CASANDRA FARGAS GARCÍA Universidad Autónoma de Madrid
2. El fenómeno de la caza de brujas y el concepto de bruja
This section provides information required to troubleshoot intra-RAT handover faults due to poor Uu quality. The information includes fault descriptions, background information, possible causes, fault handling method and procedure, and typical cases.
5.1 Definitions of Intra-RAT Handover Faults
If an intra-RAT handover fault occurs, UEs have difficulty performing intra-RAT handovers due to system faults.
5.2 Background Information
This section describes counters and alarms related to intra-RAT handover faults. In addition, this section provides intra-RAT handover procedures.
Related Counters
l Outgoing Handover Measurement (Cell)(HO.eRAN.Out.Cell) l Incoming Handover Measurement (Cell)(HO.eRAN.In.Cell) For details, see eNodeB Performance Counter Reference.
Related Alarms
l Board overload alarm
– ALM-26202 Board Overload l Alarms related to RF modules
– ALM-26529 RF Unit VSWR Threshold Crossed
– ALM-26522 RF Unit RX Channel RTWP/RSSI Unbalanced l Cell capability degraded alarm
– ALM-29243 Cell Capability Degraded l Alarms related to CPRI links
– ALM-26235 RF Unit Maintenance Link Failure – ALM-26234 BBU CPRI Interface Error
– ALM-26233 BBU CPRI Optical Interface Performance Degraded – ALM-26506 RF Unit Optical Interface Performance Degraded l Alarms related to clock sources
– ALM-26263 IP Clock Link Failure – ALM-26264 System Clock Unlocked – ALM-26538 RF Unit Clock Problem – ALM-26260 System Clock Failure
– ALM-26265 Base Station Frame Number Synchronization Error
Handover Procedures
Handovers are classified as coverage-based, load-based, frequency-priority-based, service-based, and UL-quality-based. For details, see eRAN Mobility Management in Connected Mode Feature Parameter Description.
5.3 Troubleshooting Method
This section describes how to identify and troubleshoot the possible cause.
Possible Causes
There are various causes of handover faults, such as incorrect data configuration, hardware faults, interference, and poor Uu quality. Therefore, to effectively diagnose a handover fault, you need to carry out a pertinent analysis based on the actual situation.
Table 5-1 shows possible causes of handover faults.
Table 5-1 Possible causes of handover faults
Scenario Fault Description Possible Causes
The whole network
l The UE cannot receive handover commands from the network.
l Hardware is faulty.
l Parameters are set to inappropriate values.
l The target cell is congested.
l The Uu quality is poor.
Fault Analysis
The following measures are effective in locating a handover fault:
l Analyzing handover-related performance counters l Investigating TopN cells
l Checking alarms related to devices or data transmission l Checking the configurations of neighboring cells
l Checking handover algorithm configurations l Investigating interference and cell coverage
To locate an intra-RAT handover fault, you are advised to select TopN cells with handover faults and then follow the troubleshooting procedure shown in Figure 5-1.
Figure 5-1 Troubleshooting flowchart for intra-RAT handover faults
Troubleshooting Procedure
1. Check whether the hardware is faulty.
Hardware faults are the most likely cause if handovers suddenly become abnormal without recent modifications to the configurations of the abnormal cell and its neighboring cells.
Yes: Hardware faults are often accompanied by alarms. You are advised to handle the fault by following the instructions on how to troubleshoot handover faults due to hardware faults.
Go to 2.
No: Go to 3.
2. Check whether the fault is rectified.
Yes: End.
No: Go to 3.
3. Check whether handover parameters are incorrectly configured.
Specifically, check whether handover thresholds and neighboring cell configurations are incorrect.
Yes: Follow the instructions on how to troubleshoot handover faults due to incorrect data configurations. Go to 4.
No: Go to 5.
4. Check whether the fault is rectified.
Yes: End.
No: Go to 5.
5. Check whether the service channel of the target cell is severely congested.
Check the service satisfaction rates to determine whether the service channel of the target cell is severely congested.
Yes: Follow the instructions on how to troubleshoot handover faults due to target cell congestion. Go to 6.
No: Go to 7.
6. Check whether the fault is rectified.
Yes: End.
No: Go to 7.
