UMTS CELL CELECTION AND RESELECTION: A DETAILED ANALYSIS

The purpose of cell reselection is to continuously re-select the most suitable cell in the network when UE is not operating on a dedicated channel.There are three types of cell reselection
1. Intra frequency cell reselection : occurs between cells on the same radio frequency.
2. Inter frequency cell reselection : occurs between cells on different radio frequenies with no interruption of its connectivity with current cell.
3. Inter-RAT cell reselection : : happens between cells on different radio access technologies.

UE performs location registration if the new cell belongs to a different registration area.
During cell reselection UE takes measurement of the serving cell as well as he neighbour cells provided in SIB-11 or 12.

The S-criteria (Suitable Criteria) and R-criteria (Ranking Criteria) are calculated for each measured cells. All suitable cells are ranked based on R criteria. If highest ranked cell is a cell other than current serving cell for Treselection second and UE has camped on current serving cell for at leas one second then that cell is chosen for cell reselection.

S-Criteria

It determines whether a cell is suitable. Cell must meet the following criteria to be suitable.

Srxlev > 0     and      Squal > 0

Where, Srxlev = Qrxlevmin - Pcompensation

             Squal = Qqualmeas - Qqualmin

refer: 3GPP TS 25.331

R-Criteria

It determine the ranking of the serving cell and other suitable neighbour cell. The highest ranked cell is chosen for cell reselection. The definition of R criteria for serving cell is given as

                  Rs = Qmeas,s + Qhyst

Qmeas,s = measured quality of serving cell

Qhyst,s = hysteresis applied to serving cell.

Qmeas,s stands for measured CPICH RSCP and CPICH Ec/I0 .

UE chooses the highest ranking cell to camp on if the cell ranks higher than the current serving cell for Treselection seconds and UE has camped on current serving cell for at least one second. The parameter Treselection is defined by UTRAN in SIB-3 or SIB-4.