LTE Idle Mode – Cell Reselection

After cell selection UE performs measurement and carry out cell reselection of new candidate cell. This is based on certain criterion. Reselection process involves 2 steps

1.    Measurements Condition:  Specifies when to start measuring neighboring cells. Once condition is met UE measures the RSRP level of the E-UTRA cells on the current frequency and any inter-frequency and inter-RAT frequency carriers.

2.     Reselection Condition: Specifies reselection criteria for candidate cell. There are two different mechanisms for cell reselection:

 Legacy Based

Priority Based

Cell reselection can be either Legacy or Priority based.

Legacy Based

 This is classical Ranking based system and is applied to intra-frequency reselection.

• It is also applied to inter frequencies which are equally prioritized.

• UE applies cell ranking R criterion on serving cell and neighboring cell.

Priority Based

Priority values in the range [0..7] are used, where 0 indicates lowest priority and 7 indicates highest priority.

Priorities can be set for:

Inter Frequencies: Lower, Equal or Higher Priority

Inter-Rat Frequencies: Higher or Lower Priority

Equal priorities between different RATs are not supported

Idle Mode – Cell Reselection: Priority Based

CellReselectionPriority    :           Absolute reselection priority for E-UTRA or IRAT frequency

sNonIntraSearch :           Threshold for inter and IRAT frequency

threshXLow                    :           Threshold for Srx value of serving cell below which UE performs cell

 reselection towards a lower priority cell.

threshXLow                    :           Threshold for Srx value of target cell for cell reselection towards a

                                                lower priority Inter or IRAT frequency.

tReselectionEutra           :           Cell reselection timer value for EUTRAN

tReselectionUtra :           Cell reselection timer value for UTRAN