cell selection and reselection小区选择与小区重选

CELL SELECTION AND RESELECTIONKEY WORDSCell selection Idle Mode Cell search Cell selection: S-criteria Initial cell selection Stored information cell selection Cell reselection: R-criteria Legacy Cell Reselection Priority-Based Cell ReselectionCELL SELECTIONWhat: look for a suitable cell Why: obtain access to certain services When: at power on return to the coverage after leaving Cell_DCH after a number of failed RRC Connection requests when entering Idle mode after completion of an emergency call when entering Idle mode after a number of failed cell update attempts Idle mode: UE is powered on, but does not have a connection to the Radio Network Idle mode Services Limited service: emergency calls onlyon a acceptable cell: The S-criteria are fulfilled Normal service: public use on a suitable cell: The cell belongs to the selected PLMN. The S-criteria are fulfilled. The cell is not a part of a forbidden registration area Operator-related services: for operators only on a reserved cellThe purpose of camping on a cell in Idle Mode Enable the UE to receive SI Enable the UE to access the network when it needs to establish an RRC connection Enable the PLMN to find the UE when there is a call for it . Send “paging” message Enable the UE to receive cell broadcast serviceHow: Cell Search ProcedureI. synchronizationII. read system information on the BCCH to select a PLMN and to find a suitable cell.*system information: is mapped to one MIB and some SIBsthe PLMN ID is in the MIBSteps Channel ContentsSlot synchronization P-SCH primary synchronization codeFrame synchronizationdetermines the scrambling code group S-SCH secondary synchronization code sequences Detecting DL primary scrambling codeP-CPICHReading System InformationP-CCPCH system information Cell Selection Procedure Initial cell selection procedure: the UE does not know the WCDMA radio frequency channelsCase: the UE does not find a suitable cell of the PLMN on which it was previously registered/does not find a suitable cell in stored information cell selection procedure i. search the strongest cell scan all WCDMA RF channels search for the cell with the highest signal levelii. find the PLMN reads the system information (cell search) reads the PLMN ID, determines the PLMN to which the cell belongs If the cell belongs to the selected PLMN, it is the suitable celliii. Fail to find a suitable cellto attempt to camp on the strongest acceptable cell and try regularly to find a suitable cell Stored information cell selection procedure: the UE knows the carrier frequencies that have previously been used(stored on the USIM)Case: the UE does not find a suitable cell of the PLMN on which it was previously registeredi. cell search Synchronization the UE reads system information on the BCCHii. if all requirements for a suitable cell are fulfilled the UE selects that cell and tries to registeriii. Fail to find a suitable cellturn to initial cell selection procedurecamped normally: In this state, the UE monitors paging and SI, performs periodical radio measurements and evaluates R-criteria. S-criteria:Squal 0 and Srxlev 0quantities: Squal: evaluates CPICH SIR, expressed in CPICH Ec/NoSqual = Qqualmeas- qQualMin (only for WCDMA cells)Srxlev: cell selection RX level valueSrxlev = Qrxlevmeas- qRxLevMin - Pcompensation (for all cells)Calculate:Qqualmeas: the received signal quality expressed in CPICH Ec/No measured by the UEQrxlevmeas: the received signal strength expressed in CPICH RSCP measured by the UE Pcompensation= max (maxTxPowerUl - P, 0) qQualMin: cell parameter the minimum required quality value in the cell sent in SIB3 for the serving cell, and in SIB11 for adjacent cellsqRxLevMin: cell parameter the minimum required signal strength in the cell sent in SIB3 for the serving cell and in SIB11 for adjacent cellsCELL RESELECTIONWhat: look for a more suitable cellWhy: obtain access to certain services in a better cellWhen: The cell is no longer suitable The cell become barred The UE has found a better neighboring cell The UE is in limited service state on an acceptable cellHow: Legacy (or ranking based) cell reselection Priority based cell reselectionLEGACY CELL RESELECTIONCase: intra-frequency cell reselection within WCDMA inter-frequency cell reselection within WCDMA cell reselection from WCDMA to GSMMeasurement Rules Whether the UE perform intra-frequency measurementi. Squal sIntraSearch: NOii. Squal sInterSearch: NOii. Squal sRatSearch: NOii. Squal threshHigh (for a duration of treSelection seconds) Cell reselection to a lower priority LTE frequency Srxlev(s)threshLowSrxlev(LTE) is defined as: Srxlev = Qrxlevmeas qRxLevMin(for a duration of treSe