problem analysis

Guide to TD-LTE Handover Problem Analysis

Contents

Handover Procedure Description Concept of Handover Optimization Summary of Handover-Related Parameters Handover Optimization FAQs and Cases

内部资料妥善保管▲

Handover Procedure Description

UE Target eNBSource eNB

2. Measurement Report

3. HO Request

4. HO Request Ack

EPC

1. RRCConnection Reconfiguration

(MeasurementControl)

Packet data Packet data

UL allocation（PDCCH）

Buffer packets from S_eNB

Deliver buffered packets to T_eNB

6. SN Status Transfer

7. Random Access Preamble

8. Random Access Response

5. RRC Connection Reconfiguration(HO Command)

9. RRC Connection Reconfiguration Complete(HO Confirm)

Path Switch

10. Release Resource

Flush DL Buffer

Release Resources

Measurement Control is usually brought in the “RRC Connection Reconfiguration”message for the random access procedure or last handover process. Measurement Report:The UE reports the cells which RSRP reach the handover threshold according the measurement control information of the current cell. HO RequestAfter receiving a Measurement Report, Source eNodeB applies to Target eNodeB for resources and configuration information .HO Request AckTarget eNodeB feeds back the information received by the UE and other configuration information to Source eNodeB. RRC Connection ReconfigurationSource eNodeB transfers the information received by Target eNodeB and configuration information to the UE and informs the UE that Target eNodeB is ready for UE access. The RRC Connection Reconfiguration message contains Measurement Control. SN Status TransferSource eNodeB transfers the cache of UE services to Target eNodeB. Random Access PreambleAfter receiving an RRC Connection Reconfiguration (handover command) in Step 5, the UE gets connected to the eNodeB by using the information in the reconfiguration message.Random Access ResponseTarget eNodeB sends ”RRC connection reconfiguration complete” to responds the UE access. When the UE receives this command, the access process is complete. Afterwards, the UE sends “RRC Connection Reconfiguration Complete” on the RRC layer (Step 9).RRC Connect Reconfiguration complete （ HO Confirm ）The UE reports an RRC Connection Reconfiguration Complete. The handover is complete. Release ResourceAfter the UE successfully accesses to Target eNodeB, Target eNodeB informs Source eNodeB to delete the context information of UE.

内部资料妥善保管▲ Handover Procedure Description Handover categories

Intra-site handover

X2 interface handover

S1 interface handover

内部资料妥善保管▲ Handover Procedure DescriptionHandover Categories—Intra-site Handover

UE eNB

Measurement Report

MSG1(Random Access Preamble)

RAR(Random Access Response)

RRC Connection Reconfiguration(HO Command)

RRC Connection Reconfiguration Complete(HO Confirm)

Intra-site handover procedure need not to apply to the core network for changing the data transmission route.

内部资料妥善保管▲ Handover Procedure DescriptionHandover Categories—X2 Interface Handover

UE Target eNBSource eNB

Measurement Report

EPC

X2AP HandoverRequest

X2AP HandoverRequestAcknowledge

RRCConnectionReconfiguration(HO Command)

X2AP SNStatusTransfer

S1AP PathSwitchRequest

S1AP PathSwitchRequestAcknowledge

X2AP UEContextRelease

RRCConnectionReconfigurationComplete(HO Confirm)



Neighboring cells attributable to different eNBs, and eNBs has been established X2 port link.

As shown in the figure on the right: After receiving a Measurement Report, Source eNodeB sends a X2AP Handover Request to Target eNodeB through X2 interface (Step 3) . Target eNodeB feeds back by returning a X2AP Handover Request Acknowledge. After receiving the feedback (Step 4), Source eNodeB sends a RRC Connection Reconfiguration to the UE and sends a SNStatus Transfer containing package cache and package cache number information to Target eNodeB. After the UE accesses the target eNodeB, Target eNodeB sends a Path Switch Request to the core network to inform the network to transfer the UE service to Target eNodeB. The priority of X2 interface handover is higher than that of X2 interface handover.

