UMTS To LTE Reselection

LTE 2 reselection mechanisms are used in UMTS

Legacy Based:

This is ranking based system and it applies to intra-frequency, inter-frequency as well as for GSM reselection.

Priority Based:

This mechanism applies for cell reselection from WCDMA to LTE 

Priority based method results into 2 different scenarios:

High Priority UMTS to Low Priority LTE 

Low Priority UMTS to High Priority LTE 

Both legacy based and priority based cell reselection can be active at the same time.

Reselection from Low Priority UMTS to High Priority LTE

If LTE frequency is of higher priority than the current frequency, measurements are always performed.

Measurement Condition:

The UE searches every layer of higher priority at every Thigher_priority_search = (60*Nlayers) seconds.

Nlayers is the total number of configured higher priority E-UTRA frequencies

Reselection Condition:

Cell reselection occurs if UE finds a cell with an Srxlev value greater than ThreshHigh. 

Srxlev (target) > ThreshHigh

Reselection from High Priority UMTS to Low Priority LTE

Measurement Condition:

The UE only starts measurement for candidate LTE when following threshold is met.

Srxlev (serving) < SPrioritySearch1

Squal (serving) < SPrioritySearch2

Reselection Condition:

If Srx value of the serving cell falls below the threshServingLow value, UE attempts to reselect a cell on LTE. Cell reselection occurs if UE finds a cell with an Srxlev value greater than threshLow value for that frequency.

Srxlev (source) < ThreshServing

Low Srxlev (target) > ThreshLow