EUTRA CELL RESELECTION

EPS Mobility management (EMM) comprises the functions and procedures that maintain the connectivity between UE and EPS as UE moves between the coverage areas of different base station or access networks.

When registered to the EPS network UE can move from one cell to another cell, from one registered area to other or even leaves the EPS network and return back in other time.

UE performs Cell Reselection in ECM_IDLE state in order to ensure that it is camped on the best available cell.

Cell Reselection procedure : SIB-3

SIB-3 contains the information related to cell reselection procedure. The parameters directly related to procedure are:
1. Qhyst = hysteresis value for ranking criteria (0 to 24 dbm).
2. Treselection EUTRA = Cell reselection timer value for EUTRAN (0....7s)

While others are related to measurement ans evaluation process.

threshServingLow = The threshold for the serving frequency used in reselection evaluation towards towards lower priority EUTRAN frequencies o
r RAT.

q-RxLevMin = The minimum required Rx level in cell in dBm.
S-IntraSearch = the threshold (in dB) for intra frequency measurements. The UE can omit measurements of intra-frequency neighbour cells if the serving cell is above a certain threshold.


SIB-4 contains neighbour cell information.

Cell Reselection procedure- SIB-4

physCellId - Physical layer cell identity of the neighbour cells.
Qoffset = the offset value used for cell reselection process between serving and neighbour cells (-24..24 dB)


Now,

Rs=Qmeas,s + Qhyst,s          //For serving cell
Rn=Qmeas,n + Qhyst,n         //For neighbour cell

When Rn becomes greater than Rs cell reselection is completed.

Rn > Rs -> Cell Reselection.