内部资料妥善保管▲ Handover Procedure DescriptionHandover Categories—S1 Interface Handover

UE EPCSource eNB

Measurement Report

Target eNB

S1AP HandoverRequest

S1AP HandoverRequestAcknowledge

RrcConnectionReconfiguration(HO Command)

S1AP UEContextReleaseCommand

RrcConnectionReconfigurationComplete(HO Confirm)



S1AP HandoverRequest

S1AP HandoverRequestAcknowledge

S1AP_EnbStatusTransferMsg

S1AP MMEStatusTransfer

S1AP HandoverNotify

S1AP UEContextReleaseComplete

Neighboring cells attributable to the different eNBs, but did not establish the X2 port link.

The procedure of S1 interface handover is almost the same as the procedure of X2 interface handover, except the interaction signaling is transferred between eNodeBs through the S1 interface of the core network which has a longer delay than that of X2 interface.

内部资料妥善保管▲ Handover Procedure Description UE Measurement Mechanism

When the terminal measurement results satisfy the formula conditions and the state continued for a period of time,UE reports a Measurement Report

Mn+Ofn+Ocn-Hys>Ms+Ofs+Ocs+Off

the length of time:Time to Trigger

Measured amount

Serving cell

Neighbor cell

The report meets the conditions

Time

内部资料妥善保管▲ Handover Procedure DescriptionForeground Signaling Resolution

内部资料妥善保管▲ Handover Procedure DescriptionForeground Signaling Resolution—Measurement Control

Measurement control information is contained in reconfiguration messages that are generally sent for initial access and handover commands.

Measurement control information includes neighboring cell list, event judgment threshold, delay and reporting interval.

内部资料妥善保管▲ Handover Procedure DescriptionForeground Signaling Resolution—Measurement Control

Measurement Report reports to all cells that meet event trigger conditions.

内部资料妥善保管▲ Handover Procedure DescriptionForeground Signaling Resolution—Handover Command

内部资料妥善保管▲ Handover Procedure Description Foreground Signaling Resolution-- MSG1

UE accesses the target eNodeB by using the access configuration carried in handover commands sent by the source eNodeB.

内部资料妥善保管▲ Handover Procedure DescriptionForeground Signaling Resolution--eNodeB Returning an RAR Message

内部资料妥善保管▲ Handover Procedure DescriptionForeground Signaling Resolution—UE Feeding Back a Reconfiguration Complete Message

The reconfiguration complete message is already packetized when UE receives a handover command. Random access to the target eNodeB can be deemed as access initiated by the reconfiguration complete message contained in MSG3.

Contents



To analyze abnormal processes, first check eNodeB, transmission and UE statuses. If there are problems, solve them before analysis. The analysis of abnormal handover process include: Check whether the UE receives a handover

command after sending a measurement report (procedure 1).

Check whether the UE sends a MSG1 to the target eNodeB after receiving a reconfiguration command (procedure 2).

Check whether the UE receives a MSG2 after sending the MSG1 (procedure 3).

Concept of Handover OptimizationMeasurement report

Check whether the handover command is

received.

Check whether the MSG1 is successfully

sent.

Procedure 1

No

Yes

Check whether the RAR is received.

Yes Procedure 2

No

Procedure 3

No

End

The delivery of reconfiguration is complete.

(MSG3)

Yes

内部资料妥善保管▲ Concept of Handover Optimization Procedure 1

Check whether the serving cell has

received the measurement report.

Yes

Check whether there is any cell not being

configured as a neighbor.

Check whether the test points are properly covered.

No

Check whether there is interference with the cell

uplink.

Yes

Check the optimization coverage and handover

parameters.

Check and resolve the interference.

Yes

Optimize the neighbor cells.

Yes

Check whether the problem is resolved after the optimization.

No

No

Yes

Procedure 1

End

Check whether the cell state, admission parameters, and

transmission are proper and whether the UE

works properly.

