UE synchronization and cell identification

The UE synchronize to the eNB reading the primary and secondary synchronization signals (PSS and SSS respectively). By reading both signals the UE achieves time and frequency synchronization (frame, subframe, slot and symbol synchronization in the time domain; and it identifies the center of the channel bandwidth in the frequency domain) and calculates the Physical Layer Cell Identity or PCI.

The Physical Layer Cell Identity (PCI) is used to identify a cell within a network. The UE can differentiate cell from cell by means of its PCI. But it should not be confused with e the Cell ID which is used to identify the cell on an O&M perspective.

Coming back to the PCI, there are only 504 possible physical layer cell identities. So, chances are that each of the 504 PCIs are used more than one time in the network.

The Physical Layer Cell identity is composed of NID1 extracted from the SSS and the NID2 extracted from the PSS. The formula to calculate the PCI is the following:

PCI = (3 x NID1) + NID2

NID1: Physical Layer Cell Identity group. Defines SSS sequence in a range that goes from 0 to 167.

NID2: Identity within the group. Defines PSS sequence in a range that goes from 0 to 2.

If you work with Ericsson RAN the following CLI command can give you both components, to calculate the PCI:

cmedit get * EUtranCellFDD.(physicalLayerSubCellId,physicalLayerCellIdGroup) -t

PCI confusion

PCI Confusion

PCI confusion happened to Cell with PCI 20 (in grey) where it has 2 neighboring cells that shares the same PCI (PCI 31, in blue). Notice that all cells have to be in the same EARFCN to generate a true issue in the network. What you can expect on a situation of PCI confusion? Poor handover success rate, and RRC/RAB drops are common in this particular scenario.

PCI Collision

PCI Collision

This scenario although is very uncommon, is possible to find. It happened when 2 neighboring cells shares the same PCI (like here with PCI 20). Neighboring does not refer in this case to neighbor relation. It is just that both cells are close enough geographically. And also in the same EARFCN. In the overlapping area between these two cells synchronization and demodulation could be a challenge. What you can expect on a situation of PCI collision? Primary there will be high RRC and RAB drop rate.

Workaround

Poor planning and big finger mistakes are responsible for causing any of these scenarios. There are multiple tools to deal with this issues. Some vendors like Ericsson have PCI collision reports based on X2 measurements. It require that the neighbor relations are bidirectional for that purpose. But there are in the market many tools, mostly comes with planning software.

Cheers!

Diego Goncalves Kovadloff

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