gNodeB Cell Outage Detection and Recovery

Cell outage refers to scenarios where a UE cannot access cell services or where there is significant key performance indicator (KPI) deterioration due to faults or alarms. Cell outage significantly affects network performance and user experience, especially on single-frequency and single-RAT networks. Cell outage detection enables Huawei gNodeBs to automatically detect cell outages, shortening the cell outage duration. Cell outage detection is used throughout the network lifecycle.

Cell outage may be caused by:

• Software or hardware faults detected by the gNodeB, which include faults in an RRU, baseband processing unit, CPRI port, feeder, power supply system, and transmission link (such as an S1 link).

• Unknown faults, which include the software or hardware faults that cannot be detected by a gNodeB and do not trigger any alarms, such as faults on common or physical channels.

A gNodeB detects cell outage based on:

• Alarms: The gNodeB checks software and hardware and determines cell outage upon detecting cell unavailability alarms.

• Abnormal KPIs: The gNodeB determines cell outage based on major KPIs including the service setup success rate, abnormal release rate, and RRC setup success rate.

1) Based on Cell Unavailability Alarms

Detection Methods

The gNodeB monitors its hardware and software status. Once detecting a cell unavailability fault, it reports ALM-29841 NR Cell Unavailable to the U2020. After receiving the alarm, the U2020 performs one of the following operations:

• If the Enable cell unavailability fault detection check box is selected, the U2020 considers the cell to be an outage cell due to a cell unavailability fault and adds the cell to the outage cell list for you to query. After the fault is rectified either before or after cell reactivation, the U2020 removes this cell from the list and adds this cell to the list of recovered outage cells.

• If the Enable cell unavailability fault detection check box is deselected, the U2020 does not determine whether the cell is an outage cell. After the fault is rectified either before or after cell reactivation, the U2020 adds the cell to the list of recovered outage cells.

Important Notes

• Deselect the Enable cell unavailability fault detection check box on the U2020 before performing any operations on the entire network.

  If you perform operations on the entire network, a large number of cells may become outage due to the volume of cell unavailability faults being reported.

• Select the Generate alarm on cell outage check box on the U2020 to effectively process cell outage caused by cell unavailability faults.

  Cell unavailability faults greatly affect user experience. To ensure the reliability of cell unavailability fault-based cell outage detection, the U2020 can detect cell outage even when the ALM-29841 NR Cell Unavailable alarm is manually inappropriately masked, or is lost due to an alarm storm.

  This setting helps you obtain outage cell data in cases where the alarm is masked. Therefore, you are advised to enable alarm generation by selecting the Generate alarm on cell outage check box on the U2020

  If you deactivate the outage cell caused by cell unavailability faults, ALM-29841 NR Cell Unavailable will be cleared. However, if such alarms caused by cell unavailability faults persist, the outage cell is not added to the list of recovered outage cells and remains in the outage cell list.

2) Based on Abnormal Preset KPIs

The gNodeB determines whether a cell is an outage cell based on the KPIs service setup success rate, abnormal release rate, and RRC setup success rate. Cell outage detection based on the RRC setup success rate applies only to SA networking. Cell outage detection based on the service setup success rate and cell outage detection based on the abnormal release rate apply to both NSA networking and SA networking. The gNodeB tracks the values of these KPIs for the cell over consecutive performance measurement periods to identify cell outage. This feature cannot identify cell outage based on KPI changes caused by UE faults or coverage problems, such as weak coverage or coverage holes. If any KPI for a cell returns an unexpected value, the gNodeB determines that the cell is an outage cell and notifies the U2020 of the outage cell. Table 1 describes the criteria for cell outage detection based on abnormal preset KPIs. The U2020 considers the cell recovered only after KPIs return to normal.

The measurement period is the period by which cell outage detection based on abnormal preset KPIs is performed. For details on checking the measurement periods, see Measurement Period Check.

Table 1 Criteria for determining cell outages based on abnormal preset KPIs

Table 2 describes the preset KPI thresholds.

Table 2 Preset KPI thresholds

Table 3 and Table 4 describe the counters used in the following formulas for calculating these KPIs.

In NSA networking,

• Service setup success rate = Number of successful DRB additions for LTE-NR NSA DC UEs on the SgNB/Number of DRB addition requests for LTE-NR NSA DC UEs on the SgNB

• Abnormal release rate =Number of abnormal DRB releases for LTE-NR NSA DC UEs on the SgNB/Number of DRB releases for LTE-NR NSA DC UEs on the SgNB

Where,

• The number of DRB addition requests for LTE-NR NSA DC UEs on the SgNB indicates the cumulative total of Service setup number.