No

Resolve the cell exceptions and transmission problems and

replace the UE for another test. Yes

Report the fault symptoms and check

whether the UE or equipment has any

potential fault.

No

内部资料妥善保管▲ Concept of Handover OptimizationProcedure 2

Procedure 2

Check whether the test points are properly

covered.

Check whether there is interference with

the cell uplink.

1. Check whether the eNodeB and UE work

properly. 2. Check the optimization

coverage and handover parameters. Check whether

the problem is resolved after the optimization.

End

Yes

Yes

Check and resolve the interference.

Yes

No

Report the fault symptoms and check

whether the UE or equipment has any

potential fault.

No

内部资料妥善保管▲ Concept of Handover OptimizationProcedure 3

Check whether the eNodeB and the UE are

abnormal.

Procedure 3

Check the optimization coverage, handover

parameters and target cell access parameters.

Check whether the problem is resolved

after the optimization.

End

Yes

No

If the fault lies with the eNodeB, remove the eNodeB

problems; if the fault lies with the UE, replace the UE

to proceed with the test.

Report the fault symptoms and check whether the UE or equipment has any potential

fault.

No

Contents



问题案例Parameter Name Value Range Unit Adjustment Step Length

Cell-specific reference signals power

-60~50 dBm 0.1

Default Value Transfer Path Scope Source of Parameter

12 ENB->UE Cell 3GPP

Configuration path

OMCR setting interface: Serving Cell Configuration>Base Information>Cell-specific reference signals power

Summary of Handover-Related Parameters


Parameter Name Value Range UnitAdjustment Step Length

The Offset Between PDSCH EPRE and Cell-specific RS EPRE (P_A_DTCH) of center user

-6, -4.77, -3, -1.77, 0, 1, 2, 3

dB

Default Value Transfer Path ScopeSource of Parameter

0 ENB->UE Cell 3GPP

Configuration path

OMCR setting interface: Serving Cell Configuration>MAC ALG C >The offset Between PDSCH EPRE and Cell-specifc RS EPRE(P_A_DTCH) of centre user




Sel_Qrxlevmin -140~-44 dBm 2


-128dBm ENB->UE Cell 3GPP

Configuration path

OMCR setting interface: Serving Cell Configuration>Cell Selection and Reselection >Sel_Qrxlevmin




Filter Coefficient for RSRP0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 11, 13, 15, 17, 19



Configuration path

OMCR setting interface: Serving Cell Configuration >Parameters of Measurement Configuration > Filter Coefficient for RSRP




Event Identity A1, A2, A3,A4,A5Default Value Transfer Path Scope Source of ParameterEvents with measurement quantity of RSRP: A1, A2, A3, A4 and A5Cycles with measurement quantity of RSRP: -Events with measurement quantity of RSRQ: A1, A2, A3,A4 and A5 Cycles with measurement quantity of RSRQ: -

ENB->UE Cell 3GPP

Configuration pathOMCR setting interface: Base Station Radio Resource Management>Measurement Configuration >IntraFreq Measurement for Handover> Event Identity




Cell individual offset -24 ～ 24 dB 1


0 ENB->UE CELL 3GPP

Configuration path

OMCR setting interface ； Serving Cell Configuration>ENodeB Neighbouring Relation > Cell individual offset



Parameter Name

Value Range UnitAdjustment Step Length

Time to Trigger

0, 40, 64, 80, 100, 128, 160, 256, 320, 480, 512, 640, 1024, 1280, 2560, 5120

ms

Default Value Transfer Path ScopeSource of Parameter

256 ENB->UE eNb 3GPP

Configuration path

OMCR setting interface: Base Station Radio Resource Management>Measurement Configuration >IntraFreq Measurement for Handover> Time to Trigger



Parameter Name

Value Range Unit Adjustment Step Length

Hysteresis 0, …, 15 dB 0.5


0 ENB->UE Cell 3GPP

Configuration path

OMCR setting interface: Base Station Radio Resource Management>Measurement Configuration >IntraFreq Measurement for Handover> Hysteresis



