6 User Identification and RRC Connection Mobility

25.3013GPPRadio interface protocol architectureTS

6.1 UE identification on the radio interface

A Radio Network Temporary Identity (RNTI) is used as an UE identifier on RACH/FACH, or, for TDD mode, also on DSCH by the RRC, or on PCH by the RRC, when a RRC connection exists. For the HS-DSCH the UE identification is included by the physical layer with the help of a UE-specific CRC.

Definition of UE identifiers

Several types of RNTIs exist. One is used within the Serving RNC and it is denoted by Serving RNC RNTI (S-RNTI). A second type is used within a cell controlled by a CRNC, when applicable, and it is denoted by Cell RNTI (C-RNTI). A third type is used only for TDD within a cell controlled by a CRNC when a DSCH is allocated and it is denoted by DSCH-RNTI. A fourth type is used within a cell controlled by a CRNC when an HS-DSCH is configured and it is denoted by HS-DSCH-RNTI (H-RNTI). A fifth type is used within a cell when an E-DCH is configured to a UE and it is denoted by E-RNTI.

S-RNTI is allocated for all UEs having a RRC connection. It is allocated by the Serving RNC and it is unique within the Serving RNC. S-RNTI is reallocated always when the Serving RNC for the RRC connection is changed and deallocated when the RRC connection is released.

In addition for each UE having an RRC connection, there is an identifier of its current serving RNC, which is denoted as SRNC identifier. The SRNC identifier together with S-RNTI is a unique identifier of the RRC connection within PLMN. The combination of SRNC identifier and S-RNTI is referred to as U-RNTI (UTRAN Radio Network Temporary Identity), which is used on the radio interface.

C-RNTI for a UE is allocated by a controlling RNC and has a scope of either one cell or (FDD only) one URA. For the one-cell scope, it is unique within one cell controlled by the allocating CRNC. For the one-URA scope, it may or may not be unique within one URA controlled by the allocating CRNC. C-RNTI can be reallocated when a UE accesses a new cell with the cell update procedure or enters a new URA with the URA update procedure.

In TDD mode, DSCH-RNTI for a UE is allocated by controlling RNC when a DSCH channel is configured. DSCH-RNTI is unique within the cell carrying the DSCH.

When a UE is in CELL_DCH state, H-RNTI for the UE is allocated by a controlling RNC and it is unique within one cell controlled by the allocating CRNC. H-RNTI is reallocated when an HS-DSCH cell change is performed. In FDD and 1.28 Mcps TDD mode, when a UE is in CELL_FACH, CELL_PCH or URA_PCH state, H-RNTI for the UE is allocated by a controlling RNC and has a scope of either one cell or (FDD only) one URA. For the one-cell scope, it may be unique or shared among users within one cell controlled by the allocating CRNC. For the one-URA scope, it may be unique within one URA controlled by the allocating CRNC.

E-RNTI for a UE has a scope of either one cell or (FDD only) one URA. For the one-cell scope, it is allocated by the Node-B and it is valid within one cell. E-RNTI may be reallocated when the serving E-DCH cell changes. Several UEs may use the same E-RNTI. The E-RNTI may be allocated to a UE in CELL_DCH, CELL_FACH (FDD and 1.28 Mcps TDD only), and CELL_PCH (FDD and 1.28 Mcps TDD only) and URA_PCH (FDD only). For the one-URA scope, it is allocated by the CRNC and unique within one URA controlled by the allocating CRNC.

Usage of UE identifiers

U-RNTI is allocated to an UE having a RRC connection. It identifies the UE within UTRAN and is used as a UE identifier in cell update, URA update, RRC connection reestablishment and (UTRAN originated) paging messages and associated responses on the radio interface. The SRNC identifier within the U-RNTI is used by the Controlling RNC to route the received uplink messages towards the Serving RNC.

C-RNTI is used as a UE identifier in all other DCCH/DTCH common channel messages on the radio interface.

In TDD mode, DSCH-RNTI is used as a UE identifier for DTCH and DCCH in downlink when mapped onto DSCH transport channel.

H-RNTI is used as a UE identifier for the HS-DSCH.

E-RNTI is used as a UE identifier when controlling UE uplink resources on the E-DCH.

NAS identifiers are used as the UE identifier in the initial access CCCH message on the radio interface.

6.2 UE connection to UTRAN

The different levels of UE connection to UTRAN are listed below:

– No signalling connection exist
The UE has no relation to UTRAN, only to CN. For data transfer, a signalling connection has to be established.

– Signalling connection exist
There is a RRC connection between UE and UTRAN. The UE position can be known on different levels:

– UTRAN Registration Area (URA) level
The UE position is known on UTRAN registration area level. URA is a specified set of cell, which can be identified on the BCCH.

– Cell level
The UE position is known on cell level. Different channel types can be used for data transfer:

– Common transport channels (RACH, FACH, DSCH, HS-DSCH, E-DCH);

– Dedicated transport channels (DCH, E-DCH).