CELL RESELECTION

First is what is cell reselection.

When Ue is in connected state and inactive for some time, the Enb release the bearer and radio resources and moves Ue to idle mode. If the Ue is moving or the current cell signal strength is becoming poor then ue have to select another good cell. This selecting of other cell is called Cell Reselection.

Here 3 things taken in consideration.
1. Comparing the Srxlev with reselecting parameters.
2.On priority.
3.On Ranking serving and neighboring cell.

1. Comparing the Srxlev with reselecting parameters to measure or not to measure the other cell.

For intra frequency cell to measure this criteria must met. If this criteria not met Ue should not measure other cell and will stay in that cell.

Srxlev< SintrasearchP and Squal< SintrasearchQ

Srxlev= Qrxlevmeans - (Qrxlevmin + Qrxlevminoffset) - Pcompensation
Squal= Qqualmeans - (Qqualmin + Qqualminoffset)

Srxlev               =Cell selection RX level value (dB)

SintrasearchP   =specifies the Srxlev threshold (in dB) for intra-frequency measurements.

Squal                =Cell selection Quality level value (dB)

SintrasearchQ   =specifies the Srxlev threshold (in dB) for intra-frequency measurements.

Qrxlevmeans    =Measure cell Rx level (RSRP)

Qrxlevmin        =is the minimum required receive power level in this cell, given in dBm

Qrxlevminoffset = is an offset to Q_rxlevmin that is only taken into account as a result of a periodic search for a higher priority PLMN while camped normally in a Visitor PLMN (VPLMN)

Qqualmeans     =Measure cell Quality level (RSRQ)

Qqualmin         =is the minimum required receive quality level in this cell, given in dB

Qqualminoffset=is an offset to Qqualmin that is only taken into account as a result of a periodic search for a higher priority PLMN while camped normally in a Visitor PLMN (VPLMN)

Pcompensation   =P_EMAX- P_UMAX  or 0, is the value used for P_Compensation. P_EMAX [dBm] is the maximum power a UE is allowed to use in this cell, whereas P_UMAX [dBm] is the  maximum transmit power of an UE according to the power class the UE  belongs too. At the moment only one power class is defined for LTE, which corresponds to Power Class 3 in WCDMA that specifies +23 dBm.  P_EMAX is defined by higher layers and corresponds to the parameter P-_MAX defined in [2]. Based on this relationship, P_EMAX can take values between -30 to +33 dBm. Only  when P_EMAX > +23 dBm P_Compensation is it considered when calculating S_rxlev. The P-_MAX information element (IE) is part of SIB Type 1 as well as in the "RadioResourceConfigCommon" IE, which is part of the SIB Type 2.

For inter frequency cell to measure this criteria must met. If this criteria not met Ue should not measure other cell and will stay in that cell.

Srxvel < SnonintrasearchP
Squal < SnonintrasearchQ

SnonintrasearchP= specifies the Srxlev threshold (in dB) for inter-frequency measurements.SnonintrasearchQ=specifies the Srxlev threshold (in dB) for inter-frequency measurements.

2.On Priority

Priority of the present cell and the neighboring cell are described in-

i) System Informationii) RRC Connection Release messageiii) Inheriting from another RAT

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 (SIB1),             where Qrxlevemeas = measured RSRP level, qRxLevMin = minimum RSRP level for campingSrxlev of WCDMA cell  > threshXLowP (SIB6),      where Srxlev = Qrxlevmeas - qRxLevMin (SIB1),                If we express using dB/dBm, Srxlev = Qrxlevmeas [dBm] - (2 x qRxLevMin (SIB1)) [dBm]             where Qrxlevemeas = measured RSCP level, qRxLevMin = minimum RSCP level for campingReselection Criteria  (From Lower Priority LTE Cell to Higher 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 WCDMA cell  > threshXHighP (SIB6),      where Srxlev = Qrxlevmeas - qRxLevMin (SIB1),             where Qrxlevemeas = measured RSCP level, qRxLevMin = minimum RSCP level for camping

3.On Ranking serving and neighboring cell.

In some condition 2 or more neighboring cell met all cell relection criteria. But Ue have to rank each neighboring cell by the present cell. The cell which have more rank among the neighboring cell Ue select that cell.

Rs = Qmeas,s + QHyst
Rn = Qmeas,n - Qoffset

Qmeas= RSRP measurement quantity used in cell reselections.
Qoffset= For intra-frequency: Equals to Qoffsets,n, if Qoffsets,n is valid, otherwise this equals to zero. For inter-frequency: Equals to Qoffsets,n plus Qoffsetfrequency, if Qoffsets,n is valid, otherwise this equals to Qoffsetfrequency.

Rs < Rn If this condition met Ue moves to that Cell.