LTE Cell Selection

Cell Selection Criterion

Cell Selection procedure allows the UE to camp on to a cell. There are three types of Cell selection in LTE:

1. Initial cell selection: UE scans all frequencies
2. Stored information cell selection: Stored list available with UE (earlier camed on cell). This can be also vendor specific implementation; like how each one of them would like to optimise the stored information cell selection
3. "Redirected carrier information" in RRC_Connection_Release message

Working:
  • RRC asks PHY to scan frequencies and report cells.
  • Once when UE PHY scans the UE supported frequencies and reports the found out cells during the scan, the results are reported to UE RRC along with their Cell IDs and the Cell specific Power values (RSRP, RSRQ).
  • UE RRC will select the strongest Cell ID from the list and then will go for Cell validation procedure
  • When the selected Cell ID passes the cell validation procedure, the UE camps onto it and proceeds to decode the broadcast information (MIB, SIBs). It is during this procedure, that the below mentioned calculations take place:
The cell selection criterion S is fulfilled when:

Srxlev > 0  AND  Squal > 0 

where:
Srxlev = Qrxlevmeas – (Qrxlevmin + Qrxlevminoffset) – Pcompensation
Squal = Qqualmeas – (Qqualmin + Qqualminoffset)

where:
Srxlev
Cell selection RX level value (dB)
Squal
Cell selection quality value (dB)
Qrxlevmeas
Measured cell RX level value (RSRP)
Qqualmeas
Measured cell quality value (RSRQ)
Qrxlevmin
Minimum required RX level in the cell (dBm)
Qqualmin
Minimum required quality level in the cell (dB)
Qrxlevminoffset
Offset to the signalled Qrxlevmin taken into account in the Srxlev evaluation as a result of a periodic search for a higher priority PLMN while camped normally in a VPLMN
Qqualminoffset
Offset to the signalled Qqualmin taken into account in the Squal evaluation as a result of a periodic search for a higher priority PLMN while camped normally in a VPLMN
Pcompensation
max(PEMAXPPowerClass, 0) (dB)
PEMAX
Maximum TX power level an UE may use when transmitting on the uplink in the cell (dBm) 
PPowerClass
Maximum RF output power of the UE (dBm) according to the UE power class as defined in

The signalled values Qrxlevminoffset and Qqualminoffset are only applied when a cell is evaluated for cell selection as a result of a periodic search for a higher priority PLMN while camped normally in a VPLMN. During this periodic search for higher priority PLMN the UE may check the S criteria of a cell using parameter values stored from a different cell of this higher priority PLMN.

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