UMTS Cell Selection and Reselection

Content:

• Strategy of PLMN Selection
• Strategy of Cell Selection and Reselection
• Strategy of LA and RA Update
• Strategy of URA Update
• Strategy of CELL Update

Strategy of PLMN Selection

Setting Up a Cell and Public Channels

Strategy of PLMN Selection

• The USIM maintains a list of allowed PLMN types. During PLMN selection and reselection, based on the list of allowed PLMN types and a list of PLMN identities in priority order, the particular PLMN may be selected either automatically or manually. The UE will select a PLMN when it registers at power-on or re-registers after leaving connected mode. In addition, the UE can make periodic PLMN selection.

Support for PLMN Selection

• The UE scans all RF channels according to its capabilities to find available PLMNs. On each carrier, the UE shall search for the strongest cell and read its system information. The allowed PLMN information in USIM determines which PLMN will be selected finally. 
• Whenever a UE is switched on or re-registers after leaving connected mode, it attempts to camp on the last registered PLMN. If there is no registered PLMN stored in the USIM, the UE selects and attempts registration on other PLMNs using either the automatic mode or the manual mode. 
• Once the UE has selected a PLMN, the cell selection procedure will be performed in order to select a suitable cell of that PLMN to camp on.

PLMN Selection-Automatic Mode

Manual PLMN Selection Mode

• The UE displays all PLMNs that it finds by scanning all frequency carriers. The UE displays those PLMNs that are allowed as well as those that are not allowed. The user makes a manual selection, according to the available access technology for the chosen PLMN, and the UE attempts registration on this PLMN. 
• If the user selects an available PLMN in the forbidden PLMN list, the UE attempts to register and may receive a positive acknowledgement from the CN. In this case, the PLMN is removed from the forbidden list.

Roaming

• Roaming is a service through which a UE is able to obtain services from another PLMN than the HPLMN. The behavior that the UE must follow is specified by agreements among the network operators. A UE in automatic mode, having selected and registered on a VPLMN, periodically attempts to return to its HPLMN. The time interval between consecutive attempts is stored in the USIM. The timer may have a value of between 6 minutes and 8 hours, with a step size of 6 minutes. In the absence of a fixed value, a default value of 30 minutes is used by the UE.

Strategy of Cell Selection and Reselection

Strategy of Cell Selection and Reselection

• For normal service, the UE has to camp on a suitable cell, tune to that cell's control channel(s) so that the UE can: 
• Receive system information from the PLMN; 
• Receive registration area information from the PLMN, e.g., location area and routing area; and 
• Identify to which service area as specified by NAS that the serving cell belongs. 
• Receive other AS and NAS Information; 
• Receive paging and notification messages from the PLMN; 
• Initiate call setup for outgoing calls or other actions from the UE.

Cell selection criteria-Rule S

Srxlev > 0 AND Squal > 0

Squal ＝ Qqualmeas － Qqualmin

Srelev ＝ Qrxlevmeas－Qrxlevmin－Pcompensation

• Qqualmeas：Measured cell quality value. The quality of the received signal expressed in CPICH Ec/N0 (dB). 
• Qrxlevmeas：Measured cell RX level value. The quality of the received signal expressed in CPICH RSCP (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) 
• UE_TXPWR_MAX_RACH：Maximum TX power level on RACH (dBm) 
• P_MAX：Maximum RF output power of the UE (dBm)

Camped Normally State

• Select and monitor the indicated PICH and PCH of the cell according to information sent in system information. 
• Monitor relevant system information. 
• Perform necessary measurements for the cell reselection evaluation procedure. 
• Execute the cell reselection evaluation process.

