LTE Handover Fault Diagnosis

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LTE Handover Fault Diagnosis

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LTE Handover Fault Diagnosis

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LTE Handover Fault Diagnosis

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� Inter-frequency handover and inter-RAT handover can be coverage-based, load-based or

LTE Handover Fault Diagnosis

service-based handover. This course focuses on coverage-based handover.

� A3/A4/A5 event can be used to trigger coverage-based inter-frequency handover. eNodeB

can configure which event is used.

� B1/B2 event can be used to trigger coverage-based inter-RAT handover. eNodeB can

configure which event is used.

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� A1 leaving condition：Ms + Hys < Thresh

LTE Handover Fault Diagnosis

� A2 leaving condition：Ms – Hys > Thresh

� A3 leaving condition：Mn + Ofn + Ocn + Hys < Ms + Ofs + Ocs + Off

� A4 leaving condition：Mn + Ofn + Ocn + Hys < Thresh

� A5 leaving condition：Ms - Hys > Thresh1 or Mn + Ofn + Ocn + Hys < Thresh2

� B1 leaving condition：Mn + Ofn + Hys < Thresh

� B2 leaving condition：Ms - Hys > Thresh1 or Mn + Ofn + Ocn + Hys < Thresh2

� The variables in the formula are defined as follows:

� Ms is the measurement result of the serving cell, not taking into account any

offsets.

� Mn is the measurement result of the neighbouring cell, not taking into account any

offsets.

� Ofn is the frequency specific offset of the frequency of the neighbour cell.

� Ocn is the cell specific offset of the neighbour, and set to zero if not configured for

the neighbour cell.

� Ofs is the frequency specific offset of the serving frequency.

� Ocs is the cell specific offset of the serving, and is set to zero if not configured for

the serving cell.

� Hys is the hysteresis parameter for the events.

� Thresh1 and Thresh2 is the threshold parameter for the events.

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� Currently intra-frequency handover is most popular in networks. So intra-frequency

LTE Handover Fault Diagnosis

handover parameters are discussed here.

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LTE Handover Fault Diagnosis

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� Step 1: UE measures source cell and target cells. If the trigger condition is fulfilled, UE

LTE Handover Fault Diagnosis

reports measurement results.

� Step 2 and step 3: After receiving measurement result, eNodeB makes decision to trigger

handover. Source eNodeB sends Handover Request message to target eNodeB, including

target cell ID and some parameters for handover preparation. After receiving handover

request, target eNodeB tries access control and prepares resource for handover, then

sends Handover Request Acknowledge to source eNodeB.

� Step 4: Source eNodeB sends RRC Connection Reconfiguration including handover

parameters for handover.

� Step 5: Source eNodeB sends SN Status Transfer message to target eNodeB.

� Step 6: UE triggers non-contention based random access in target cell.

� Step 7: After successful random access in target cell, UE sends RRC Connection

Reconfiguration Complete message to target eNodeB.

� Step 8 and step 9: Target eNodeB sends Path Switch message to MME. MME informs S-

GW to switch S1 connection to target eNodeB. After successful switch MME sends Path

Switch Acknowledge message to eNodeB.

� Step 10: Target eNodeB sends UE Context Release message to source eNodeB to release

the resource of the UE.

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� Measurement control includes measurement ID, measurement quantity, measurement

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object, measurement report configuration and etc.

� The figure in the slide is an example of measurement object.

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� Measurement report configuration contains report configuration ID, measurement quantity,

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report mode and etc.

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� Measurement report contains measurement ID and measurement result.

LTE Handover Fault Diagnosis

� Measurement ID in measurement report is same with the ID in the corresponding

measurement control.

� The following is RSRP and RSRQ measurement report mapping:

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Reported value Measured quantity value Unit

RSRP_00 RSRP < -140 dBm

RSRP_01 -140 ≤ RSRP < -139 dBm

RSRP_02 -139 ≤ RSRP < -138 dBm

… … …

RSRP_95 -46 ≤ RSRP < -45 dBm

RSRP_96 -45 ≤ RSRP < -44 dBm

RSRP_97 -44 ≤ RSRP dBm

Reported value Measured quantity value Unit

RSRQ_00 RSRQ < -19.5 dB

RSRQ_01 -19.5 ≤ RSRQ < -19 dB

RSRQ_02 -19 ≤ RSRQ < -18.5 dB

… … …

RSRQ_32 -4 ≤ RSRQ < -3.5 dB

RSRQ_33 -3.5 ≤ RSRQ < -3 dB

RSRQ_34 -3 ≤ RSRQ dB

RSRP measurement report mapping

RSRQ measurement report mapping

� Handover command contains PCI of target cell and some related configuration information.

LTE Handover Fault Diagnosis

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� If whole-network handover KPI is not good, first transmission and equipment alarms

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should be checked, then cell-level handover KPI is analyzed.