Parameter Name Value Range Unit Adjustment Step Length

Amount of Reporting for event

1, 2, 4, 8, 16, 32, 64, Infinity


1 ENB->UE Cell 3GPP

Configuration path

OMCR setting interface ： Base Station Radio Resource Management>Measurement Configuration >IntraFreq Measurement for Handover> Amount of Reporting for event



Parameter Name

Value Range Unit Adjustment Step Length

Reporting Interval for Periodical

120, 240, 480, 640, 1024, 2048, 5120, 10240, 60000, 360000, 720000, 1800000, 3600000

ms

Default Value

Transfer Path Scope Source of Parameter


Configuration path

OMCR setting interface ： Base Station Radio Resource Management>Measurement Configuration >IntraFreq Measurement for Handover> Reporting Interval for Periodical




Maximum Cell Number reported

1, 2, …, 8


3 ENB->UE Cell 3GPP

Configuration path

OMCR setting interface ： Base Station Radio Resource Management>Measurement Configuration >IntraFreq Measurement for Handover> Maximum Cell Number reported




Power Ramping Step for PRACH

[0, 6] dB 2


2 ENB->UE Cell 3GPP

Configuration path




Max Transmit Number for Prach

0: 3, 1: 4, 2: 5, 3: 6, 4: 7, 5: 8, 6: 10

7: 20, 8: 50, 9: 100, 10: 200


5: 8 ENB->UE Cell 3GPP

Configuration path




Initial Power for Preamble of Prach

[-120, -90] dBm 2


-100 ENB->UE Cell 3GPP

Configuration path


Contents


内部资料妥善保管▲ Handover Optimization FAQs and Cases Abnormalities Caused by Missing Configuration of Neighbor Cells

Multiple measurement reports

No response to measurement report

内部资料妥善保管▲ Handover Optimization FAQs and Cases Abnormalities Caused by Missing Configuration of

Neighbor Cells-- Multiple Measurement Reports In a drive test shown in the figures

on the right, the first three measurement reports target PCI=28 (the same PCI, slightly different RSRPs). The fourth measurement report targets PCI=28 and 19. The measurement value of PCI=28 is 3dB higher than that of PCI=19.UE receives a handover command.

The handover command carrying the PCI of the target cell is not 28 but 19 , so PCI=28 is presumed to be a missing configured neighbor cell.

内部资料妥善保管▲ Handover Optimization FAQs and Cases Abnormalities Caused by Missing Configuration of Neighbor Cells-- No response to measurement report

The results of a drive test shows the UE does not receive a handover command after sending a measurement report. The UE initiates a reestablishment procedure after the radio link fails.

内部资料妥善保管▲ Handover Optimization FAQs and Cases Abnormalities Caused by Missing Configuration of Neighbor Cells-- Confirmation of Missing Configured Neighbor Cells

You can confirm whether the neighbor cells are configured by checking eNodeB configurations on the background.

You can also check the neighbor cell information contained in measurement control information sent by the source eNodeB.

内部资料妥善保管▲ Handover Optimization FAQs and Cases Difficult Access to Target eNodeB Due to Uplink Interferences-Problem Phenomenon

Total KPI Type Correspond Attempt Ratio

1Random Access Success[%]

207 215 96.28 %

2 RRC Connect Success[%] 38 41 92.68 %

3Initial Access Success[%]

0 0 0.00 %

4E-RAB Connect Success[%]

44 44 100.00 %

5 Call Drop[%] 26 44 59.09 %6 HO Success[%] 106 130 81.54 %

The test of Fukuoka network indexes shows access failure frequently occurs and call dropping phenomenon appears after handover. Accesses sometimes succeed and sometimes fail. There is no rule about the phenomenon.

内部资料妥善保管▲ Handover Optimization FAQs and Cases Difficult Access to Target eNodeB Due to Uplink Interferences--Problem Analysis To analyze the problem, we select some cells and do tests at fixed points. We find the UE cannot connect to the network and the status indicator on the Qualcomm UE sometimes blinks red (abnormal) and sometimes blinks green. Our self-developed UE also cannot connect to the network.

