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UMTS Cell Selection Procedure
Different Type of Cells
Acceptable Cell
An UE can always attempt an emergency call on a Acceptable Cell. The acceptable cell satisfies certain criteria.
Suitable Cell
Suitable cell is a cell on which UE can camp on to provide normal service. Suitable cell satisfies some criteria:
Barred Cell
The UE is not allowed to camp on a barred cell. System information 3 indicates whether a cell is barred or not.
Reserved Cell
This is a cell on which camping is not allowed. Only particular UE are allowed to camp on a reserved cell.
Cell Selection
A new PLMN is selected taking PLMN selection criteria into account.
Celle Selection Process
Whenever PLMN has been selected by NAS, the UE shall attempt to camp on the selected cell.
NAS may control the cell selection by:
If a suitable cell is found, UE shall select this cell to camp on and report this event to NAS so that NAS can do the necessary registration procedure.
For cell selection UE can use one of the following search procedures:
Initial cell selection
This procedure requires no prior knowledge of which RF channels are UTRA carriers. The UE shall scan all RF channels in the UTRA bands according to its capabilities to find a suitable cell. On each carrier, the UE need only search for the strongest cell. Once a suitable cell is found this cell shall be selected.
Stored information cell selection
This procedure requires stored information of carrier frequencies and optionally also information on cell parameters, e.g. scrambling codes, from previously received measurement control information elements. Once the UE has found a suitable cell the UE shall select it. If no suitable cell is found the Initial cell selection procedure shall be started.
Criteria
The cell selection S is full filled when:
Srxlev > 0 and Squal > 0
Srxlev = Qrxlevmeas – Qrxlevmin – Pcompensation
Squal = Qqualmeas – Qqualmin
Where:
Squal
Cell Selection quality value (dB)
Srxlev
Cell Selection RX level value (dB)
Qqualmeas
Measured cell quality value.
The quality of the received signal expressed in CPICH Ec/N0 (dB)
Qrxlevmeas
Measured cell RX level value.
This is received signal, CPICH RSCP for FDD cells (dBm)
Qqualmin
Minimum required quality level in the cell (dB).
Qrxlevmin
Minimum required RX level in the cell (dBm)
Pcompensation
max(UE_TXPWR_MAX_RACH – P_MAX, 0) (dB)
UE_TXPWR_MAX_RACH
Maximum TX power level an UE may use when accessing the cell on RACH (read in system information) (dBm)
P_MAX
Maximum RF output power of the UE (dBm)
Where to get these parameters?
Qqualmeas (EcNo): Ratio of energy per modulating bit to the noise spectral density the quality. Tells the quality of the cell and is reported from physical layer
Qqualmin: Received in SIB3/4 for serving cell or SIB11/12 for neighbor cell.
Qrxlevmeas (RSCP): Received Signal Code Power (RSCP) is the received power on one code measured on the primary CPICH and is reported from physical layer.
UE_TXPWR_MAX_RACH: Received in SIB3/4 for serving cell or SIB11/12 for neighbor cell
P_MAX: It is UE dependent value.
Reference
3GPP TS 25.304: User Equipment (UE) procedures in idle mode and procedures for cell reselection in connected mode
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