The document discusses LTE handover fault diagnosis. It describes typical handover flows, measurement control processes, handover request messages, KPIs for measuring handover success rates, and various fault scenarios and solutions. Common faults include poor coverage causing signaling failures, incorrect neighbor cell configurations, and transmission issues on the X2 or S1 interfaces. Diagnosis involves analyzing call traces, RSRP/RSRQ measurements and troubleshooting potential causes like RF adjustments, parameter optimizations or transmission resource limitations.

1. LTE eRAN3.0 Handover Fault Diagnosis
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2. LTE eRAN3.0 Handover Fault Diagnosis
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5.  Typical X2 Handover flow
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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 cell
Deliver buffered and in transit
packets to target eNB
Buffer packets from
Source eNB
9.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
Handover
Completion
Handover
Execution
Handover
Preparation
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)

6.  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 .
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7.  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
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8.  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
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12.  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.
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15.  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.
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18.  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
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19.  Scenario 1: After UE sends measurement report, it doesn’t receive any handover
commands.
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20.  Scenario 2: UE doesn’t send measurement report, RLF happens in source cell, and RRC
establishment setup to target cell also fails.
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22.  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
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25.  If we don’t enable the basic coverage based intra/inter frequency handover swith, then
eNdoeB doesn’t send any measurement control message.
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27.  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
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31.  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
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Should be “Permit
HO”

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34.  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
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35.  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
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36.  Scenario 2 : Due to the poor transmission quality, leading very long handover prepare time,
 Solution: Contact transmission engineer to check the problem
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38.  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
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39.  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
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