Qualcomm QXDM interface Self-developed UE LMT interface


We check the call drop procedure and reestablishment procedure on Qualcomm QCAT, the Qualcomm analysis software.

UE triggers MSG1 because DCI0 does not arrive at “rach reason=UL_DATA” and the SR transmission attempts reach the maximum. RRC reestablishment is triggered after the MSG1 fails to be sent to the eNodeB in eight transmission attempts.

Handover Optimization FAQs and Cases Difficult Access to Target eNodeB Due to Uplink Interferences-Problem Analysis


To verify whether it is a regular problem, we carry out DT tests near the sites. Test areas are shown in the following figure: In the area enclosed by blue ellipse, the success rate of accesses and handovers is relatively high.

Some sites in the area enclosed by red ellipse are difficult for UE to access.

RRC reestablishment fails in the HO test. Then UE access fails again after RRC reestablishment is once again rejected.



According to the above analysis, it is presumed that the problem results from uplink data abnormalities caused by interferences. Currently Fukuoka site adopts GC office, namely, one BBU with several RRUs forming a site. All sites in Japan are omnidirectional sites. The site distribution is shown in the figure: Call drops concentrate in the area enclosed by red ellipse. Currently tests are performed only in Ref area (the area enclosed in blue ellipse) .We found call drops have a certain relationship with BBUID. Non-call drop areas belong to BBUID=400010 and call drop areas belong to BBUID=400011 and 400012, so we doubted there are some problems in the correlation of BBUID=400010, 400011 and 400012.



GPSti me RxPow0(dBm) RxPow1 RxPow2 RxPow3 TxDPow0(dBm) TxDPow1 TxDPow2 TxDPow32011-11-11 11: 02: 02: 740 -84. 6 -81. 55 -88. 1 -84. 13 37. 17 37. 02 37. 78 37. 212011-11-11 11: 02: 08: 750 -84. 6 -81. 55 -88. 1 -84. 13 37. 17 37. 02 37. 78 37. 212011-11-11 11: 02: 15: 450 -84. 18 -81. 51 -88. 1 -84. 13 37. 13 37. 16 37. 78 37. 212011-11-11 11: 02: 21: 350 -84. 18 -81. 51 -88. 1 -84. 13 37. 13 37. 16 37. 78 37. 212011-11-11 11: 02: 27: 340 -84. 18 -81. 51 -88. 1 -84. 13 37. 13 37. 16 37. 78 37. 212011-11-11 11: 02: 33: 960 -84. 62 -81. 51 -88. 1 -84. 15 37. 3 37. 16 37. 78 37. 112011-11-11 11: 02: 39: 950 -84. 62 -81. 51 -88. 1 -84. 15 37. 3 37. 16 37. 78 37. 112011-11-11 11: 02: 46: 610 -84. 62 -81. 53 -80. 87 -84. 11 37. 3 37. 21 37. 79 36. 992011-11-11 11: 02: 52: 560 -84. 62 -81. 53 -80. 87 -84. 11 37. 3 37. 21 37. 79 36. 992011-11-11 11: 02: 58: 570 -84. 62 -81. 53 -80. 87 -84. 11 37. 3 37. 21 37. 79 36. 992011-11-11 11: 03: 05: 150 -84. 62 -81. 53 -80. 3 -83. 41 37. 3 37. 21 38. 09 372011-11-11 11: 03: 11: 150 -84. 62 -81. 53 -80. 3 -83. 41 37. 3 37. 21 38. 09 372011-11-11 11: 03: 17: 780 -84. 6 -81. 53 -80. 3 -83. 69 37. 15 37. 21 38. 09 37. 182011-11-11 11: 03: 23: 750 -84. 6 -81. 53 -80. 3 -83. 69 37. 15 37. 21 38. 09 37. 18

We checked the equipment alarms on the OMC based on the previous analysis. We first check the GPS status and found the GPS at 400011 site is unlocked.

