Idle Cell selection in LTE

Cell Selection and Re-Selection
Cell selection entity handles the cell selection/ reselection functionality, it is responsible for selecting the most suitable cell to camp on for the UE. The cell selections are made on different parameters. These parameters include the Power parameters and the parameters included in the system information Broadcasts.

Cell Selection Criteria
In order to be the candidate for a suitable or acceptable cell, the following criterion is to be met:
Srxlev  > 0
Srxlev = Qrxmeasured – Qrxlevmin – Pcomposition

Cell Re-Selection Criteria
The cell reselection is performed on the basis of the ranking of the current and the neighboring cells. The ranking is performed on the basis of the following R values
Rs = Qmeas,s + Qhysts
Rn = Qmeas,n – Qoffsets,n ­

Cell Selection Process Diagram
In the Idle mode the CSE performs the following tasks in order to select a cell:
1. When a PLMN is selected the cell selection is made through one of the cell selection procedures
2. If suitable cell is found the UE camp normally on the cell.
3. In camp normally state the cell reselection is made depending upon the triggers described later in this document.
4. If no suitable cell found in 2, the UE searches for an acceptable cell and enters the camp on any cell state
5. In camp on any state the cell reselection is made depending upon the triggers, if no suitable cell found the UE move to any cell state
6. If suitable cell found in 5 the UE move to the camp normally state.
7. In either state i.e. camped normally or camped on any when ever the UE returns from the connected mode to the idle mode the UE tries to camp on the last cell used for camping in the connected mode.
8. In case last cell in 5 is not a suitable or acceptable cell, procedure 1 and 4 are repeated depending upon the camped state.

The following figure shows the process diagram of the cell selection and re-selection
in idle mode for LTE.