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LTE UE PLMN selection

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 AS layer to report available PLMN.

•  STEP-2:The LTE NAS layer is responsible for selecting a PLMN from these list of reported PLMN.

STEP-1: Let us see how NAS layer gets requested information from AS layer:

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

STEP-2: Let us see how NAS layer selects PLMN from reported list. It utilizes USIM information for PLMN selection:

•  IMSI defines Home PLMN (i.e. HPLMN)
•  HPLMN selector with access technology:Defines priorities of each RAT associated with HPLMN.
•  User controlled PLMN selector with access technology allows end user to prioritise the PLMN and RAT
•  Operator controlled PLMN selector with access technology allows the operator to prioritise the PLMN and access technology.
•  Forbidden PLMNs: Defines PLMN which UE does not automatically attempt to access. Forbidden PLMNs are added to the list when network will reject an ATTACH REQUEST using cause value as 'PLMN not allowed'. •  EHPLMN(Equivalent HPLMN) defines set of PLMN which are equivalent to PLMN with which LTE UE is registering. This list can be treated or deleted during the ATTACH or TRACKING AREA UPDATE procedures. EHPLMN are ordered in terms of their priority.

PLMN automatic and manual selection types

PLMN can either be selected manually or automatically as described below:

In the case of automatic selection, the LTE UE selects the PLMN and RAT(Radio Access Technology) using following order of priority:

•  1. HPLMN or the highest priority EHPLMN
•  2. PLMN and RAT combinations defined within USer controlled PLMN selector
•  3. PLMN and RAT combinations defined within operator controlled PLMN selector
•  4. Other PLMN reported as high quality PLMN, selected in random order.
•  5. Other PLMN selected in order of decreasing signal quality
The LTE UE searches all supported access technologies befire PLMN selection using last two steps 4 and 5 mentioned above.

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

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


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