LTE UE Cell Selection and Reselection Explained

Cell selection allows an LTE-compliant UE to camp on a cell. There are three types of cell selection:

• Initial cell selection
• Stored information cell selection
• Cell selection when leaving RRC connected mode

All these types are depicted in the figure below.

Initial Cell Selection

This type of selection does not require any prior knowledge. The UE scans all RF carriers while searching for a suitable cell.

Stored Information Cell Selection

This method uses RF carrier and cell parameter information to identify a suitable cell. If stored information cell selection fails, the initial cell selection procedure is initiated.

Cell Selection When Leaving RRC Connected Mode

This applies to UEs transitioning from RRC CONNECTED to RRC IDLE mode. The eNodeB can direct the UE toward a specific RF carrier by including ‘redirected carrier information’ within the RRC CONNECTION RELEASE MESSAGE; otherwise, the UE is free to search any RF carrier.

All three types of cell selection techniques involve the UE searching for a suitable cell. A suitable cell is defined as a cell that has the following characteristics:

• Belongs to the selected PLMN, registered PLMN, or an equivalent PLMN.
• It is not barred.
• Belongs to at least one tracking area which is not forbidden.
• Satisfies the cell selection criteria.

The cell selection criteria is defined as follows:

$S_{rxlev} > 0$

Where,

$S_{rxlev} = Q_{rxlevmeas} - (Q_{rxlevmin} + Q_{rxlevminoffset}) - P_{compensation}$

• $Q_{rxlevmeas}$ = RSRP measured by UE in dBm
• $Q_{rxlevmin}$ = Minimum required RSRP signaled within SIB-1
• $Q_{rxlevminoffset}$ = Usually included in SIB-1. If not included, a value of zero is set.
• $P_{compensation}$ = MAX($P_{EMAX} - P_{UMAX}$, 0)

Priorities between RF carriers and different RATs are not used during cell selection.

REFERENCE: 3GPP TS 36.304

LTE UE Cell Reselection

The UE is free to do cell reselection either when the serving cell’s RF carrier strength is poor or when a suitable high-strength neighbor cell is available in the same RAT or a different RAT. RAT stands for Radio Access Technology. It can be GSM, UMTS, or LTE.

The UE only performs cell reselection evaluation for LTE RF carriers and inter-RAT RF carriers that are defined within the SI and for which the UE has a priority. Blacklisted cells are excluded from the list of candidate cells for cell reselection. Absolute priorities can be allocated to RF carriers belonging to both LTE and other RATs.

REFERENCE: 3GPP TS 36.304, 3GPP TS 36.331