Rel 16 Feature – NR Mobility enhancements

NR Mobility enhancements: To reduce latency during mobility

The work item on NR mobility enhancements specifies solutions to reduce interruption time during HO (by Dual Active Protocol Stack (DAPS) handover), and to improve HO/SCG change reliability and robustness (by Conditional Handover (CHO), Conditional PSCell Change (CPC) and T312 based fast failure recovery).

Dual Active Protocol Stack (DAPS) handover

DAPS Handover is a handover procedure that maintains the source gNB connection after reception of RRC message for handover and until releasing the source cell after successful random access to the target gNB.

– The UE maintains DL reception and UL transmission for user data with source upon receiving DAPS HO command before successful RACH in target (UL switching);

– Upon receiving the indication on UL switching, for UL the PDCP layer will only forward the user data to target path;

– The UE will continue the reception of DL from both source and target and provides UL to both source (PDCP will not forward the user data to low layer) and target node before release of source.

– Upon HO failure, the UE can use source link for recovery instead of reestablishment if the source link is still valid;

Conditional Handover (CHO)

Conditional Handover (CHO) is a handover procedure that is executed only when the configured execution condition(s) are met.

To improve the robustness, the network can provide the up to 8 candidate cell configuration(s) associated with execution condition (s) to UE. The UE maintains connection with source gNB after receiving CHO configuration and starts evaluating the CHO execution conditions for the candidate cell(s). If at least one CHO candidate cell satisfies the corresponding CHO execution condition, the UE detaches from the source gNB, applies the stored corresponding configuration for that selected candidate cell, synchronises to that candidate cell and completes the RRC handover procedure by sending RRCReconfigurationComplete message to the target gNB. The UE releases stored CHO configurations after successful completion of RRC handover procedure.

Upon failure, the UE will perform CHO if the selected cell is CHO candidate cell, and it is the first time of recovery and if the network allows CHO based recovery.

Conditional PSCell Change (CPC)

Conditional PSCell Change is a PSCell change procedure that is executed only when PSCell execution condition(s) are met.

To improve the robustness, the network can provide the up to 8 candidate cell configuration(s) associated with execution condition (s) to UE. If CPC is configured in the RRCReconfiguration, the UE maintains connection with source PSCell after receiving CPC configuration and starts evaluating the CPC execution conditions for the candidate PSCell(s). If at least one CPC candidate PSCell satisfies the corresponding CPC execution condition, the UE detaches from the source PSCell, applies the stored corresponding configuration for the selected candidate PSCell and synchronises to that candidate PSCell. The UE completes the CPC execution procedure by an ULInformationTransferMRDC message to the MN which includes an embedded RRCReconfigurationComplete message to the new PSCell if SRB3 is not configured, otherwise the UE sends RRCReconfigurationComplete message to the new PSCell directly.

Upon SCG failure, same as MR-DC, the UE will transmit the SCG Failure Information message to the MN.

In Rel-16, conditional handover-based NR PSCell addition/change for any architecture option with NR PSCell is supported, but limit to intra SN change without MN involvement.

T312 based fast failure recovery

T312 based solution (same as LTE) is used for both PCell and PSCell. The motivation of T312 is to speed up RLF recovery procedure by triggering re-establishment procedure sooner using a shorter timer than T310. T312 on PSCell can be configured for SCG measurement configurations provided over SRB3 or SRB1.

Reference – 3GPP