Once the UE reads PSS and SSS, the next step is to select a PLMN in the initial acquisition procedure.
UE will first scan all the available frequencies, and selects a cell with strongest frequency and reads SIB1 to identify a network that a cell belongs to.
Then it will register to that network called as Registered PLMN. (Public Land Mobile Network)
Different types of PLMN as defined by specification:
Home PLMN: A PLMN where the Mobile Country Code (MCC) and Mobile Network Code (MNC) of the PLMN identity are the same as the MCC and MNC of the IMSI.
Available PLMN(s): One or more PLMN(s) for which the UE has found at least one cell and read its PLMN identity(ies).
EHPLMN: Any of the PLMN entries contained in the Equivalent HPLMN list .
Equivalent PLMN list: List of PLMNs considered as equivalent by the UE for cell selection, cell reselection and handover according to the information provided by the NAS.
Registered PLMN: This is the PLMN on which certain Location Registration outcomes have occurred .
Selected PLMN: This is the PLMN that has been selected by the NAS, either manually or automatically.
Different modes of PLMN:
There are 2 modes:
1. Automatic Mode: In this mode UE automatically selects suitable PLMN
2. Manual Mode: In manual mode UE will be presented with list of PLMN available to the user. And a manual selection can be made.
Below is how a PLMN is selected in E-UTRA case:
1. UE will scan all the available RF channels in E-UTRA to find available PLMN.
2. On each carrier, UE will search for the strongest cell and reads its system information to get the PLMN it belongs to.
3. UE shall report the PLMN identity with strongest cell to NAS.
4. The RSRP (Reference Signal Received Power) value should be greater than or equal to -110dBm for a PLMN to be selected.
5. Once the PLMN selection has been completed, cell selection procedure shall be performed in order to select a suitable cell of that PLMN to camp on.
Reference 3GPP TS 36.304