7. Check whether the Uu quality is poor.
Poor Uu quality will cause abnormal signaling exchanges, leading to handover failures.
Yes: Follow the instructions on how to troubleshoot handover faults due to poor Uu quality.
Go to 8.
No: Go to 9.
8. Check whether the fault is rectified.
Yes: End.
No: Go to 9.
9. Contact Huawei technical support.
5.4 Troubleshooting Intra-RAT Handover Faults Due to Hardware Faults
This section provides information required to troubleshoot intra-RAT handover faults due to hardware faults. The information includes fault descriptions, background information, possible causes, fault handling method and procedure, and typical cases.
Fault Description
Typical hardware faults include faulty or overloaded boards, as well as abnormal radio frequency (RF) module or clock sources. If a hardware fault occurs, the cell will degrade in capability or even become out of service, in addition to the following symptoms:
l Abnormal cell-level performance counters – Increased service drop rate
– Decreased handover success rate – Decreased access success rate l Related alarms
Background Information
Related Alarms
l Board overload alarm
– ALM-26202 Board Overload l Alarms related to RF modules
– ALM-26529 RF Unit VSWR Threshold Crossed
– ALM-26522 RF Unit RX Channel RTWP/RSSI Unbalanced l Cell capability degraded alarm
– ALM-29243 Cell Capability Degraded l Alarms related to CPRI links
– ALM-26235 RF Unit Maintenance Link Failure – ALM-26234 BBU CPRI Interface Error
– ALM-26233 BBU CPRI Optical Interface Performance Degraded – ALM-26506 RF Unit Optical Interface Performance Degraded l Alarms related to clock sources
– ALM-26263 IP Clock Link Failure – ALM-26264 System Clock Unlocked – ALM-26538 RF Unit Clock Problem – ALM-26260 System Clock Failure
– ALM-26265 Base Station Frame Number Synchronization Error
Possible Causes
Possible hardware faults that will cause handover faults are listed as follows:
l A board is overloaded.
l An RF module is faulty.
l A common public radio interface (CPRI) link is faulty.
l A clock source is faulty.
Fault Handling Flowchart
Figure 5-2 shows the fault handling flowchart for intra-RAT handover faults due to hardware faults.
Figure 5-2 Fault handling flowchart for intra-RAT handover faults due to hardware faults
Fault Handling Procedure
1. Check whether a hardware fault alarm is reported.
Yes: Handle the hardware fault alarm. Go to 2.
No: Go to 3.
2. Check whether the fault is rectified.
Yes: End.
No: Go to 3.
3. Contact Huawei technical support.
Typical Cases
Fault Description
Handovers between cell 0 and cell 2 under an eNodeB were normal with a high success rate, but the handovers from cell 1 under the eNodeB to its neighboring cells were abnormal with a relatively low success rate (7%) during busy hours.
Fault Diagnosis
1. Alarms about the eNodeB were checked. Cell 1 had reported ALM-26529 RF Unit VSWR Threshold Crossed.
2. As engineers of the customer confirmed, the eNodeB had been reconstructed recently.
Therefore, it was highly probable that the RF connections became abnormal during the site reconstruction.
3. At the site, it was found that the jumper was not securely connected to the feeder, which had caused the cell malfunction.
Fault Handling
The jumper was securely connected to the feeder. According to the KPI log, the inter-cell handover success rate was restored.
5.5 Troubleshooting Intra-RAT Handover Faults Due to Incorrect Data Configurations
This section provides information required to troubleshoot intra-RAT handover faults due to incorrect data configurations. The information includes fault descriptions, background information, possible causes, fault handling method and procedure, and typical cases.
Fault Description
l Handovers to neighboring cells are seldom initiated.
According to drive test results or signaling tracing results, the UE experiences relatively low signal quality in its serving cell. The signal level of neighboring cells meets the threshold for a handover, but handovers occur with a low probability This leads to a high service drop rate.
l Handovers to neighboring cells are frequently initiated.
The signal level and quality of neighboring cells are almost the same as those of the serving cell, but handovers to the neighboring cells are frequently initiated. This leads to poor quality of voice services and a high probability of service drops.
Background Information
None
Possible Causes
l Configurations of neighboring cells are incorrect.
