Sunday, 21 August 2016

LTE PCI Self-Optimization Feature



Introduction
There are 504 physical cell identifiers (PCIs) in an LTE network. They are divided into 168 groups and each group consists of three PCIs. PCIs are essential for successful signal synchronization and signal demodulation. Each E-UTRAN cell maps only one PCI When excessive E-UTRAN cells exist on the LTE network, multiple E-UTRAN intra-frequency cells inevitably use the same PCI. If PCIs are improperly planned or manually modified, cell frequencies are changed, or neighboring cell parameters are modified, PCIs may conflict between E-UTRAN intra-frequency cells
Developed the PCI conflict detection and self-optimization feature, which provides the following functions:
·         PCI conflict detection PCI conflict detection is divided into the following types:
1.       Distributed detection Distributed detection is performed by the eNodeB. Configuration parameters on the eNodeB, such as neighboring cell information, PCIs, and frequencies, change due to manual operations, ANR, or X2 message interaction, causing the eNodeB to perform PCI conflict detection.
2.       Centralized detection Centralized detection is performed by the SONMaster. Based on the obtained NE configuration data, engineering parameter data, and MRs and performance data to which you subscribe, the SONMaster performs PCI conflict detection manually or periodically.
·         Distributed and centralized detection results are displayed on the SONMaster in a unified way and involved in PCI self-optimization.PCI self-optimization.Conflicting cells are re-assigned with proper PCIs based on the detected PCI conflicts combined with the network topology and neighboring cell configuration data. PCI self-optimization supports PCI mod 3, mod 6, and mod 30 optimization.
Benefits
PCI conflict detection and self-optimization provided by automatically detects PCI conflicts between neighboring cells on the LTE network and assigns proper PCIs to conflicting cells using PCI self-optimization to eliminate or reduce PCI conflicts, thereby reducing service drop rates and improving handover success rates. In addition, this feature optimizes PCI mod 3 conflicts, enhancing the downlink throughput of cells and improving the SINR

PCI Conflict Types
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.

PCI Confusion
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.
The PCI confusion occurs in the following scenarios :

1. Intra-frequency confusion between LTE cells
In this scenario, cells A, B, and C are LTE cells with the same downlink frequency. Cells A and B have the same PCI. Therefore, cells A and B create confusion for cell C
Note : According to LTE neighboring cell configuration principles, the frequency and PCI of cell C must not be the same as those of cell A or cell B. Otherwise, cell A or cell B cannot be added to the NCL of cell C LTE

 



In this scenario, cells A, B, and C are LTE cells with the same downlink frequency. Cells A and B have the same PCI. Therefore, cells A and B create confusion for cell C
Note : According to LTE neighboring cell configuration principles, the frequency and PCI of cell C must not be the same as those of cell A or cell B. Otherwise, cell A or cell B cannot be added to the NCL of cell C LTE

2. Inter-frequency confusion between LTE cells

 
In this scenario, cells A, B, and C are LTE cells. Cells A and B have the same frequency and PCI and are inter-frequency neighboring cells of cell C. Therefore, cells A and B create confusion for cell C. 3

3.Inter-RAT confusion created by LTE cells for a UMTS cell

 
In this scenario, two LTE cells create PCI confusion for the UMTS cell, thereby affecting the handover of the UMTS cell. 

According to RNC configuration principles, two LTE cells with the same frequency and PCI must not be added to the NCL of the UMTS cell. Therefore, you can add only cell A or cell B to the NCL of cell C.
4) Inter-RAT confusion created by LTE cells for a GSM cell
The principle diagram for LTE is the same as that for UMTS. LTE-to-GSM handovers seldom occur on the network. Therefore, LTE-to-GSM confusion is seldom created and is not considered in this version temporarily

PCI Mod 3 Interference

On the LTE network, if PCIs of two neighboring cells are different and the mod 3 remainders are the same, this constructs the PCI mod 3 interference, as shown in

