LTE SON Feature - PCI Conflict Detection Feature Proposal
CMPak 4G North Optimization Team
Jan 2016
HUAWEI TECHNOLOGIES CO., LTD.
Content
1
PCI Conflict Detection Feature Description
2
PCI Conflict Detection Types
3
PCI Conflict Detection Scenarios
4
Implementation Plan
5
Conclusion
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PCI Conflict Detection Description:
•
Physical cell identifiers (PCIs) are used for signal synchronization and demodulation. There are 504 PCIs in the LTE system. These PCIs
are divided into 168 groups, each containing 3 PCIs.
•
Each evolved universal terrestrial radio access network (E-UTRAN) cell is allocated a PCI. When many cells exist in an LTE system, some
cells with the same frequency must share one PCI.
•
A PCI conflict may occur between cells with the same frequency if PCIs are planned inappropriately or changed manually, or if neighboring
cell parameters are modified.
•
Huawei developed the PCI conflict detection and self-optimization feature, which provides the following functions:
PCI conflict detection.
Detects PCI conflicts in the E-UTRAN. The eNodeB triggers PCI conflict detection when its neighboring cell, PCI, or frequency changes.
The change may be caused by manual operation, ANR, or message exchanges over the X2 interface.


PCI self-optimization
Reallocates appropriate PCIs to conflicting cells based on the PCI conflict detection result. (Not Recommended).
Note:
Delivering PCI optimization suggestions will deactivate and then activate this cell, during which the number of online subscribers is reduced.
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PCI Conflict Detection Types:
PCI Conflict Types
The PCI conflict is classified into PCI collision and PCI confusion.

PCI Collision
A PCI collision occurs between two intra-frequency cells that use an identical PCI but are insufficiently isolated. In this case, UEs in the
overlapping area of the two cells cannot implement signal synchronization or decoding.
•
•
If a cell has the same frequency and PCI as one of its neighboring cells, there is a PCI collision
between the cell and the neighboring cell.
Figure-A shows a PCI collision between cell A and cell B.
 PCI Confusion
Fig A.
A PCI confusion occurs between a detected cell and a neighboring cell if the two cells have the same frequency and PCI and if the
reference signal received power (RSRP) of the two cells reaches the handover threshold.
•
The PCI confusion may lead to UE handover failures or service drops.
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PCI Conflict Detection Types:
A PCI confusion occurs in the following scenarios:
•
•
The detected cell is a configured neighboring cell of the serving cell. Fig A.
The detected cell is not a configured neighboring cell of the serving cell. Fig B.
Fig A.
Fig B.
 Methods of Triggering PCI Conflict Detection
Any changes in the PCI conflict parameters will drive the eNodeB to check for PCI conflicts. The following are possible changes:
•
•
•
The PCI or frequency of an eNodeB's local cell is changed.
The PCI or frequency of an external E-UTRAN cell is changed, or an external E-UTRAN cell is added or removed.
A neighboring cell in the intra-frequency or inter-frequency NRT is added or removed.
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PCI Conflict Detection scenarios:
An eNodeB detects PCI conflicts in the following scenarios:

PCI conflict detection based on X2 messages
•
The eNodeB receives an X2 message. If an X2 connection has been set up between the eNodeBs and GlobalProcSwitch.
X2BasedUptENodeBCfgSwitch is turned on, one eNodeB will update the neighboring cell parameters based on another eNodeB's cell
information carried in the message.

PCI conflict detection triggered by manual operations
•
The PCI conflict parameters are changed due to manual modifications to the PCI, frequency or other parameters.

PCI conflict detection triggered by ANR
•
ANR adds or removes a neighboring cell, or it updates the PCI of the neighboring cell.
Note:
If the eNodeB detects or resolves a PCI conflict, it reports the PCI conflict information to the U2000 and this information will be updated in the
PCI Optimization Task window on the U2000. The U2000 also decides whether to allow the eNodeB to report the ALM-29247 Cell PCI Conflict
alarm to the U2000 alarm console or LMT alarm interface, depending on the setting of the ENodeBAlgoSwitch. PciConflictAlmSwitch
parameter.
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PCI Conflict Detection scenarios:
PCI conflict detection based on X2 messages:
If an X2 interface is set up or an eNodeB parameter changes after the X2 interface is set up, the eNodeBs involved are notified through the X2
interfaces.
During the setup of an X2 interface, the source eNodeB sends an X2 Setup Request message to the target eNodeB. If the request is
acceptable, the target eNodeB sends an X2 Setup Response message to the source eNodeB.
•
•
•
After the X2 interface is set up, the source eNodeB sends an X2 eNodeB Configuration Update message to the target eNodeB if the X2-related
configuration, such as serving cell PCI, TAC, and neighboring cells, of the source eNodeB changes.
when an eNodeB receives an X2 message (X2 Setup Request, X2 Setup Response, or X2 eNodeB Configuration Update) from the peer
eNodeB, it updates corresponding parameters in the NCL for the cells under the peer eNodeB, adds cells to or removes cells from the
NCL, or adds neighbor relation to or removes neighbor relation from the NRT based on the message. This will trigger PCI conflict
detection.
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Implementation Plan:
Recommendation:
•
Based on the Scenario’s of PCI Conflict detection, It is recommended to enable PCI conflict detection based on X2 messages, as PCI
conflict detection based on ANR affects system capacity. To obtain ECGIs, a UE in connected mode needs to be changed to work in
discontinuous reception (DRX) mode, during which the UE cannot perform any services. This affects uplink and downlink throughput.
Trial Areas:
It is recommended to implement this trial in following steps.
1)
2)
3)
In First step, we will select 1 TAC (Proposal TAC: 5031) and turned on the switches and check if there is any PCI conflict between cells
reported or not.
If no PCI conflict reported in 3 days, then after customer approval we manually generate PCI Conflict issue to verify the feature working
properly.
After analysis of outcomes of first steps, we can decide to roll-back or implement the trial globally.
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Implementation Plan:
Monitoring Method:
•
•
3 days monitoring is required to check the PCI Conflict detection between cells..
Complete KPI benchmark will be shared after the implementation to ensure all KPI’s are working normally.
Expected Outcome:
•
•
After removal of PCI conflict issues between cells, which identified by PCI Conflict detection feature will improve the outgoing Handover
Success Rate and Service Call Drop Rate in CMPAK North Network Sites Improve User Experience.
Interference between cells due to PCI Conflict issue will also be fixed.
Roll-Back criteria:
•
If there is any major degradation the trial will be rolled back.
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Conclusion:
• This will increase overall User Experience and Network Performance.
• By Monitoring the performance of the PCI conflict detection feature it will improve the service
drop rates and outgoing handover success rates.
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Thank you
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