What is cell re selection and how UE performs it?

Answer 1

If the UE finds a “better” cell, UE reselects it and camps on it. After camping on, UE monitors the system information to get the quality threshold and performs measurements for the cell reselection evaluation procedure. The UE evaluates whether or not a better cell exists.

The E-UTRAN controls the quality measurements for LTE cells to be reselected. The UE measurements are triggered according to the serving cell quality level and the threshold indicated in the system information. The measurement must satisfy different requirements for intra frequency, inter frequency or inter RAT (Radio Access Technology) quality estimations.

Cell selection is performed in RRC idle mode. The camping on a cell in idle mode enables the UE to receive information from the LTE network. UE stays in idle mode until it transmits a request to establish an RRC connection. After receiving the RRC connection set up, the mode changes into connected mode.

I hope it helps :)

Answer 2

On Behalf of Bart -
Cell Reselection is like a Handover, but it happens when UE is in IDLE mode. When the Reselection finishes the UE is still in IDLE mode, through the whole procedure the UE did not change its state.

Imagine scenario when you have already Selected Cell. UE is doing/comparing measurements and now if RSRP/RSRQ is very poor or undetectable, the UE will have to perform Cell Selection procedure again for finding other cell Although when the RSRP/RSRQ is measurable at least, it evaluate "Cell Reselection Criteria" and perform Cell Reselection if the criteria is met, or stay in the current cell if the criteria is not met.