WCDMA RRC States and Procedures Explained

This page covers the WCDMA RRC (Radio Resource Control) state diagram for a UE (User Equipment) compliant with the UMTS standard. It describes the RRC states, including both idle and connected modes, and the procedures associated with each.

RRC States and Procedures Overview

The RRC layer is responsible for controlling the radio resources between the UE and the UTRAN (UMTS Terrestrial Radio Access Network). It manages the establishment, maintenance, and release of radio connections.

Image alt: WCDMA UMTS RRC Procedures in Idle Mode

Idle Mode

When a UE is powered on, it initially enters idle mode. In this mode, the UE performs essential functions such as location registration. Even after camping onto a UTRAN network, it continues to monitor system information and paging messages broadcast by the UTRAN.

UTRAN Selection and Reselection

The Non-Access Stratum (NAS) is responsible for selecting a suitable UTRAN cell. Ideally, the UE operates on its Home UTRAN. However, when the UE moves out of its Home UTRAN coverage area, a visited UTRAN cell will be selected.

Cell Selection and Reselection

The UE selects the most suitable cell based on idle mode measurements and established cell selection criteria. The NAS controls the cell selection based on a list of forbidden registration areas and a list of NAS-defined service areas, taking priority into account.

Even after camping on a cell, the UE continuously performs measurements and searches for a better cell. Once a better cell is found, it connects to that cell based on the cell selection criteria.

Image alt: UMTS RRC connection establish procedure

Connected Mode

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

During this connection establishment, the WCDMA RRC configures the Layer-1 (Physical Layer) and Layer-2 (Data Link Layer) using parameters received from the UTRAN. Once the configuration is complete, the UE’s RRC initiates the RLC (Radio Link Control) signaling link.

Following successful RRC connection establishment, the signaling connection establishment resumes. Once the signaling connection is established, higher-layer peer-to-peer signaling data transfer occurs. The UE remains in connected mode as long as the WCDMA RRC connection persists.

The level of UE connection to the UTRAN within the connected mode depends on the QoS (Quality of Service) requirements of the active radio access bearers 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, the WCDMA UE can be in one of several RRC states: CELL_DCH, CELL_FACH, CELL_PCH, and URA_PCH.

CELL_DCH State

The WCDMA UE enters the CELL_DCH state when it is assigned a DPCH (Dedicated Physical Channel). This assignment can occur either in response to an RRC Connection Request or through an RRC-controlled Bearer Re-configuration procedure.

A UE in this mode can be identified by three parameters: frequency, scrambling code, and channelization code. When the UE releases the RRC connection, it transitions back to idle mode.

CELL_FACH State

The UE transitions to the CELL_FACH state when it has been assigned a common channel by the UTRAN network. This assignment typically occurs as part of the RRC connection establishment procedure and is managed by the RNC (Radio Network Controller) or through an RB (Radio Bearer) re-configuration procedure.

CELL_PCH State

While the UE is in either CELL_FACH or CELL_DCH state, if the flow of traffic ceases, it will transition to the CELL_PCH state. In this state, the UE does not have any uplink channel and monitors the paging channel using DRX (Discontinuous Reception) parameters. The primary purpose of this state is to conserve power and extend battery life.

URA_PCH State

URA (UTRAN Registration Area) is essentially a collection of cells, similar to a GSM LA (Location Area). The URA_PCH state is comparable to the CELL_PCH state, but in this case, the location of the UE is known at the URA level. This state is typically used by fast-moving UEs that require DRX to conserve battery power.

WCDMA-compliant UEs will publish gaps during compressed mode for measurement purposes. This is used by other RATs (Radio Access Technologies) such as GSM and LTE to perform cell measurements, which helps in cell reselection as well as handover procedures.

Summary of UMTS RRC Functions

The following is a summary of the key functions performed by the WCDMA/UMTS RRC layer:

• 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 the 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 (Protocol Data Units).
• Control of requested QoS.
• Reporting and control of UE measurements.
• Outer loop power control.
• Control of ciphering.