Measurement rules for cell re-selection

• If Sx > SIntraSearch, UE does not measure intra-frequency cells. 
• If Sx <= Sintrasearch, UE measure intra-frequency cells. 
• If Sintrasearch is not broadcast, UE measure intra-frequency cells. Whether to broadcast Sintrasearch or not is controlled by SintrasearchPre. 
• If Sx > Sintersearch, UE does not measure inter-frequency cells. 
• If Sx <= Sintersearch, UE measure inter-frequency cells. 
• If Sintersearch is not broadcast, UE measure inter-frequency cells. Whether to broadcast Sintersearch or not is controlled by SintersearchPre. 
• If Sx > SSearchRat, UE does not measure inter-RAT cells. 
• If Sx <= SSearchRat, UE measure inter-RAT cells. 
• If SsearchRat is not broadcast, UE measure inter-RAT cells. Whether to broadcast SsearchRat or not is controlled by OtherRATInfoPre. 
• Where, Sx is Squal.

Cell reselection criteria-Rule R

• Qmeas,s：Quality measurement result of serving cell.
• Qmeas,n：Quality measurement result of neighbor cell.
• Qhyst1s ：Hysteresis value when measurement value is CPICH RSCP.
• Qhyst2s ：Hysteresis value when measurement value is CPICH Ec/No.
• Qoffset1s,n ：Offset of two cells when measurement value is CPICH RSCP.
• Qoffset2s,n ：Offset of two cells when measurement value is CPICH Ec/No.
• The new cell is better ranked than the serving cell during a time interval Treselection according to the R criteria.
• More than 1 second has elapsed since the UE camped on the current serving cell.

Quality measure for Cell Selection and Reselection

Strategy of LA and RA Update

Strategy of LA and RA Update

• If the LAI or RAI read from system information are different from those stored in USIM before the UE switches off, the UE will perform LA or RA update. 
• When the UE in IDLE mode moves to a new LA or RA or PLMN, it will perform update. 
• The process of LA or RA update is controlled by CN and is transparently transmitted as the RAN is concerned.
• There are three types of update: normal update, periodic update and IMSI attach and detach.

LA and RA Structure

The LA is an area to which the CN sends a paging message for circuit switched service and the RA is an area to which the CN sends a paging message for packet switched services. Each area, LA or RA, may consist of several cells of one or more RNC, which are connected to the same core network. Each cell could belong to only one LA and one RA. Normal LA and RA Updating

Normal update is performed when the UE moves in a cell belonging to a new LA or RA. The UE reads the LAI and the RAI in the system information and detects that one or both of the received area identities differ from the ones stored on the USIM. If the LAI or RAI received from the system information is not in the forbidden LAIs or RAIs list, an LA or RA update request is sent by the UE.

Periodic LA and RA Updating

Periodic LA and RA updating is used to notify the network of the UEs availability, and to avoid unnecessary paging attempts for a UE that has lost coverage and is not able to inform the CN that it is inactive. The periodic LA update procedure is controlled by a timer, called t3212, whose value is sent to UEs with broadcast message. The periodic RA update is controlled by a timer, called t3312, whose value is sent by the CN to the UE in the IMSI attach or in the routing area update message accept.

Location Update Flow

IMSI Attach/Detach

The UE sends an “attach” or “detach” message when it is powered on or off indicating whether the UE is active or inactive. The CN avoids performing paging attempts when IMSI detach is applied and the UE is switched off. When the UE is switched on, the UE performs a location registration request, indicating IMSI attach, if it is in the same LA or RA in which it was switched off. If the registration area is changed, a normal LA update is performed by the UE.

Strategy of URA Update

Strategy of URA Update

• If the new cell that the UE camps on is not in the same URA area as the former cell that the UE camps on, the URA Update is triggered.
• The UE enters a cell that has no URAs defined. This will trigger a release of the RRC Connection and the UE enters Idle mode.

Process of URA Update

Strategy of CELL Update

Strategy of CELL Update

• Cell reselection in state CELL_FACH.
• Radio link failure in state CELL_DCH.
• T305 periodic cell update. 
• The UE in state URA_PCH sends data in uplink.
• The UE in state CELL_FACH/CELL_DCH detects that an RLC unrecoverable error of AM RLC entity.

Process of CELL Update

End of Course