Chapter 2

Idle Mode Procedure

RRC_IDLE is a UE AS state in which the UE is switched on but does not have any established RRC connection. No RRC connection means that the presence of the UE is, in general, not known to the network at the cell level because eNBs do not have any context for the UE. The location of the UE in RRC_IDLE is known to the network at the level of tracking areas, which consist of cells. The UE AS behaviors applied in RRC_IDLE are defined in [1].

The motivation behind using RRC_IDLE is to reduce UE power consumption by utilizing, for example, discontinuous reception. The number of processes that the UE is required to perform in RRC_IDLE is significantly smaller than in RRC_CONNECTED. For example, the UE in RRC_IDLE is required sometimes to wake up for a short time to perform a limited number of processes, for example, monitoring incoming calls and measurements for mobility, but the UE is soon allowed to sleep again and stays inactive most of the time. Such a long inactive time in RRC_IDLE significantly reduces battery consumption in the UE.

The term “camp on” is often used when UE behaviors in RRC_IDLE are described. “Camp on” is the UE state in which the UE stays on a cell and is ready to initiate a potential dedicated service or to receive an ongoing broadcast service. Camping on a cell by the UE in RRC_IDLE has several purposes:

  • reception of system information for the camped cell (see Section 3.2);
  • initiation of an RRC Connection Establishment on the camped ...

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