8.3.8 Inter-RAT cell reselection to UTRAN

25.3313GPPProtocol specificationRadio Resource Control (RRC)Release 17TS

8.3.8.1 General

The purpose of the inter-RAT cell reselection procedure to UTRAN is to transfer, under the control of the UE and to some extent the source radio access technology, a connection between the UE and another radio access technology (e.g. GSM/GPRS, but not UTRAN) to UTRAN.

8.3.8.2 Initiation

When the UE makes an inter-RAT cell reselection to UTRAN according to the criteria specified in [4], it shall initiate this procedure. The inter-RAT cell reselection made by the UE may use system information broadcast from the source radio access technology or UE dedicated information.

If the NAS procedures associated with inter-system change specified in [5] require the establishment of an RRC connection, the UE shall:

1> set the variable ESTABLISHMENT_CAUSE to "Inter-RAT cell reselection";

NOTE: This value of ESTABLISHMENT_CAUSE has priority over the cause requested by upper layers.

1> initiate an RRC connection establishment procedure as specified in subclause 8.1.3;

1> after initiating an RRC connection establishment:

2> release all resources specific to the other radio access technology.

If the NAS procedures associated with inter-system change specified in [5] do not require the establishment of an RRC connection, the UE shall:

1> enter idle mode in the target cell without accessing the cell; and

1> release all resources specific to the other radio access technology.

8.3.8.2a Initiation of inter-RAT cell reselection from GERAN Iu mode

When the UE performs an inter-RAT cell reselection from GERAN Iu mode Cell_Shared state, the UE shall:

1> initiate the cell update procedure as specified for the cell reselection case in CELL_FACH and CELL_PCH states, using the cause "cell reselection"and setting the G-RNTI in the IE "U-RNTI".

When the UE performs an inter-RAT cell reselection from GERAN Iu mode GRA_PCH state, the UE shall:

1> compare the GRA identity which the MS had been assigned to in GERAN against the URA identities which are broadcast in the UTRAN cell.

1> If the assigned GRA identity is not present in the list of URA identities that are broadcast in the UTRAN cell:

2> initiate the URA update procedure as specified for the URA reselection case in URA_PCH state, using the cause "change of URA" and setting the G-RNTI in the IE "U-RNTI".

The UE shall:

1> set the following variables equal to the corresponding variables in GERAN Iu mode:

CIPHERING_STATUS

ESTABLISHED_RABS

ESTABLISHED_SIGNALLING_CONNECTIONS

INTEGRITY_PROTECTION_INFO

INTER_RAT_HANDOVER_INFO_TRANSFERRED

LATEST_CONFIGURED_CN_DOMAIN

START_THRESHOLD

UE_CAPABILITY_TRANSFERRED.

1> set the new uplink and downlink HFN of RB2 to MSB₂₀(MAX(uplink HFN of RB2, downlink HFN of RB2)).

NOTE: MSB₂₀() operation provides the HFN mapping from GERAN Iu mode to UTRAN. In GERAN Iu mode the length of HFN component of the COUNT-C of RB2 is longer than 20 bits.

1> initialise the variable TIMERS_AND_CONSTANTS to the default values and start to use those timer and constants values.

8.3.8.3 UE fails to complete an inter-RAT cell reselection

If the inter-RAT cell reselection fails before the UE has initiated the RRC connection establishment the UE may return back to the other radio access technology.

If the RRC connection establishment fails, the UE shall enter idle mode.

8.3.8.3a UE fails to complete an inter-RAT cell reselection from GERAN Iu mode

When the UE performs an inter-RAT cell reselection from GERAN Iu mode to UTRAN, and the cell reselection fails:

1> the UE may return back to the GERAN Iu mode state from which it initiated the inter-RAT cell reselection.