LTE And LTE Advanced Cell Selection Procedure

The principles of PLMN selection in [amazon_link id=”012385489X” target=”_blank” ]E-UTRA[/amazon_link] are based on the 3GPP PLMN selection principles. Cell selection is required on transition from EMM_DETACHED to EMM-REGISTERED and from ECM-IDLE or ECM-CONNECTED.

Cell selection

• The UE NAS layer identifies a selected PLMN and equivalent PLMNs

The NAS can control the RAT(s) in which the cell selection should be performed, for instance by indicating RAT(s) associated with the selected PLMN, and by maintaining a list of forbidden registration area(s) and a list of equivalent PLMNs. The UE shall select a suitable cell based on idle mode measurements and cell selection criteria.

• The UE searches the E-UTRA frequency bands and for each carrier frequency identifies the strongest cell. It reads cell system information broadcast to identify its PLMN(s)
• The UE may search each carrier in turn (“initial cell selection”) or make use of stored information to shorten the search (“stored information cell selection”).
• The UE seeks to identify a suitable cell; if it is not able to identify a suitable cell it seeks to identify an acceptable cell. When a suitable cell is found or if only an acceptable cell is found it camps on that cell and commence the cell reselection procedure
• A suitable cell is one for which the measured cell attributes satisfy the cell selection criteria; the cell PLMN is the selected PLMN, registered or an equivalent PLMN; the cell is not barred or reserved and the cell is not part of a tracking area which is in the list of “forbidden tracking areas for roaming”
• An acceptable cell is one for which the measured cell attributes satisfy the cell selection criteria and the cell is not barred

Transition to RRC_IDLE

On transition from RRC_CONNECTED to RRC_IDLE, a UE should camp on the last cell for which it was in RRC_CONNECTED or a cell/any cell of set of cells or frequency be assigned by RRC in the state transition message.

Recovery from out of coverage

The UE should attempt to find a suitable cell in the manner described for stored information or initial cell selection above. If no suitable cell is found on any frequency or RAT the UE should attempt to find an acceptable cell.



Cell Selection Process

Initial Cell Selection

This procedure requires no prior knowledge of which RF channels are E-UTRA carriers. The UE shall scan all RF channels in the E-UTRA bands according to its capabilities to find a suitable cell. On each carrier frequency, the UE need only search for the strongest cell. Once a suitable cell is found this cell shall be selected.

Stored Information Cell Selection

This procedure requires stored information of carrier frequencies and optionally also information on cell parameters, from previously received measurement control information elements or from previously detected cells. Once the UE has found a suitable cell the UE shall select it. If no suitable cell is found the Initial Cell Selection procedure shall be started.

Cell Selection Criterion

The cell selection criterion S is fulfilled when:

Srxlev > 0 AND Squal > 0

Where:

Srxlev = Qrxlevmeas – (Qrxlevmin + Qrxlevminoffset) – Pcompensation

Squal = Qqualmeas – (Qqualmin + Qqualminoffset)

Where:

Srxlev	Cell selection RX level value (dB)
Squal	Cell selection quality value (dB)
Qrxlevmeas	Measured cell RX level value (RSRP)
Qqualmeas	Measured cell quality value (RSRQ)
Qrxlevmin	Minimum required RX level in the cell (dBm)
Qqualmin	Minimum required quality level in the cell (dB)
Qrxlevminoffset	Offset to the signalled Qrxlevmin taken into account in the Srxlev evaluation as a result of a periodic search for a higher priority PLMN while camped normally in a VPLMN
Qqualminoffset	Offset to the signalled Qqualmin taken into account in the Squal evaluation as a result of a periodic search for a higher priority PLMN while camped normally in a VPLMN
Pcompensation	max(PEMAX –PPowerClass, 0) (dB)
PEMAX	Maximum TX power level an UE may use when transmitting on the uplink in the cell (dBm)
PPowerClass	Maximum RF output power of the UE (dBm) according to the UE power class

References

1. Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network
   (E-UTRAN); Overall description; Stage 2: 3GPP TS 36.300
2. Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) procedures in idle mode : 3GPP TS 36.304