UMTS Cell Selection Procedure

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Different Type of Cells

Acceptable Cell

An UE can always attempt an emergency call on a Acceptable Cell. The acceptable cell satisfies certain criteria.

• The cell is not barred
• Cell selection criteria are fulfilled

Suitable Cell

Suitable cell is a cell on which UE can camp on to provide normal service. Suitable cell satisfies some criteria:

• The cell should be part of the selected PLN or registered PLMN or equivalent PLMN.
• The cell should not be barred
• The cell is part of at least one LA which is not part of the forbidden LA list.
• The cell selection criteria are full filled.

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.

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Cell Selection

Prerequisite
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:

• Providing information on RAT(s) associated with the selected PLMN
• Maintaining set of forbidden registration areas.
• Providing list of equivalent PLMN

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)
• 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.

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Reference

• 3GPP TS 25.304: User Equipment (UE) procedures in idle mode and procedures for cell reselection in connected mode

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