If neighboring cells are not configured or incorrectly configured, handovers cannot be triggered even after the UE reports measurements of these neighboring cells.
l The X2 link is incorrectly configured.
If an X2 interface is incorrectly configured, handovers to some neighboring cells cannot be successfully executed. For example, if the IP path for an X2 interface is incorrectly configured, X2-based inter-eNodeB handovers cannot be executed; or, if the IP path from the target eNodeB to the source serving gateway (S-GW) is not configured, X2-based inter-S-GW handovers cannot be executed.
l Parameters such as handover thresholds, hysteresis, and time-to-trigger are inappropriately configured.
In the preceding handover scenario, a handover is triggered only when the signal level of a neighboring cell is higher than that of the serving cell by at least a certain amount. As a result, if handover parameters (such as the threshold, cell individual offsets [CIOs], hysteresis, and time-to-trigger) are inappropriately set, the probability of triggering handovers is either significantly low or significantly high.
Fault Handling Flowchart
Figure 5-3 shows the fault handling flowchart for intra-RAT handover faults due to incorrect data configurations.
Figure 5-3 Fault handling flowchart for intra-RAT handover faults due to incorrect data configurations
Fault Handling Procedure
1. Check whether the X2 link is incorrectly configured.
Yes: Correct the X2 link configuration. Go to 2.
No: Go to 3.
2. Check whether the fault is rectified.
Yes: End.
No: Go to 3.
3. Check whether there are missing configurations of neighboring cells.
Yes: Complete neighboring cell configurations. Go to 4.
No: Go to 5.
4. Check whether the fault is rectified.
Yes: End.
No: Go to 5.
5. Check whether handover parameters are incorrectly configured.
Yes: Correct their configurations.
No: Go to 7.
6. Check whether the fault is rectified.
Yes: End.
No: Go to 7.
7. Contact Huawei technical support.
Typical Cases
Fault Description
During a drive test, a UE did not receive any handover commands after sending A3 measurement reports to the eNodeB. Ultimately, the service is dropped.
Fault Diagnosis
1. According to Huawei maintenance personnel, these A3 measurement reports were successfully received by the source eNodeB. Later, the source eNodeB sent a Handover Request message through the X2 interface to the target eNodeB, but the target eNodeB responded with a Handover Failure message containing a cause value indicating unavailable transport resources.
2. The signaling over the X2 interface was traced and was found to be normal.
3. The configuration of the IPPATH MO for the X2 interface was checked and an
inconsistency was found. The adjacent node ID specified in the IPPATH MO was different from the X2 interface ID, which caused a resource request failure and ultimately a handover failure.
Fault Handling
The configuration of the IPPATH MO was corrected. Then, the test was conducted again and the UE was successfully handed over to the target cell.
5.6 Troubleshooting Intra-RAT Handover Faults Due to Target Cell Congestion
This section provides information required to troubleshoot intra-RAT handover faults due to target cell congestion. The information includes fault descriptions, background information, possible causes, fault handling method and procedure, and typical cases.
Fault Description
The service satisfaction rate in the target cell is lower than the admission threshold for handed-over services, due to which the target eNodeB rejects the requests of handhanded-overs to the target cell.
The service satisfaction rate in a cell can be viewed on the M2000.
Background Information
None
Possible Causes
l UEs in the target cell surge due to assemblies or activities.
l A large number of UEs have been handed over to the target cell due to inappropriate parameter configurations.
Fault Handling Flowchart
Figure 5-4 shows the fault handling flowchart for intra-RAT handover faults due to target cell congestion.
Figure 5-4 Fault handling flowchart for intra-RAT handover faults due to target cell congestion
Fault Handling Procedure
1. Check whether the handover fails due to target cell congestion.
Yes: Expand the capacity of the target cell or tune the network optimization parameters of the target cell. Go to 2.
No: Go to 3.
2. Check whether the fault is rectified.
Yes: End.
No: Go to 3.
3. Contact Huawei technical support.
Typical Cases
Fault Description
During a period, all handovers to a cell failed.
Fault Diagnosis
1. The cell coverage was checked. No coverage hole was found.
2. The RF module serving the cell was checked. No fault was found.
3. As signaling tracing for a single UE indicated, the service satisfaction rate in the cell was always low (lower than the admission thresholds for handed-over services with QCIs ranging from 1 to 4) when a handover failure message appeared. Therefore, these handovers failed because the traffic channel was so congested in the cell that there were no resources available for new handed-over services.