The sites of 400011 and 400012 concentrates and the 400011 has no GPS lock, which may lead to GPS interferences in the cells belonging to 400011 and peripheral cells and uplink access failures. For cells where UE handover fails, we query the uplink receive power traced on the MTS, and find that the eNodeB receive power is too high (normally about -96-99dbm) and the receive power evaluation is about -80dbm.

According to the above analysis, the problem is presumed to occur because GPS out-of-lock of BBUID=400011 have impacts on other cells. We check on the background and find the GPS of 400010 and 400012 is normal, but the GPS of 400011 is not powered on by BBU. However, electrical signal are already sent and have impacts on cells around. Afterwards we carry out tests by blocking all the cells belonging to 400011 and did tests and found in some cells of 400010, UE handover also fails. Then we query the uplink receive power and find there are certain interferences.


内部资料妥善保管▲ Handover Optimization FAQs and Cases Difficult Access to Target eNodeB Due to Uplink Interferences--Solution and Verification

From the analysis of access methods for BBUs in Fukuoka area connecting to each cell, it can be found that the sites in Fukuoka area adopt GC office, that is ,a BBU manages all cells belonging to it and every BBU shares a GPS signal source. Although the GPS of the site(BBUID=400010) is already powered on, sometimes GPS out-of-lock occurs and the cells that near to the site(BBUID=400010) are affected. Therefore, we block all the cells belonging to the BBUs of 400011 and 400012 and carry out tests, and finally find UE handover in all the cells belonging to 400010 succeeds. The problem is solved.

内部资料妥善保管▲ Handover Optimization FAQs and Cases Handover Problems Caused by Complicated Environments

Characteristics of the TI project network coverage: All sites are omnidirectional. All sites are densely distributed. Radio propagation environment is complex 1) The engineering parameters cannot be adjusted because TD-

LTE eNodeBs are co-sited with existing PHS sites and using omni-directional antennas.

2) The eNodeBs are densely deployed to ensure indoor coverage.The urban environments environment is complicated, resulting in preferable RSRP but poor SINRs.The pilot pollution is particularly serious nearby the intersections.

The focus of our work is to optimize the failure of these business areas.

内部资料妥善保管▲ Handover Optimization FAQs and Cases Handover Problems Caused by Complicated Environments — Overall Problem Handling Concept

Avoid UE handovers at intersections: If a UE must be redirected to another cell at an intersection, advance or postpone the handover through adjustment or, if necessary, set the one-way neighbor relationship. This approach is only used for routes where the KPI problems cannot be resolved. Radio parameters must be adjusted flexibly based on the alleviation of effects affecting other proper cells during the handover optimization.

Reduce UE handovers brought about by the cross coverage by means of coverage adjustment. If the adjustment fails, try to avoid the UE from being redirected. If the problem still persists, add neighbor cells and adjust the handover parameters.

内部资料妥善保管▲ Handover Optimization FAQs and Cases Handover Problems Caused by Complicated Environments — Case 1

Normally, the UE moves along the blue line and hands over from one cell with PCI=336 to another with PCI=83. However, when the UE moved to the place marked by a red X, the RSRP of the cell with PCI=4 increased suddenly and when the UE handed over to the cell with PCI=4, the RSRP of the cell with PCI=4 dropped unexpectedly. Consequently, the call was dropped because the UE did not receive any handover command.

To prevent the UE from redirection to cells with PCI=336 and PCI=83 at the intersection, we strengthened the RS power of the cell with PCI=83 to keep the handover point between cells with PCI=336 and PCI=83 away from the intersection.


Normally, the UE moves along the blue line and hands over from one cell with PCI=45 to another with PCI=61. However, as the intersection was covered by the cell with PCI=34, the RSRP of the cell with PCI=34 decreased unexpectedly when the UE passed through this intersection. Consequently, the call was dropped because the UE did not receive any handover command after sending a measurement report.