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� L.HHO.IntraeNB.IntraFreq.ExecSuccOut：Number of successful intra-eNodeB intra-

LTE Handover Fault Diagnosis

frequency outgoing handovers in a cell

� L.HHO.IntraeNB.InterFreq.ExecSuccOut：Number of successful intra-eNodeB inter-

frequency outgoing handovers in a cell

� L.HHO.IntereNB.IntraFreq.ExecSuccOut：Number of successful inter-eNodeB intra-

frequency outgoing handovers in a cell

� L.HHO.IntereNB.InterFreq.ExecSuccOut：Number of successful inter-eNodeB inter-

frequency outgoing handovers in a cell

� L.HHO.IntraeNB.IntraFreq.ExecAttOut：Number of intra-eNodeB intra-frequency outgoing

handover attempts in a cell

� L.HHO.IntraeNB.InterFreq.ExecAttOut：Number of intra-eNodeB inter-frequency outgoing

handover attempts in a cell

� L.HHO.IntereNB.IntraFreq.ExecAttOut：Number of inter-eNodeB intra-frequency outgoing

handover attempts in a cell

� L.HHO.IntereNB.InterFreq.ExecAttOut：Number of inter-eNodeB inter-frequency outgoing

handover attempts in a cell

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LTE Handover Fault Diagnosis

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� The result is from a network with HUAWEI equipments.

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� Possible scenarios for handover triggered too early:

LTE Handover Fault Diagnosis

� After receiving handover command, UE fails to handover to the target cell because

of the bad signal in target cell, then UE triggers RRC connection reestablishment in

source cell.

� UE handovers to target cell successfully but downlink synchronization fails in the

target cell, then UE triggers RRC connection reestablishment.

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LTE Handover Fault Diagnosis

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� Possible scenarios for handover triggered too late:

LTE Handover Fault Diagnosis

� The source cell signal is bad, so UE can not receive handover command. Then UE

triggers RRC connection reestablishment in target cell. If the UE context is setup

successfully in the target cell, the RRC reestablishment can be done.

� The source cell signal drops too fast, UE has no time to report measurement result.

Then UE triggers RRC connection reestablishment in target cell. The UE context can

not be setup successfully in the target cell, the RRC reestablishment can not be

done.

� There is something wrong with the X2 transmission, HANDOVER REQUEST

message can not be transferred to the target cell.

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LTE Handover Fault Diagnosis

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� In real networks, most handover failures are caused by Uu interface problems, resulting in

LTE Handover Fault Diagnosis

the abnormal signaling over Uu interface.

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� These are just possible reasons for abnormal Uu signaling. More information and trace

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result should be collected for deep analysis.

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� RSRP: RSRP is the downlink reference signal power received by UE. RSRP and SINR roughly

LTE Handover Fault Diagnosis

reflect the downlink quality. Generally the RSRP is more than -85dBm for UE near an

eNodeB, equal to -95dBm for UE at the middle of eNodeB coverage, and less than -

105dBm for UE at cell edge.

� SINR: SINR is the downlink reference signal SINR. If SINR is less than 0dB, the downlink

channel quality is poor. If SINR is less than -3dB, the downlink channel quality is very poor

and close to the demodulation threshold, which can easily cause loss of handover

messages. The uplink SINR can be obtained from the user performance tracing on the LMT.

� IBLER: Generally IBLER should be near the target value of 10%. If the channel quality is

very good, IBLER is close to 0%. If IBLER is very high, it means the channel quality is poor

and call drop may happens.

� Uplink scheduling grants and downlink scheduling assignments over the PDCCHs: It is the

number of PDCCHs correctly demodulated by a UE in each TTI. If there is enough uplink or

downlink data source and the channel quality is very good, the number of the grants or

assignments could be very high, even close to 1000. If the number of uplink grants is too

low, it may be caused by bad channel quality.

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� For example, poor downlink channel quality not only affects demodulation of the downlink

LTE Handover Fault Diagnosis

signaling, the incorrectly demodulated PDCCH also affects the uplink scheduling, leading

to loss of uplink signaling.

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LTE Handover Fault Diagnosis

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� Abnormal signaling over X2 interface is usually caused by X2 transmission problem or

LTE Handover Fault Diagnosis

internal problem of site. Internal problem of site is not popular. So it is very possible that

X2 transmission causes abnormal signaling over X2 interface. It is not easy to analyze

transmission problem. In this slide transmission problem is not the focus.

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LTE Handover Fault Diagnosis

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� Possible solutions:

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� Adjusting the antenna to change the coverage of serving cell or neighboring cell

� Adjusting CIO to make handover happen earlier

� To increase neighboring cell CIO or decrease serving cell CIO could make

handover happen earlier.

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� RAR: Rand Access Response

LTE Handover Fault Diagnosis

� In handover area, RSRP is good but SINR and IBLER is bad. So it is very possible that the

problem is caused by downlink interference.

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� UE can not receive random access response because of the downlink interference.

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� The figure shows the RSRP, SINR and IBLER during handover. IBLER and SINR is poor, so

LTE Handover Fault Diagnosis

handover fails.

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� The figures shows that after adjustment handover is triggered successfully. SINR and IBLER

LTE Handover Fault Diagnosis

are better.

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