WCDMA RRC

This page covers WCDMA RRC state diagram for UE compliant to UMTS standard. It covers RRC states idle and connected.

Procedures in the UE

Procedures in Idle Mode

When UE is powered on it will be in idle mode. In this mode, UE will perform location registration. In the idle mode, UE will receive and monitor system information and paging messages from the UTRAN network. It does this even after camped on to the UTRAN network.

UTRAN Selection and Reselection

In this system,non access stratum(NAS) will select a suitable UTRAN cell. Usually UE operates on its Home UTRAN. When UE goes out of Home UTRAN coverage, visited UTRAN cell will be selected.

Cell Selection and Reselection

The UE shall select the most suitable cell based on idle mode measurements and cell selection criteria. The NAS will control the cell selection based on list of forbidden registration areas and a list of NAS defined service areas based on priority set. Even after camped on , UE will perform measurements and always search for better cell. Once found it will connect with the best UTRAN cell. This is done based on cell selection criteria.

CONNECTED MODE

For UE initiated calls, the higher layers of the UE send a request to RRC layer to establish a signaling connection establishment. Upon receiving this, the RRC of the UE initiates a connection establishment procedure with the peer RRC of the UTRAN by sending an RRC connection request message through the MAC. The UE enters connected mode once the RRC connection is established.

During this connection establishment procedure, the WCDMA RRC configures the layers Layer-1 and Layer-2 using the parameters received from the UTRAN network. Once the configuration is complete the RRC of the UE initiate the RLC signaling link. Once the RRC connection establishment is successful, the signaling connection establishment will be resumed. Once the signaling connection is established there will be higher layer peer-to-peer signaling data transfer.The UE enters connected mode once the WCDMA RRC connection is established. Within the connected mode the level of UE connection to UTRAN will be determined by the QoS requirements of the active radio accessbearers and the characteristics of the traffic on those bearers.

Procedures in Connected Mode

•  Radio Access Bearer Establishment
•  Radio Access Bearer Release
•  Radio Access Bearer and Signaling Link Configuration
•  Transport Channel Reconfiguration
•  Transport Format Combination Control
•  Physical Channel Reconfiguration
•  Data Transmission

In connected mode, WCDMA UE will have cell_DCH, cell_FACH, cell_PCH and URA_PCH RRC states.

CELL_DCH state

WCDMA UE will be in the CELL_DCH state when UE is assigned a DPCH(dedicated physical channel) either in response to RRC Connection Request or through RRC controlled Bearer Re-config procedure. UE can be identified in this mode by three parameters viz. frequency,scrambling code and channelization code. When UE release the RRC connection it will move to IDLE mode.

CELL_FACH state

UE moves to CELL_FACH state when it has been assigned common channel by the UTRAN network. This happens as part of RRC connection establishment procedure and assigned by RNC or through RB reconfig. procedure.

CELL_PCH state

While UE is in CELL_FACH or CELL_DCH state and flow of traffic is stopped, it will move to CELL_PCH state. Here UE will not have any uplink channel and UE monitors paging channel using the DRX parameters. The purpose of this state is to save the power and enhance the battery usage for long time.

URA_PCH state

URA is basically collection of cells and it is similar to GSM LA. This state is similar to CELL_PCH state but here location of the UE is known at URA level. This state is used by fast moving UEs needing DRX to save the battery power.

WCDMA compliant UE will publish the gaps during the compressed mode for measurement purpose. This is used by other RATs such as GSM and LTE to perform cell measurements. This helps in cell reselection as well as handover. Following are the summary of functions performed by WCDMA/UMTS RRC layer.

Summary of UMTS RRC functions

•  Reception of broadcast information provided by the non-access stratum (Core Network)
•  Reception of broadcast information related to the access stratum
•  Establishment, maintenance and release of an RRC connection between the UE and UTRAN
•  Establishment, reconfiguration and release of Radio-Access Bearers
•  Assignment, reconfiguration and release of radio resources for the RRC connection
•  RRC connection mobility functions
•  Paging/notification
•  Routing of higher layer PDUs
•  Control of requested QoS
•  Reporting and control of UE measurements
•  Outer loop power control
•  Control of ciphering

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