 
As shown in the preceding figure, cells A, B, and C have the same frequency but different PCIs. The PCI mod 3 remainders for cells A and B are 1, which constructs the PCI mod 3 interference. If a cell is in dual antenna mode, the carriers transmitted in the reference signal   will generate intra-frequency interference, thereby decreasing the SINR. (Note: If cells are in single antenna mode and the PCI mod 6 remainders are the same, the carriers transmitted in the reference signal will generate intra-frequency interference, thereby decreasing the SINR.) According to the research results of PCI mod 3 conflicts on the network, the PCI mod 3 conflicts have the following impact on the user SINR and throughput:
·         The SINR decreases most when the cell has no load and decreases less when the network load grows. The PCI mod 3 conflicts have little impact on user throughput.
·         In highly-loaded scenarios, the PCI mod 3 conflicts bring insignificant impact and are not the main cause of interference problems. Therefore, the PCI mod 3 conflicts in the case of certain load need to be considered.

PCI Self-Optimization

For distributed detection, the eNodeB reports PCI conflict information to the SONMaster after detecting PCI conflicts. After receiving distributed detection results, the SONMaster combines them with centralized detection results, performs centralized PCI self-optimization, and assigns conflicting cells with proper PCIs. PCI self-optimization results take effect only when they are delivered to the eNodeB
PCI self-optimization is also supported in scenarios where some network element (NE) engineering parameters are not configured or the cells use compressed bandwidth. These parameters include longitude, latitude, azimuth, and beamwidth.

Prioritization Rules
If multiple cells experience PCI conflicts, the SONMaster adheres to the following prioritization rules to prioritize the conflicting cells and then selects the cells for PCI optimization in descending order.
1. Prioritizes cells with higher optimization priorities specified by users. The optimization priority of an LTE cell can be specified on the SONMaster as High priority, Low priority, or Locked. During PCI self-optimization, conflicting cells with a high priority are assigned with new PCIs in precedence. A conflicting cell whose priority is Locked will not be assigned with a new PCI.
 2. Prioritizes cells experiencing the most frequent PCI conflicts with other cells. Changing the PCI of a cell experiencing the most frequent PCI conflicts improves the PCI optimization efficiency. If multiple conflicting cells have the same optimization priority, the cell that experiences the most frequent PCI conflicts is preferentially optimized.
 3. Prioritizes cells that are newly deployed or cells whose PCIs are recently changed. This is because: − The probability of data errors for these cells is high. − Changing the PCI of a cell involved in network reconfiguration has a minor impact on the live network. If multiple conflicting cells have the same optimization priority and experience the same PCI conflict level, newly deployed cells or cells whose PCIs are recently changed are preferentially optimized.
4. Prioritizes micro cells. A new PCI takes effect after it is delivered to the eNodeB. This PCI delivery blocks the cell under the eNodeB, thereby interrupting services. Changing the PCI of a micro cell has a minor impact on the live network because a micro cell usually covers a small area. If multiple conflicting cells have the same optimization priority, experience the same PCI conflict level, and have the same PCI modification property (cells are deployed recently or their PCIs are changed recently or neither), micro cells are preferentially optimized.
5. Prioritizes cells that have a small number of neighboring cells. Blocking a cell with few neighboring cells has a minor impact on the live network. If multiple conflicting cells of the same type have the same optimization priority, experience the same PCI conflict level, and have the same PCI modification property, cells that have a small number of neighboring cells are preferentially optimized.

Budi Prasetyo

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1 comments:

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altaf sk
AUTHOR
27 June 2017 at 03:17 delete

Hi,
In this article you mentioned that "Changing the PCI of a cell involved in network reconfiguration has a minor impact on the live network". Can you please explain a little more on this. What will happen to connected UE's when enodeB changes it pci: does it give a rrc release to them or make a handover. Can you also provide a source of reference for this article. I am interested in learning about the PCI optimization feature. Your inputs will be a great help to me.
Thanks

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