LTE UE PLMN Selection: Process and Priorities

PLMN stands for Public Land Mobile Network. The LTE compliant UE is responsible for selecting a PLMN for subsequent cell selection. A PLMN is identified by its PLMN identity broadcast within SIB-1. A single cell can belong to multiple PLMNs, so SIB-1 may broadcast a list of PLMN identities.

Following are two steps for PLMN selection:

• STEP-1: The LTE UE NAS layer requests the AS layer to report available PLMNs.
• STEP-2: The LTE NAS layer is responsible for selecting a PLMN from the list of reported PLMNs.

STEP-1: NAS Layer Requesting Information from AS Layer

Let’s examine how the NAS layer receives the requested information from the AS layer:

• The UE scans all the RF channels within its supported RF bands.
• The UE searches for the strongest cell carrier and reads System Information (SI) to identify PLMNs.
• PLMNs are reported to NAS as high quality if RSRP >= -110dBm.
• PLMNs not meeting the high-quality criteria are reported along with their RSRP to NAS.
• The UE can optimize the PLMN search procedure using stored information such as RF carriers and cell parameters. The NAS layer can stop the search at any instant, for example, after finding the home PLMN.

STEP-2: NAS Layer PLMN Selection

Let’s see how the NAS layer selects a PLMN from the reported list. It utilizes USIM information for PLMN selection:

• IMSI (International Mobile Subscriber Identity): Defines the Home PLMN (HPLMN).
• HPLMN selector with access technology: Defines the priorities of each Radio Access Technology (RAT) associated with the HPLMN.
• User controlled PLMN selector with access technology: Allows the end-user to prioritize the PLMN and RAT.
• Operator controlled PLMN selector with access technology: Allows the operator to prioritize the PLMN and access technology.
• Forbidden PLMNs: Defines PLMNs which the UE does not automatically attempt to access. Forbidden PLMNs are added to the list when the network rejects an ATTACH REQUEST using the cause value ‘PLMN not allowed’.
• EHPLMN (Equivalent HPLMN): Defines a set of PLMNs that are equivalent to the PLMN with which the LTE UE is registering. This list can be treated or deleted during the ATTACH or TRACKING AREA UPDATE procedures. EHPLMNs are ordered in terms of their priority.

PLMN Automatic and Manual Selection Types

PLMNs can either be selected manually or automatically, as described below:

In the case of automatic selection, the LTE UE selects the PLMN and RAT using the following order of priority:

• 1. HPLMN or the highest priority EHPLMN
• 2. PLMN and RAT combinations defined within the User controlled PLMN selector
• 3. PLMN and RAT combinations defined within the operator controlled PLMN selector
• 4. Other PLMNs reported as high-quality PLMNs, selected in random order.
• 5. Other PLMNs selected in order of decreasing signal quality

The LTE UE searches all supported access technologies before PLMN selection using the last two steps (4 and 5) mentioned above.

In the case of manual selection, the UE presents the end-user with available PLMNs, listing them in the order as mentioned above.

REFERENCES

3GPP TS 36.304, TS 23.122, TS 31.102, TS 24.301