Thursday, 3 September 2015

LTE Radio Resouse Control


1.1 UE Status and Status Transition (Both Intra- and Inter-RAT)

A UE is in RRC_CONNECTED mode when an RRC connection has been established. If an RRC connection has not been established, the UE is in RRC_IDLE mode. The RRC status can be further characterized as follows:
l   A UE specific discontinuous reception (DRX) may be configured by upper layers.
l   UE controlled mobility
l   The UE:
       Monitors a paging channel to detect incoming calls, system information changes, and Earthquake and Tsunami Warning System (ETWS) notifications (for ETWS-supported UEs only).
       Performs neighboring cell measurements and cell selection and reselection.
       Obtains system information.
l   Transfer unicast data to or from UE.
l   A UE may be configured with a UE specific DRX at lower layers.
l   The mobility is controlled on the network side, which means the network sends the order for a handover or a network assisted cell change (NACC) to GSM/EDGE radio access network (GERAN).
l   The UE:
       Monitors a paging channel or contents of system information block (SIB) type 1 to detect system information changes and ETWS notifications (for ETWS-supported UEs only).
       Monitors control channels associated with the shared data channel to determine whether data must be scheduled for it.

       Performs neighboring cell measurements and reports measurement results.
       Obtains system information.
Figure 1-1 shows RRC states in evolved universal terrestrial radio access (E-UTRA) and illustrates the mobility support between evolved universal terrestrial radio access network (E-UTRAN), universal terrestrial radio access network (UTRAN), and GERAN.

1.2 Signaling Radio Bearer

A signaling radio bearer (SRB) is defined as a radio bearer (RB) that is used only for the transmission of RRC and non-access stratum (NAS) messages. Specifically, the following three SRBs are defined:
l   SRB0 is used for transmitting RRC messages using the common control channel (CCCH).
l   SRB1 is used for transmitting RRC messages (which may include piggybacked NAS messages) and NAS messages, both using the dedicated control channel (DCCH).
l   SRB2 is used for transmitting NAS messages using the DCCH logical channel. SRB2 has a lower-priority than SRB1 and is always configured by E-UTRAN after security activation.
In the downlink, NAS message piggybacking is used only for a dependent procedure, for example, bearer establishment, modification, or release. A dependent procedure is with joint success or failure. In the uplink, NAS message piggybacking is used only for transferring the initial NAS message during connection setup.
Note: RRC messages include the NAS messages transferred on SRB2, but not include any RRC protocol control information.
Once security is activated, all RRC messages on SRB1 and SRB2, including those containing NAS or non-3GPP messages, are integrity-protected by the Packet Data Convergence Protocol (PDCP) and encrypted by the PDCP. NAS independently applies integrity protection and encryption to NAS messages.

1.3 Functions

The RRC protocol provides the following functions:

Broadcast of System Information

  • NAS common information

  • Information applicable to UEs in RRC_IDLE mode, for example, cell selection and reselection parameters, neighboring cell information

  • Information applicable to UEs in RRC_CONNECTED mode, for example, common channel configuration information.

  •   ETWS notification

RRC Connection Control

  •  Paging

  • RRC connection establishment, modification, or release, including UE identity (C-RNTI) assignment or modification, SRB1 and SRB2 establishment, modification or release, and access class barring.

  • Initial security activation, for example, initial configuration of AS integrity protection (SRBs) and AS encryption (SRBs and DRBs)

  • RRC connection mobility, including intra- and inter-frequency handover, associated security handling, key or algorithm changes, and specification of RRC context information transferred between network nodes

  • Data radio bearer (DRB) establishment, modification, and release

  •  Radio configuration control, including assignment and modification of ARQ configuration, HARQ configuration, and DRX configuration

  • QoS control, including assignment and modification of the following items:

  1. Semi-persistent scheduling (SPS) configuration information for downlink and uplink
  2. Uplink rate control parameters in the UE, which is allocation of a priority and a prioritized bit rate (PBR) for each RB

  • Recovery from radio link failures

  •  Inter-RAT mobility, including security activation and transfer of RRC context information
  • Measurement configuration and result reporting
  1. Measurement establishment, modification, and release, including intra-frequency, inter-frequency, and inter-RAT measurements 
  2.  Measurement gap establishment and release
  3.  Measurement result reporting
  • Other functions, including transfer of dedicated NAS information and non-3GPP dedicated information, transfer of UE radio access capability information, and support for E-UTRAN sharing (multiple PLMN identities)
  • Generic protocol error handling
  •   Support of self-configuration and self-optimization

Budi Prasetyo

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