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.

      LTE and LTE Advanced Cell Selection And Reselection LTE And LTE Advanced Cell Selection Procedure

      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:

      SrxlevCell selection RX level value (dB)
      SqualCell selection quality value (dB)
      QrxlevmeasMeasured cell RX level value (RSRP)
      QqualmeasMeasured cell quality value (RSRQ)
      QrxlevminMinimum required RX level in the cell (dBm)
      QqualminMinimum required quality level in the cell (dB)
      QrxlevminoffsetOffset 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
      QqualminoffsetOffset 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)
      PEMAXMaximum TX power level an UE may use when transmitting on the uplink in the cell (dBm)
      PPowerClassMaximum 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

About Prashant Panigrahi

I am Prashant Panigrahi, live in Sweden. Now working as wireless professional and this website is just a reflection of what I learned. You can connect me on LinkedIn, Twitter
This entry was posted in LTE, LTE Advanced and tagged , , , , . Bookmark the permalink.