RRC States in LTE

In LTE, the RRC connection has two states idle and connected is in RRC_CONNECTED when an RRC connection has been established. If this is not the case means if no RRC connection is established then UE is in RRC_IDLE state. The RRC states can further be characterized as follows.

RRC-Idle

• A UE specific DRX may be configured by upper layers.
• UE controlled mobility; (Cell selection/reselection, TA update)
• The UE:
  • Monitors a Paging channel to detect incoming calls, system information change, for ETWS capable UEs, ETWS notification, and for CMAS
  •  capable UEs, CMAS notification;
  • Performs neighboring cell measurements and cell (re-)selection;
  • Acquires system information.

RRC-Connected

• Transfer of unicast data to/from UE.
• At lower layers, the UE may be configured with a UE specific DRX.
• Network controlled mobility, i.e. handover and cell change order with optional network assistance (NACC) to GERAN;
• The UE:
  • Monitors a Paging channel and/ or System Information Block Type 1 contents to detect system information change, for ETWS capable UEs, ETWS    notification, and for CMAS capable UEs, CMAS notification;
  • Monitors control channels associated with the shared data channel to determine if data is scheduled for it;
  • Provides channel quality and feedback information;
  • Performs neighboring cell measurements and measurement reporting;
  • Acquires system information.

Differences between LTE RRC states and UMTS RRC States

 In LTE there is only 2 RRC states i.e. RRC_IDLE and RRC_CONNECTED whereas in UMTS system RRC has a 5 state i.e. IDLE, CELL_FACH, CELL_DCH, CELL_PCH and URA_PCH.

One of the reasons why we don’t have CELL_FACH and CELL_DCH state is because there is no concept of common and dedicated transport channel in LTE. In LTE the data transfer will be done through the defined shared transport channel. Therefore this will simplifies the RRC State machine handling and improves RRC performance.

The following figure not only provides an overview of the RRC states in E-UTRA, but also illustrates the mobility support between E-UTRAN, UTRAN and GERAN.