Cell Reselection is a mechanism to change cell after UE is camped on a cell and stay in IDLE mode (let UE get connected to cell which has the best condition among all the cells to which the UE is allowed to camp on).
UE uses a set of pretty complicated criteria and algorithms for this reselection process.
These are
First Level Criteria : Absolute Priority
Second Level Criteria : Radio Link Quality
Third Level Criteria : Cell Accessibility
When UE is powered on , usually it goes on with the following sequence.
i) Cell Detection/Search
ii) Cell Selection
iii) RACH and Completion of Registration
iv) < In IDLE >
v) Keep measuring RSRP/RSRQ for the cell it registered
vi) if RSRP/RSRQ is very poor or undetectable, go to step i) for finding other cell
if RSRP/RSRQ is measureable at least, it evaluate < Cell Reselection Criteria >
perform Cell Reselection if the criteria is met
stay in the current cell if the criteria is not met
Step vi) is the most complicated process going on during the idle stage and in this page I will mostly deal with < Cell Reselection Criteria >. Understanding this criteria is the most imporant thing in implementing and testing Cell Reselection.
Credit: http://www.sharetechnote.com/html/Handbook_LTE_Cell_Reselection.html
