Huawei LTE Handover events

LTE eRAN3.0 Handover Fault Diagnosis

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Typical X2 Handover flow



Legend

packet data packet data

UL allocation

2.Measurement Reports

(RRC_MEAS_RPRT)

3.HO decision

4.Handover Request

(HANDOVER_REQUEST)

5.Admission Control

6.Handover Request Ack

(HANDOVER_REQUEST_ACKNOWLEDGE)

7.RRC Conn. Reconf. incl.

mobilityControlinformation

(RRC_CONN_RECFG(HO_CMD))

DL allocation

Data Forwarding

11.RRC Conn. Reconf. Complete

(RRC_CONN_RECFG_CMP(HO_CMP))

17. UE Context Release

(UE_CONTEXT_RELEASE)

12.Path Switch Request

(S1AP_PATH_SWITCH_REQ)

UE Source eNB Target eNB Serving Gateway

Detach from old cell and

synchronlze to new cellDeliver buffered and in transit

packets to target eNB

Buffer packets from Source eNB9.Synchronisation

10.UL allocation+TA for UE

packet data

packet data

L3 signalling

L1/L2 signalling

User Data

1.Measurement Control

(RRC_CONN_RECFG)

16.Path Switch Request Ack

(S1AP_PATH_SWITCH_REQ_ACK)

18.Release Resources

Ha

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Co

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Ha

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Exe

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Ha

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r

Pre

pa

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MME

0.Area Restriction Provided

13.User Plane update

request

15.User Plane update

response

14.Switch DL path

8.SN Status Transfer

(SN_STATUS_TRANSFER)

End Marker

End Marker

packet data

Measurement Control CMP

(RRC_CONN_RECFG_CMP)

Measurement Control

(RRC_CONN_RECFG)

Measurement control includes all control parameters to guide UE measurement.

Measurement consists of intra-frequency measurement and inter-frequency measurement.

Perquisite: Related handover algorithms should be enabled

For intra-frequency measurement control, this message is default sent during UE initial

attach procedure or after UE handover in a new cell. The following IEs (Information

Element) are included inside:

Measurement object, indicate the object with unique frequency and ID

Report configuration, indicate the related event to period report parameters.

A1/A2/A3 could be included.

Measurement ID, bind the ID with object and report configuration, thus eNodeB

can understand the purpose of each report.

Quantity configuration, including the measurement filter parameters

For inter- frequency measurement control, this message should be trigged by some specific

intra-frequency measurement report (A1/A2) or some special event based on handover

scenarios. Besides the above IEs, it includes one more IE: gap pattern, assist UE to measure

inter-frequency .



Once the event for report is trigged, UE periodic repeat this report until eNodeB reply the

instruction or the UE report times approach the maximum report times which is included in

measurement control message



After eNodeB receive measurement report, it select the suitable target and send handover

request message to the target cell via S1 or X2 interface. The following IEs could be

included:

eCGI of target cell

GUMMEI

UE context information

UE history information









General handover success rate = (Handover preparation success rate) × (handover

execution success rate)

Handover preparation success rate = Handover execution attempt number /Handover

preparation attempt number. This is used to evaluate the admission success rate for the

target cell, if this rate is low, that indicates the handover fault is caused by target cell

admission control problem.

Hand over execution success rate = Handover execution success number/Handover

execution attempt number. Generally, we need focus on this KPI to analyze the handover

fault caused by radio problem, such as coverage and parameters setting.







Measurement point:

L.HHO.Prep.FailOut.MME: During handover preparation in a handover over the S1

or X2 interface, the L.HHO.Prep.FailOut.MME counter is incremented by 1 each

time the source eNodeB receives a UE CONTEXT RELEASE COMMAND message

from the MME.

L.HHO.Prep.FailOut.NoReply: After the handover preparation in a handover over

the S1 or X2 interface is complete, if the source eNodeB does not receive any

messages from the target and does not cancel the handover, the

L.HHO.Prep.FailOut.NoReply counter is incremented by 1 each time the source

eNodeB does not receive a HANDOVER REQUEST ACKNOWLEDEG message or a

HANDOVER PREPARATION FAILURE message from the target eNodeB.

L.HHO.Prep.FailOut.PrepFailure: During handover preparation in a handover over

the X2 interface, the L.HHO.Prep.FailOut.PrepFailure counter is incremented by 1

each time the source eNodeB receives a HANDOVER PREPARATION FAILURE

message from the target eNodeB.

L.HHO.Prep.FailOut.HOCancel: During a handover over the S1 or X2 interface,the

L.HHO.Prep.FailOut.HOCancel counter is incremented by 1 each time the source

eNodeB decides to cancel the handover and sends a HANDOVER CANCEL message

to the MME or target eNodeB.







Early handover occurs if the target cell is not good or stable, consists of following scenarios:

Scenario 1: After receiving a handover command, a UE experiences a radio link

failure (RLF) during the handover to the target cell. Then, the UE is handed over

back to the source cell during RRC connection reestablishment.

Scenario 2: A UE camps on the target cell for a short period and the RLF happen. If

UE can reestablish the RRC connection to source cell, it is consider as a early

handover



Scenario 1: After UE sends measurement report, it doesn’t receive any handover

commands.



Scenario 2: UE doesn’t send measurement report, RLF happens in source cell, and RRC

establishment setup to target cell also fails.





Handover failure is caused by signaling exchange failure, including UU interface, X2

interface or S1 interface.

UU interface message exchange failure is usually caused by poor coverage

X2 interface or S1 interface message exchange failure is usually caused by

transmission problem







If we don’t enable the basic coverage based intra/inter frequency handover swith, then

eNdoeB doesn’t send any measurement control message.





From above figure, we can see that Due to poor DL channel quality, SINR of PDCCH is very

low, leading the demodulation of UL grant failure. Therefore, the measurement report

can’t be sent.

RSRP, RSRQ, IBLER and DL/UL grant statistic can be used to evaluate channel quality

Solution:

To increase the possibility of UL scheduling, we can enable the pre- allocation

scheduling, so that UE can get UL grant in advanced to send measurement report

If the poor quality is caused by coverage, then make some RF adjustment for

coverage

If the poor quality is caused by edge interference, we can enable ICIC feature to

mitigate the interference









Scenario: after eNodeB receive measurement report, if all reported cell is not in NRT

(neighbor relation table) and ANR is not enable or the cell in NRT is set forbidden handover,

then eNodeB doesn’t send any handover request to target .

Solution:

Enable ANR to automatic configure NCL and NRT

Configure NRT by manual

How to check neighbor configuration



Should be “Permit

HO”





Scenario 1: No transmission resource

Possible cause:

Incorrect configuration of IP path

The transmission resource is limited

Solution:

First check IP path configuration, check if all the parameters are identical



If all configuration is correct, the problems could be caused by the restriction of

transmission resource . We can extend transmission bandwidth or enable relevant

TRM algorithms



Scenario 2 : Due to the poor transmission quality, leading very long handover prepare time,

Solution: Contact transmission engineer to check the problem





Possible cause :

Poor DL coverage

Improper parameters handover parameters setting

Solution:

RF adjustment

This scenario is considered as delayed handover, we can use MRO algorithm to

optimize it



Possible cause

Poor coverage of target cell

PRACH power control setting problem

Handover parameters setting problem

Solutions:

Perform RF adjustment of target cell

Increase initial power or power step for PRACH

This problem is considered as early handover, we can enable MRO algorithm to

optimize the relevant parameters





















Documents

Huawei LTE Handover events