LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
Confidential Information of Huawei. No Spreading Without Permission
LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
Typical X2 Handover flow UE
Source eNB
Target eNB
Serving
MME
Gateway
0.Area Restriction Provided 1.Measurement Control (RRC_CONN_RECFG) packet data
packet data UL allocation
Legend
2.Measurement Reports (RRC_MEAS_RPRT)
L3 signalling L1/L2
3.HO decision
signalling
User Data
5.Admission Control 6.Handover Request Ack (HANDOVER_REQUEST_ACKNOWLEDGE)
DL allocation 7.RRC Conn. Reconf. incl. mobilityControlinformation (RRC_CONN_RECFG(HO_CMD)) Detach from old cell and synchronlze to new cell
Handover Preparation
4.Handover Request (HANDOVER_REQUEST)
Deliver buffered and in transit packets to target eNB
Handover Execution
8.SN Status Transfer (SN_STATUS_TRANSFER) Data Forwarding
9.Synchronisation
Buffer packets from Source eNB
10.UL allocation+TA for UE 11.RRC Conn. Reconf. Complete (RRC_CONN_RECFG_CMP(HO_CMP))
packet data
packet data
End Marker
14.Switch DL path packet data
End Marker 17. UE Context Release (UE_CONTEXT_RELEASE)
16.Path Switch Request Ack (S1AP_PATH_SWITCH_REQ_ACK)
15.User Plane update response
18.Release Resources Measurement Control (RRC_CONN_RECFG) Measurement Control CMP (RRC_CONN_RECFG_CMP)
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Handover Completion
12.Path Switch Request (S1AP_PATH_SWITCH_REQ) 13.User Plane update request
LTE eRAN3.0 Handover Fault Diagnosis
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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LTE eRAN3.0 Handover Fault Diagnosis
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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LTE eRAN3.0 Handover Fault Diagnosis
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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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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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LTE eRAN3.0 Handover Fault Diagnosis
Scenario 1: After UE sends measurement report, it doesn’t receive any handover commands.
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LTE eRAN3.0 Handover Fault Diagnosis
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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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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”
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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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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LTE eRAN3.0 Handover Fault Diagnosis
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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LTE eRAN3.0 Handover Fault Diagnosis
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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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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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LTE eRAN3.0 Handover Fault Diagnosis
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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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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LTE eRAN3.0 Handover Fault Diagnosis
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