5.4 Inter-RAT mobility

38.3313GPPNRProtocol specificationRadio Resource Control (RRC)Release 15TS

5.4.1 Introduction

Network controlled inter-RAT mobility between NR and E-UTRA is supported, where E-UTRA can be connected to either EPC or 5GC.

5.4.2 Handover to NR

5.4.2.1 General

Figure 5.4.2.1-1: Handover to NR, successful

The purpose of this procedure is to, under the control of the network, transfer a connection between the UE and another Radio Access Network (e.g. E-UTRAN) to NR.

The handover to NR procedure applies when SRBs, possibly in combination with DRBs, are established in another RAT. Handover from E-UTRA to NR applies only after integrity has been activated in E-UTRA.

5.4.2.2 Initiation

The RAN using another RAT initiates the handover to NR procedure, in accordance with the specifications applicable for the other RAT, by sending the RRCReconfiguration message via the radio access technology from which the inter-RAT handover is performed.

The network applies the procedure as follows:

– to activate ciphering, possibly using NULL algorithm, if not yet activated in the other RAT;

– to re-establish SRBs and one or more DRBs;

5.4.2.3 Reception of the RRCReconfiguration by the UE

The UE shall:

1> apply the default L1 parameter values as specified in corresponding physical layer specifications except for the parameters for which values are provided in SIB1;

1> apply the default MAC Cell Group configuration as specified in 9.2.2;

1> perform RRC reconfiguration procedure as specified in 5.3.5;

NOTE: If the UE is connected to 5GC of the source E-UTRA cell, the delta configuration for PDCP and SDAP can be used for intra-system inter-RAT handover. For other cases, source RAT configuration is not considered when the UE applies the reconfiguration message of target RAT.

5.4.3 Mobility from NR

5.4.3.1 General

Figure 5.4.3.1-1: Mobility from NR, successful

Figure 5.4.3.1-2: Mobility from NR, failure

The purpose of this procedure is to move a UE in RRC_CONNECTED to a cell using other RAT, e.g. E-UTRA. The mobility from NR procedure covers the following type of mobility:

– handover, i.e. the MobilityFromNRCommand message includes radio resources that have been allocated for the UE in the target cell;

5.4.3.2 Initiation

The network initiates the mobility from NR procedure to a UE in RRC_CONNECTED, possibly in response to a MeasurementReport message, by sending a MobilityFromNRCommand message. The network applies the procedure as follows:

– the procedure is initiated only when AS security has been activated, and SRB2 with at least one DRB are setup and not suspended.

5.4.3.3 Reception of the MobilityFromNRCommand by the UE

The UE shall:

1> stop timer T310, if running;

1> if T390 is running:

2> stop timer T390 for all access categories;

2> perform the actions as specified in 5.3.14.4;

1> if the targetRAT-Type is set to eutra:

2> consider inter-RAT mobility as initiated towards E-UTRA;

2> forward the nas-SecurityParamFromNR to the upper layers, if included;

1> access the target cell indicated in the inter-RAT message in accordance with the specifications of the target RAT.

5.4.3.4 Successful completion of the mobility from NR

Upon successfully completing the handover, at the source side the UE shall:

1> reset MAC;

1> stop all timers that are running except T325;

1> release ran-NotificationAreaInfo, if stored;

1> release the AS security context including the KRRCenc key, the KRRCint key, the KUPint key and the KUPenc key, if stored;

1> release all radio resources, including release of the RLC entity and the MAC configuration;

1> release the associated PDCP entity and SDAP entity for all established RBs;

NOTE : PDCP and SDAP configured by the source RAT prior to the handover that are reconfigured and re-used by target RAT when delta signalling (i.e., during inter-RAT intra-system handover when fullConfig is not present) is used, are not released as part of this procedure.

1> if the targetRAT-Type is set to eutra and the nas-SecurityParamFromNR is included:

2> indicate the release of the RRC connection to upper layers together with the release cause ‘other’.

5.4.3.5 Mobility from NR failure

The UE shall:

1> if the UE does not succeed in establishing the connection to the target radio access technology; or

1> if the UE is unable to comply with any part of the configuration included in the MobilityFromNRCommand message; or

1> if there is a protocol error in the inter RAT information included in the MobilityFromNRCommand message, causing the UE to fail the procedure according to the specifications applicable for the target RAT:

2> revert back to the configuration used in the source PCell;

2> initiate the connection re-establishment procedure as specified in subclause 5.3.7.