This is a bit of complicated topic, but I try to give you as much as information as possible.
Cell reselection criterion C2 is defined as C2 = C1 + CELL_RESELECT_OFFSET – TEMPORARY_OFFSET
For example, please look at the following senario;
Can you see, which cell is selected as a serving cell in the IDLE mode when the received level from the 900 MHz cell is -80 dBm and from 1800 MHz cell -87 dBm and cellReselectOffset = 10 dB, temporaryOffset = 20 dB and penaltyTime = 640 s are set for the 1800 MHz cell. Minumum access levels for both cells are -100 dBm.
C1 and C2 are used.
C2 criteria has two alternatives depending on the value of penalty time.
– Penalty time is set 640 and thus C2 = C1 – cellReselectOffset
– C1 is used for 900 MHz cell and C1 = -80 – (-100) = 20 dB.
– C2 is used for 1800 MHz cell.
C1 = -87 – (-100) = 13 dB
=> C2 = 13 – 10 = 3 dB
=> 20 (900 MHz) > 3 (1800 MHz)
=> 900 MHz cell is selected!