UE Idle Mode Overview _ PLMN Selection Procedure

General UE Initial Procedure


UE Idle Mode Behaviors And RRC status Connection

A UE that is powered on but does not have an RRC connection to the radio network is considered to be in idle mode

Idle mode management can increase the access success rate, improve the quality of service, and ensure that UEs camp on cells with good RX signal levels and qualities

In the idle mode, UE can perform:

1. PLMN selection
2. Cell search & selection
3. Cell reselection
4. TA update
5. Periodic paging monitoring

LTE is capable with “always online” functions, after UE initial attach, the default RRC status is active.

There is a timer(UE inactive timer) in eNodeB to control active to idle transition, when a UE has neither received nor sent data for a duration exceeding this period of time, the eNodeB releases the radio resources for the UE


  
PLMN Selection Procedure
PLMN Classification 

Home PLMN: the PLMN which the user is subscribed, there is only one PLMN

The Equivalent Home PLMN (EHPLMN): It is a list of PLMNs that can provide the same service as home PLMN. This PLMN is recoded in USIM

Visited PLMN: The Visited PLMN (VPLMN) is the PLMN where the UE is located.

PLMN selection includes two stages

UE self-PLMN selection

• Automatic  selection mode: UE will automatic select one suitable PLMN based on the priority
• Manual selection mode: UE search all the valid PLMN in the network, and the user could select one of them

PLMN register: After UE select one PLMN, it will register it to EPC during attachment flow, if this procedure is failure, UE goes back to self PLMN selection procedure. If all PLMN registration fails, UE enters the limited service state

PLMN Selection Procedure






Priority of Automatic PLMN Selection

Scenario 1: Initial PLMN selection, with no RPLMN information

• UE AS initiate cell search, read all the PLMN from SIB1, and report it to UE NAS
• UE NAS will select one of them based on the predefined priority

Scenario 2: If last VPLMN exists in USIM, UE will select this PLMN and start cell search with last frequency, if no valid cell was found all PLMN registration fails, then UE goes back to initial PLMN selection

The priority for initial PLMN selection

• The EHPLMN with the highest priority will be selected. If no such list, then use HPLMN
• Each combination of PLMN with radio access technology (RAT) in the "User Controlled PLMN Selector with Access Technology" data file in the USIM, in order of priority.
• Each combination of PLMN with RAT in the "Operator Controlled PLMN Selector with Access Technology" data file in the USIM, in order of priority Other combinations of PLMNs with RATs providing high-quality signals, in a random order. A high-quality signal is defined as the Reference Signal Received Power (RSRP) value of a cell in the combination of PLMN with RAT being greater than or equal to -110dBm. 
• Other combinations of PLMNs with RATs in order of descending RX signal quality.