If you have purchased a new LTE phone(UE) from an AT&T showroom. You walk out of the store all excited. You power on the phone. The map to go from no connection at all to your UE selecting AT&T connection to a cell works in 3 simple steps
What is Cell Selection ?
Cell selection is a procedure which allows UE to camp on to a cell. So that UE can make an actual RF connection to the network, after camping on to the cell. During cell selection off course UE , will look for a cell with better RF coverage. However , the better RF coverage is computed mathematically as shown in Selection criteria at the end below.
Conditions for camping on a Cell
UE will always prefer to camp onto a suitable cell. Therefore , it will always keep on looking for a suitable cell. If it cannot find one, then it will go and camp on an acceptable cell. The question which comes to mind is. What the heck is a suitable cell ?
A suitable cell is defined as a cell which
Types of Cell Selection
There are 3 types of cell selection
Criteria for Cell Selection
In general Cell Selection criteria is based on finding a better RF coverage cell for the UE. The coverage strength and quality of a cell in LTE is measured generally on :
S-Criteria based on RSRP (Cell Selection Criteria)
Cell Selection Criteria(also know as the S-criteria) is defined as
The above criteria is based on RSRP
Parameters for Cell Selection
Qrxlevmeas = RSRP measured by the UE in dBm
Qrxlevmin = minimum required RSRP signaled within System Information Block 1 (SIB 1)
Qrxlevminoffset = measurement offset which is only applied when a cell is evaluated for cell selection as a result of periodic search for higher priority PLMN while camped on a Visited PLMN. Qrxlevminoffset is optionally included within SIB 1 . A value of 0 dB is applied if Qrxlevminoffset is excluded from SIB 1.
Pcompensation = Defined as MAX(P emax – P umax, 0 ) where P emax is the maximum allowed uplink transmit power within the cell, and P umax is the maximum transmit power capability of the UE. If the UE transmit power capability is less than the maximum allowed transmit power , Pcompensation makes cell selection criteria more difficult to achieve . This avoids UE camping on cells for which they have insufficient transmit power to access i.e., towards cell edge
S-Criteria based on RSRQ (Cell Selection Criteria)
The newer version of cell selection criteria(S criteria ) according to release 9 of 3GPP specifications add a quality check to the cell selection criteria given as:
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