We increased the CIOs of cells with PCI=34 and PCI=61 with 3 dB, moved the handover point towards the west, and redirected the UE to the cell with PCI=61 in advance. As a result, the call drop caused by rapid signal deterioration in the cell with PCI=34 was avoided.


Normally, the UE moves along the blue line and hands over from one cell with PCI=45 to another with PCI=8. However, the UE handed over to the cell with PCI=4 at the intersection marked by a red X, and then the call was dropped when the UE was switched back to the cell with PCI=45.

To prevent the UE from redirection to the cell with PCI=4, we have successively attempted to lower the RS power of the cell with PCI=4 and adjust the CIOs of cells with PCI=45 and PCI=4. Nevertheless, the call drop persisted while the RSRP of the cell with PCI=4 was 17 dB stronger than that of the cell with PCI=45. We analyzed the RSRP distribution of cells nearby the intersection, only to find out that the northbound road was covered by the cell with PCI=4 and the RSRP of the cell with PCI=7 approximated to that of the cell with PCI=4 near the intersection.

A redirection to the cell with PCI=4 can be avoided on this test route by deleting the one-way neighbor relationship (from the cell with PCI=45 to the cell with PCI=4). In this case, the UE moving towards the cell with the PCI=4 from the intersection can be redirected to the cell with PCI=7 first, and then to the cell with PCI=4.

内部资料妥善保管▲ Handover Optimization FAQs and Cases Lessons Learnt on Dealing with Call Drops Caused by UE Out-of-Sync Behaviors — Problem Symptoms

During a test, the re-establishment of an RRC connection was denied when the UE moved to the location marked with a blue rectangle.

内部资料妥善保管▲ Handover Optimization FAQs and Cases Lessons Learnt on Dealing with Call Drops Caused by UE Out-of-Sync Behaviors — Problem Symptoms

According to the check results of the Layer 3 signaling, the measurement report was sent twice before the re-establishment without receiving any handover command, which eventually led to the UE out-of-sync and re-establishment rejection.

According to the diagnosis signaling, the UE had sent an SR before sending the measurement report, but no scheduling information was returned by the PDCCH. Namely, the SR failed.


When the delivery of SRs reached the upper limit, the source cell initiated a random access procedure.Queried the MAC RACH Trigger signaling,the cause value of the delivery of random access was “UL data arrival” — namely, the random access procedure was initiated to restore the uplink services when both SR and MR delivery failed.

During the entire random access procedure, no RAR was received upon each delivery of MSG1 by the source cell.

Handover Optimization FAQs and Cases The experience to deal with Call Drops Caused by UE Out-of-Sync Behaviors — Symptom Analysis


When the delivery of MSG1s reached the upper limit, the attempt to restore uplink services in the source UE failed and the RRC connection was redirected to the re-establishment procedure because of a radio link failure.

However, re-establishment required cell selection and the selected cell had no context of that UE. Consequently, the re-establishment was denied and the call was dropped.

Handover Optimization FAQs and Cases The experience to deal with Call Drops Caused by UE Out-of-Sync— Symptom Analysis

The cell with PCI=134 is reconstructed and has no UE context.

内部资料妥善保管▲ Handover Optimization FAQs and Cases The experience to deal with Call Drops Caused by UE Out-of-Sync Behaviors — Problem Resolution and Verification

UL data arrival often occurred in weak fields of the source cell. This problem can be addressed by redirecting a UE to another cell with better signal quality beforehand.

We checked changes in the RSRP of a trouble spot only to find out that the signal strength of the source cell dropped dramatically while that of the neighboring cell increased sharply in a very short time. In this case, we shortened the time-to-trigger of the current network as the adjustment of cell migration produced no obvious effect.

We attempted to change the "time to the trigger" parameter from 320ms to 256ms, the aim is to shorten A3 judgment time. After the modification, this problem was resolved on the basis of multiple tests.

guide to td-lte handover problem analysis

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