Evaluation process for Cell selection in LTE
The cell selection criterion S is a pre-condition for suitable cells.
The conditions for E-UTRAN, UTRAN FDD and GSM cells are listed in the figure.
R – CRITERION – CELL RESELECTION
The cell reselection evaluation process depends on whether Hierarchical Cell Structure (HCS) is used or not. In order to perform cell reselection UE measures and ranks the neighbor cells.
For each type of neighbor cells (Intra-Frequency; Inter-Frequency; Inter-RAT, i.e. GSM) thresholds are definable. Measurements of neighbor cells will be triggered if these thresholds are reached.
HIGH MOBILITY / MEDIUM MOBILITY / NORMAL MOBILITY:
For faster moving UEs the procedure alters – speed dependent scaling rules are applied. If the number of (different cells) cell reselections during the past time period TCRmax exceeds NCR_H, high mobility has been detected. If the number exceeds NCR_M, and not NCR_H, medium mobility has been detected.
In high/medium-mobility states, a UE:
• multiplies Qhyst by “Speed dependent ScalingFactor for Qhyst for mobility state” if sent.
• multiplies TreselectionRAT by “Speed dependent ScalingFactor for TreselectionRAT for mobility state for RAT cells. (RAT = EUTRAN, UTRAN, GERAN).
Cell reselection evaluation is performed according to the UE internal triggers or if the information on the BCCH used for the cell reselection evaluation procedure has been changed.
FOR INTRA-FREQUENCY AND EQUAL PRIORITY INTER-FREQUENCY CELLS:
(Re-) Selected cell is a suitable cell (e.g. fulfills the S criterion) and is the best ranked cell (has the highest R). The UE shall however reselect the new cell, only if the following conditions are met:
• the new cell is better ranked than the serving cell during a time interval Treselections
• more than 1 second has elapsed since the UE has camped on the current serving cell.
The cell-ranking criterion R is defined as shown below:
s – indicates the serving cell,
n – indicates the candidate cell.
FOR INTER-FREQUENCY AND INTER-RAT NEIGHBOUR CELLS:
If UE camps longer than 1 sec in the serving cell and:
- a higher priority neighbor fulfills (during TreselectionRAT):
SnonServingCell,x > Threshhigh -> reselect neighbor cell.
- no cell fulfills SnonServingCell,x > Threshhigh :
SServingCell < Threshserving,low and SnonServingCell,x > Threshx,low ->reselect neighbor cell.