Inter-frequency and inter-RAT handovers can be coverage, load, or service based. Coverage-based handovers are triggered by certain A3/A4/A5 events for inter-frequency and B1/B2 events for inter-RAT. The document discusses the parameters involved in measuring cells and configuring handovers, including measurement reports, handover commands, and key performance indicators for analyzing handover issues. Common causes of handover problems include poor downlink quality, interference, and abnormal X2 interface signaling.

1. LTE Handover Fault Diagnosis
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2. LTE Handover Fault Diagnosis
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3. LTE Handover Fault Diagnosis
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4. LTE Handover Fault Diagnosis
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5. 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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6. 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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7. 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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8. LTE Handover Fault Diagnosis
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9. 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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10. LTE Handover Fault Diagnosis
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11. 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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12. Measurement report configuration contains report configuration ID, measurement quantity,
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report mode and etc.
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13. Measurement report contains measurement ID and measurement result.
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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

14. Handover command contains PCI of target cell and some related configuration information.
LTE Handover Fault Diagnosis
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15. LTE Handover Fault Diagnosis
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16. If whole-network handover KPI is not good, first transmission and equipment alarms
LTE Handover Fault Diagnosis
should be checked, then cell-level handover KPI is analyzed.
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17. 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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18. LTE Handover Fault Diagnosis
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19. LTE Handover Fault Diagnosis
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20. The result is from a network with HUAWEI equipments.
LTE Handover Fault Diagnosis
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21. LTE Handover Fault Diagnosis
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22. LTE Handover Fault Diagnosis
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23. Possible scenarios for handover triggered too early:
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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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24. LTE Handover Fault Diagnosis
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25. 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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26. LTE Handover Fault Diagnosis
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27. LTE Handover Fault Diagnosis
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28. LTE Handover Fault Diagnosis
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29. 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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30. LTE Handover Fault Diagnosis
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31. These are just possible reasons for abnormal Uu signaling. More information and trace
LTE Handover Fault Diagnosis
result should be collected for deep analysis.
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32. 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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33. 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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34. LTE Handover Fault Diagnosis
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35. 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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36. LTE Handover Fault Diagnosis
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37. LTE Handover Fault Diagnosis
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38. Possible solutions:
LTE Handover Fault Diagnosis
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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39. 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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40. UE can not receive random access response because of the downlink interference.
LTE Handover Fault Diagnosis
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41. LTE Handover Fault Diagnosis
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42. LTE Handover Fault Diagnosis
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43. LTE Handover Fault Diagnosis
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44. LTE Handover Fault Diagnosis
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45. 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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46. The figures shows that after adjustment handover is triggered successfully. SINR and IBLER
LTE Handover Fault Diagnosis
are better.
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47. LTE Handover Fault Diagnosis
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48. LTE Handover Fault Diagnosis
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49. LTE Handover Fault Diagnosis
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50. LTE Handover Fault Diagnosis
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51. LTE Handover Fault Diagnosis
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52. LTE Handover Fault Diagnosis
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53. LTE Handover Fault Diagnosis
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