PLMN Selection Process in LTE
The UE scans all RF channels in the UTRAN band according to its capabilities to find available PLMNs. On each carrier, the UE searches for the strongest cell according to the cell search procedure (refer cell searching) and read its system information in order to find out which PLMN the cell belongs to.
If the UE can read the PLMN identity, the PLMN and the measured signal strength is reported to the NAS: if: signal >= – x dBm (high quality PLMN); without the measured signal strength if < x dBm.
CELL SELECTION PROCESS for LTE
After selecting a PLMN, the cell selection process starts. The UE selects a suitable cell and the radio access mode based on idle mode measurements and cell selection criteria. The UE searches a suitable cell of that PLMN to camp on according to the following steps:
1) The UE creates a candidate list of potential cells to camp on by using one of the
two search procedures:
• Initial Cell Selection UE scans all RF channels in the UTRAN band to find a suitable cell. On each carrier, UE searches for the strongest cell and reads its system information. Once the UE has found the suitable cell for the selected PLMN, the UE creates a candidate list consisting of this cell and its neighboring cells as received in measurement control information.
• Stored Information Cell Selection (optionally) This procedure requires information stored from previously received measurement control information elements (cell parameters, carrier frequencies, etc). After the UE has found a suitable cell for the selected PLMN, candidate list is created same as the initial cell selection process.
2) Each cell on the candidate list is evaluated according to the selection criteria S as described below.
3) After selecting a suitable cell (S criterion fulfilled) for camp on, UE reports this event to NAS for registration procedures. If the registration is successful, the UE enters into “camped normally“ state.
1. Camped normally state: UE obtains normal service and performs the following tasks:
• Select and monitor the PCH of the cell.
• Performs system information monitoring.
• Perform necessary measurements for the cell reselection evaluation procedure.
• Execute the cell reselection evaluation procedure.
If after cell reselection evaluation process a better cell is found, the cell reselection is performed. If no suitable cell is found, the UE enters to next state “any cell selection”.
2. Any LTE cell selection: LTE UE searches an acceptable cell of any PLMN to camp on. If an acceptable cell is found, the UE reports to NAS and camp on this cell obtaining limited service. And UE enters to “camped on any cell” state. If the UE can not find any acceptable cell, it stays in this state.
3. Camped on any cell state: LTE UE obtains limited service and periodically searches for a suitable cell in the selected PLMN. If a suitable cell is found the state changes to Camped normally.