• The number of DRB releases for LTE-NR NSA DC UEs on the SgNB indicates the cumulative total of Service release number.

In SA networking,

• Service setup success rate = Number of successful QoS flow setups/Number of QoS flow setup attempts

• Abnormal release rate =Number of abnormal QoS flow releases/(Number of abnormal QoS flow releases + Number of normal QoS flow releases)

• RRC setup success rate = Number of successful RRC connection setups/Number of RRC connection setup requests

Where,

• The number of QoS flow setup attempts indicates the cumulative total of Service setup number.

• The sum of the number of abnormal QoS flow releases and the number of normal QoS flow releases indicates the cumulative total of Service release number.

• The number of RRC connection setup requests indicates the cumulative total of RRC setup request number.

Table 3 Counters in NSA networking

Table 4 Counters in SA networking

If the KPI that causes cell outage meets the criteria described in Table 5 before or after cell reactivation, the U2020 determines that the outage cell has recovered and removes it from the outage cell list. Table 5 lists the criteria for cell outage recovery detection based on abnormal preset KPIs.

Table 5 Criteria for determining cell outage recovery based on abnormal preset KPIs

Cell outage detection based on abnormal KPIs listed in Table 1 does not apply to the following scenarios:

• Some carriers are shut down, all RF channels are shut down, or the cell is powered off at the scheduled time.

• The cell is blocked.

• No resources are available in a cell due to cell overload. In this case, cell outage detection based on the service setup success rate does not apply.

• The cell is barred. In this case, UEs cannot initiate an RRC connection setup in this cell and therefore cell outage detection based on the RRC setup success rate does not apply.

• The Enable preset KPI detection check box is deselected.

The U2020 adds an outage cell detected based on an abnormal KPI to the outage cell list. Users can view this list on the U2020.

Pay attention to the following:

• An outage cell may recover in the next performance measurement period after being detected, and such scenario cannot be avoided even if this feature is used. This is more likely to occur if users set the threshold Service setup success rate(%) or RRC setup success rate(%) to a value greater than the default value, or set the threshold Abnormal release rate(%) to a value less than the default value. To reduce this probability as much as possible, it is recommended that you set the thresholds Service setup number, Abnormal release number, and RRC setup request number to 60 or above, especially during initial network deployment where weak coverage issues or coverage holes are more likely to occur.

• If any of the optical module-related alarms exists, and if the alarm severity is major, "The current cell in outage has an optical module fault. Check it preferentially." will be displayed in the cell outage cause. Optical module-related alarms include ALM-26506 RF Unit Optical Interface Performance Degraded, ALM-26504 RF Unit CPRI Interface Error, and ALM-26232 BBU Optical Module Transmit/Receive Fault.

Cell Outage Recovery

The U2020 rectifies outage cells with or without manual confirmation, or does not rectify outage cells, depending on settings.

If automatic rectification is enabled, the U2020 performs only cell reactivation. In this process, the gNodeB reconfigures cell data and requests cell resources again.

The U2020 provides automatic and manual cell reactivation methods. After an outage cell is reactivated, the cell status is displayed as running on the U2020. The U2020 then continues monitoring the cell.

Outage cell recovery methods vary according to how cell outage is detected:

• Cell outage detection based on cell unavailability alarms

  If ALM-29841 NR Cell Unavailable is displayed on the U2020, troubleshoot the fault and clear the alarm based on the alarm information. After the alarm is cleared, check whether cell outage recovers.

• Cell outage detection based on abnormal preset KPIs

  If preset KPIs return unexpected values, observe the KPIs generated during the most recent 72 or more hours within the cell outage period and check the KPI failure causes. If the KPIs were confirmed abnormal, recover the cell as soon as possible.

Three cell outage recovery modes are available:

• Recover After Manual Acknowledgement: The U2020 waits for manual selection of a recovery method before recovering an outage cell. The following recovery methods are involved: Reactivate Cell, Reset Base Station, Reset RRU, Reset RRU Through Powering-off, Reset BBP, and Reset BBP Through Powering-off.

• Auto Recover: The U2020 automatically attempts to recover an outage cell. Only one recovery method is available: Reactivate Cell. If an outage cell is detected based on an alarm, Auto Recover does not take effect.

• No Recover: The U2020 does not recover outage cells. Cells will be recovered through other methods.

Cite from the link: http://3ms.huawei.com/hi/group/2976841/wiki_5621156.html