Reselection rules

Hello, TWH.
According to the cell reselection criteria, the UE reselects a new cell when all the following conditions are met:
During the cell reselection time specified by TreselEutran, the signal quality level of the neighboring cell is always higher than that of the serving cell.
The UE camps on the serving cell for more than 1s.
The signal quality levels are described as follows:
The signal quality rank R_s of the serving cell and the signal quality rank R_n of the neighboring cell are calculated by using the following formulas:
R_s=Qmeas,s+Qhyst
R_n = Qmeas, n – CellQoffset
In the preceding information:
Qmeas,s is the RSRP value of the serving cell measured by the UE. The unit is dBm.
Qhyst is the reselection hysteresis of the serving cell configured on the eNodeB. The unit is dB.
Qmeas,n: indicates the RSRP value of the neighboring cell measured by the UE. The unit is dBm.
CellQoffset is the offset of the neighboring cell configured on the eNodeB. The unit is dB.
Thanks!

Cell reselection Criteria :

The mobile will reselect and camp on another cell if any of the following criteria is satisfied:

The serving cell is barred.
C1 value in the current cell is below 0 for 5 seconds which indicates that the path loss is high and the mobile needs to change cell.
The Mobile has unsuccessfully tried to access the network as defined by the MAXRET parameter or MaxNumberRetransmissions.


Assuming that one of the criteria above was satisfied then the mobile will select a cell with a better C1. However if the cell belongs to a different location area then the C1 for that cell has to exceed a reselection hysterisis parameter called CRH or CellReselectHyseteris for the reselection to happen !!

If the mobile is moving in a border area between location areas, it might repeatedly change between location areas. Each change requires location updating and cause heavy signaling load and risk paging message being lost. To prevent this, a cell reselect hysteresis parameter CRH is used.
The cell in a different location area will only be selected if the C1 of that cell is higher than the C1 of the current serving cell by the value of the Reselect Hysteresis. Since the Value of CRH may be different for each cell, the CRH used for comparison will be the one broadcast by the serving cell.

If the value is set very low then the mobile will Ping-Pong between location areas which will increase signaling load. If the value is set very high the mobile may camp in the wrong cell too long.

Source: http://teletopix.org/gsm/what-is-gsm-cell-reselection-and-how-it-works/