Cell Reselection scenarios

The Cell Reselection process is described in the following stages:

The Cell Reselection process is run when:

  • When the cell on which it is camping is no longer suitable.
  • When the UE, in ”camped normally” state, has found a better neighbouring cell than the cell on which it is camping.
  • When the UE is in limited service state on an acceptable cell.
  • Measurements on intra-frequency cells are performed if Srxlev ≤ SIntraSearchP or Squal ≤ SIntraSearchQ for the serving cell.
  • If Srxlev >SIntraSearchP and Squal > SIntraSearchQ, the UE does not need to perform these measurements.

where Srxlev = Qrxlevmeas – (Qrxlevmin + Qrxlevminoffset) – Pcompensation

Squal = Qqualmeas – (Qqualmin + Qqualminoffset)

  • Measurements on E-UTRA cells on inter-frequency carriers and cells on inter-RAT frequencies with higher cell reselection priority than the current frequency are always performed.
  • Measurements on E-UTRA cells on inter-frequency carriers and cells on inter-RAT frequencies with equal or lower cell reselection priority than the current frequency are performed if Srxlev ≤ SNonIntraSearchP or Squal ≤ SNonIntraSearchQ for the serving cell. If Srxlev > SNonIntraSearchP and Squal > SNonIntraSearchQ, the UE does not need to perform these measurements

Intra frequency and Equal priority cell reselection

 

For equal priority Cell Reselection process is described in the following stages:

  • When camped on an E-UTRA cell, the UE performs a ranking of neighbor E-UTRA cells on the same frequency and other frequencies with equal cell reselection priority, taking into account the cells satisfying the cell selection criteria.
  • The cell ranking is based on the RSRP measurement quantities of the serving cell (Qmeas,s) and the neighbor cells (Qmeas,n).
  • The UE applies the cell ranking criterion Rs on the serving cell and the cell ranking criterion Rn on the intra-frequency and equal priority cells

Rs = Qmeas,s + Qhyst

Rn = Qmeas,n – Qoffset

 

  • Qhyst is a hysteresis value preventing too-frequent reselection back and forth between cells of nearly equal rank.
  • QOffset is an offset in the cell ranking criterion of neighbour E-UTRA cells. It consists of a cell individual part and a frequency specific part. The frequency specific part applies to equal priority inter-frequency cells only.
  • When a neighbor cell is ranked as better than the serving cell (Rn > Rs) during a time interval TreselectionEUTRA, the UE performs a cell reselection to the better-ranked cell.

 

LTE to WCDMA cell reselection

 

From High Priority LTE Cell to Lower Priority WCDMA Cell

 

  • Measurement Criteria (From High Priority LTE Cell to Lower Priority WCDMA Cell): When LTE cell has higher priority than WCDMA, it would stay in LTE cell but it performs measurement for the low priority WCDMA if UE is under the following condition :

Srxlev of the serving cell < sNonIntraSearch (SIB3),

If we express this using dB/dBm, Srxlev of the serving cell < (2 x sNonIntraSearch (SIB3))

where Srxlev = Qrxlevmeas – qRxLevMin (SIB3),

 

  • Reselection Criteria (From High Priority LTE Cell to Lower Priority WCDMA Cell): If UE in LTE cell is under the following condition with the duration longer than tReselectionUtra (SIB6), it should reselect to WCDMA cell.

Srxlev of LTE cell (serving cell)  < threshServingLow (SIB3),

where Srxlev = Qrxlevmeas – qRxLevMin (SIB3),

where Qrxlevemeas = measured RSRP level, qRxLevMin = minimum RSRP level for camping

Srxlev of WCDMA cell  > threshXLowP (SIB6),

where Srxlev = Qrxlevmeas – qRxLevMin (SIB3),

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