Mobile Technology 

Cell Reselection is very similar to Cell Selection except followings.

• Cell Reselection happens after UE completes cell selection and gets into idle mode.
• Cell Reselection happens while UE is being covered by a registerred cell and is able to detect the registerred cell (i.e, UE is still in Service (not Out Of Service) with the registerred cell)

Overall cell selection mechanism can be summerized as follows.

i) UE is in Idle mode (in Service, Not Out Of Service) .

ii) UE scans energy level (cell power) from all the cells around it.

iii) If it detects the power which is greater than a certain level (this level is specified in UE radio stack software and this value would vary depending on UE manufacturer and model), it decodes  a certain SIB(System Information Block) that carries Cell ReSelection parameters. Which SIB carries that kind of information is different with each radio technology. See the page linked below if you are interested in more details on these SIBs for any specific radio technology.

iv) After decoding SIBs and getting Cell ReSelection parameters, it estimates the cell selection criteria calculated from the cell selection parameter and the algorithm defined by the industry standards (e.g, 3GPP). See the page linked below if you are interested in more details on how UE estimates the criteria.

v) Based on the estimation in previous step, choose the best fitting cell and perform registration process to the reselected cell.

Overall cell Reselection mechanism is same in 3G/4G/5G, but the detailed parameters and mechanism varies a little bit depending on Radio Technology.

• For 5G Cell ReSelection : Note Not Ready Yet
• For 4G Cell ReSelection : See here  
• For 3G Cell ReSelection : See here