Fault Handling
Engineers of the customer were advised to expand the cell capacity or reduce UEs in the cell by modifying handover parameter configurations. After the correspond measure was taken, the success rate of handovers to the cell became normal.
5.7 Troubleshooting Intra-RAT Handover Faults Due to Poor Uu Quality
This section provides information required to troubleshoot intra-RAT handover faults due to poor Uu quality. The information includes fault descriptions, background information, possible causes, fault handling method and procedure, and typical cases.
Fault Description
Two symptoms may occur when the Uu quality is poor. One is that the UE cannot receive any handover commands from the eNodeB, the other is that the UE cannot access the target cell and cannot report the handover complete message.
Background Information
Checking interference
1. Start a cell interference detection task and check the performance counter indicating the uplink (UL) signal quality. If high UL modulation and coding scheme (MCS) orders seldom appear, it is highly probable that interference to the cell exists.
2. Start the UE spectral scanning function and further determine whether the interference originates from neighboring cells or external systems.
Checking cell coverage l Check for weak coverage.
If the reference signal received power (RSRP) values reported by UEs during handovers are mostly lower than -115 dB, weak-coverage areas exist in the cell.
l Check for wide coverage and cross-cell coverage.
Wide coverage and over-coverage can be checked by analyzing the actual radius of cell coverage and signal quality variation in the cell.
Checking imbalance between UL and DL quality
Imbalance between UL and DL quality is classified into two situations: lower UL quality and lower DL quality.
l Check whether the transmit power of the RRU and UE falls within link budgets.
l Check the actual UL and DL coverage by using drive tests.
Checking the antenna system
l Check whether the jumper is reversely connected to the feeder.
Analyze the drive test data. If the UL signal level is different from the DL signal level in the cell and UEs at cell edge easily encounter handover failures, the jumper is reversely connected to the feeder and needs to be corrected.
l Check whether the feeder is in poor physical condition.
If a feeder is damaged, water immersed, bending, or not securely connected, the transmit power and receive sensitivity are decreased and severe service drops occur. In this case, the feeder needs to be replaced. For details, see ALM-26529 RF Unit VSWR Threshold Crossed.
Replace faulty feeders promptly.
l Check whether the tilts and azimuths of two antennas are the same.
Possible Causes
The following Uu problems may cause handover faults:
l Interference
l Unsatisfactory coverage
l Imbalance between UL and DL quality l Antenna system faults
Fault Handling Flowchart
To effectively diagnose handover faults due to poor Uu quality, you are advised to firstly find out whether this fault is caused by interference or unsatisfactory coverage. Figure 5-5 shows the fault handling flowchart for intra-RAT handover faults due to poor Uu quality.
Figure 5-5 Fault Handling flowchart for intra-RAT handover faults due to poor Uu quality
Fault Handling Procedure
1. Check whether interference exists. By using a UE spectral scanner, check whether there is DL interference from neighboring cells or external systems. By analyzing the cell interference detection result, check whether there is UL interference.
Yes: Remove the interference. Go to 2.
No: Go to 3.
2. Check whether the fault is rectified.
Yes: End.
No: Go to 3.
3. Check whether cell coverage is abnormal.
Yes: Improve cell coverage. Go to 4.
No: Go to 5.
4. Check whether the fault is rectified.
Yes: End.
No: Go to 5.
5. Check whether there is imbalance between UL and DL quality. Specifically, check whether the transmit power of the RRU and UE falls beyond link budgets.
Yes: Remove the imbalance between UL and DL quality. Go to 6.
No: Go to 7.
6. Check whether the fault is rectified.
Yes: End.
No: Go to 7.
7. Check whether there is a fault in the antenna system.
Yes: Adjust the antenna system. Go to 8.
No: Go to 9.
8. Check whether the fault is rectified.
Yes: End.
No: Go to 9.
9. Contact Huawei technical support.
Typical Cases
None
6 Troubleshooting Service Drops
About This Chapter
This chapter describes the method and procedure for troubleshooting service drops in the Long Term Evolution (LTE) system. It also provides the definitions of service drops and related key performance indicator (KPI) formulas.