Mobility enhancement in E-UTRAN Release -16
Dual Active Protocol Stack (DAPS) handover
DAPS Handover is a handover procedure that
maintains the source eNB connection after reception of RRC message for handover
and until releasing the source cell after successful random access to the
target eNB.
- If DAPS handover is
configured, the UE continues the downlink user data reception from the source
eNB until releasing the source cell and continues the uplink user data
transmission to the source eNB until successful random access procedure to the
target eNB.
- Upon reception of the handover
command, the UE:
- Creates a MAC entity for
target cell;
- Establishes the RLC entity
and an associated DTCH logical channel for target cell for each DRB configured
with DAPS;
- For the DRB(s) configured
with DAPS, reconfigures the PDCP entity to configure DAPS with separate
security and ROHC functions for source and target and associates them with the
RLC entities configured for source and target respectively;
- Retains rest of the source
link configurations until release of the source.
- When DAPS handover fails, the
UE falls back to source cell configuration, resumes the connection with source
cell, and reports the DAPS handover failure via the source without triggering
RRC connection re-establishment if the source link is still available;
Otherwise, RRC re-establishment is performed;
Conditional Handover (CHO)
A Conditional Handover (CHO) is defined as a
handover that is executed by the UE when one or more handover execution
conditions are met.
- UE maintains connection with
source eNB after receiving CHO configuration, and starts evaluating the CHO
execution condition(s) for the CHO candidate cell(s) and executes the HO
command once the execution condition(s) are met for a CHO candidate cell.
- To improve the robustness, the
network can provide the up to 8 candidate cell configuration(s) associated with
execution condition (s) to UE. If at least one CHO candidate cell satisfies the
corresponding CHO execution condition, the UE detaches from the source eNB, applies
the stored corresponding configuration for that candidate cell and synchronises
to that candidate cell. UE stops evaluating the execution condition(s) for
other candidate cells once the handover is triggered.
- The UE accesses to the target
eNB and completes the handover procedure by sending
RRCConnectionReconfigurationComplete message to target eNB. The UE releases
stored CHO configurations after successful completion of RRC handover
procedure.
- When initial CHO execution
attempt fails or HO fails, if network configured the UE to try CHO after HO/CHO
failure and the UE performs cell selection to a CHO candidate cell, the UE
attempts CHO execution to that cell; Otherwise, RRC re-establishment is
performed.
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