During the NR Registration procedure, UE will include its support for Dual Active Protocol stack (DAPS) Handover in the UE Capability message. If there is no support for DAPS from UE, then the network will not configure DAPS handover for one more DRBs during the handover procedure.
UE Capability information IEs sent by UE are discussed in detail here
Upon receiving a handover command requesting DAPS handover, the UE suspends all active Signaling radio bearers (SRBs) at the source cells and establishes SRBs for the target cell. Please note that UE does not release the SRBs at the Source cell when performing RACH on the target cell.
The UE releases the source cell SRBs configuration upon explicitly receiving source release indication from the target cell after successful DAPS handover execution.
In case when DAPS handover to the target cell fails i.e during RACH or due to some other reason and if the source cell link is available, then the UE reverts back to the source cell configuration and resumes the source cell SRBs for the control plane signaling transmission. Further discussion on UE recovery on Source cell if DAPS handover failure occurs is discussed below in this article.
When UE receives an RRCreconfiguration with sync message for DAPS Handover, This message can include
- security key refresh and Random access configuration for target cell
- Establishment of of target cell MAC entity.
- DAPS DRB Bearer :
- Establishment of RLC entity of the target Pcell
- refresh of security and reconfiguration of PDCP to add the ciphering function and the integrity protection function of the target Pcell
- SRB Bearer :
- refresh of security and establishment of RLC and PDCP for the target Pcell for SRB bearer.
The UE can also receive RRCreconfiguration with sync message for DAPS Handover and can include below
- No security key refresh and involving RA to the target Pcell and establishment of target MAC entity.
- non-DAPS bearer:
- RLC re-establishment and PDCP data recovery (for AM DRB) triggered by explicit L2 indicators.
- DAPS bearer:
- establishment of RLC for target PCell,
- reconfiguration of PDCP to add the ciphering function and the integrity protection function of the target PCell;
- SRB Bearer:
- establishment of RLC and PDCP for the target PCell.
So when UE receives an RRC message with Reconfiguration with sync and if a DAPS bearer is configured, first the UE creates a MAC entity for the target cell group with the same configuration as the source cell group. The UE reconfigures the MAC main configuration for the target cell group in accordance with the received mac-CellGroupConfig excluding tag-ToReleaseList and tag-ToAddModList;
For each DAPS bearer configured the UE performs the below actions
- Establishes an RLC entity for the target cell group with the same configuration as used in the source cell group
- Establishes a Logical channel for the target cell group with the same configuration as used in the source cell group
For each SRB bearer configured the UE performs the below actions
- Establishes an RLC entity for the target cell group with the same configuration as used in the source cell group
- Establishes a Logical channel for the target cell group with the same configuration as used in the source cell group
- Suspend SRBs for the source cell group. Please note that UE does not releases the SRB at source cell when it receives reconfiguration with sync message.
RLF Timers & Constants configuration for DAPS
If the network has explicitly configured rlf-TimersAndConstants in the RRC message for DAPS handover, UE will configure the timers as received for the target cell. If the RRC message for DAPS handover has configured rlf-TimersAndConstants set to release, then the configured DAPS bearer will use values for timers T301, T310, T311 and constants N310, N311 for the target cell group, as included in ue-TimersAndConstants received in SIB1 of the Source cell
DAPS Handover Failure Information
If there is a handover failure that has been configured with DAPS then UE informs the handover failure to the network by sending a ‘FailureInformation‘ Message to the gNB. UE triggers FailureInformation when it detects DAPS Handover failure and sends FailureInfoDAPS IE with failureType set as daps-failure
FailureInfoDAPS-r16 ::= SEQUENCE {
failureType-r16 ENUMERATED {daps-failure, spare3, spare2, spare1}
}Radio Link Failure when DAPS is configured
If any DAPS bearer is configured to the UE and UE detects N310 consecutive “out-of-sync” indications for the source SpCell from the lower layers while timer T304 is running, UE starts T310 timer for the Source SpCell and if the Link is not restored by the UE i.e UE receives “in-sync” indication before T310 expires it declares radio link failure on the SpCell.
The UE will try to restore the link at the source cell and will trigger RRC Reestablishment on the Source cell and performs the below action
- Release Source SpCell configuration
- Reset source MAC entity
- Release the source MAC Configuration
- for each DAPS Bearer
- Release the Associated RLC entitties and associated logical channel for the soource SpCell.
- Reconfigure PDCP entity to release DAPS.
- For Each SRB Bearer
- Release PDCP entity to release DAPS.
- Release the RLC entitties and associated logical channel for the soource SpCell.
DAPS Configuration
configRestrictInfoDAPS-r16 ConfigRestrictInfoDAPS-r16
[[
configRestrictInfoDAPS-v1640 ConfigRestrictInfoDAPS-v1640
]]
ConfigRestrictInfoDAPS-r16 ::= SEQUENCE {
powerCoordination-r16 SEQUENCE {
p-DAPS-Source-r16 P-Max,
p-DAPS-Target-r16 P-Max,
uplinkPowerSharingDAPS-Mode-r16 ENUMERATED {semi-static-mode1, semi-static-mode2, dynamic }
} }Does every UE need to Support DAPS?
Dual Active Protocol stack (DAPS) is not a mandatory feature and not every UE is expected to support this feature. During the initial deployment there not be many UE’s available that support this functionality, so it will not be wise for the operators to deploy this feature. DAPS requires a lot of changes at the UE protocol stack and it needs to handle Simialteanous RX/TX from source and target cells, this might increase the complexity at the UE and might require extensive testing with Infra vendors before the feature can be widely available. As of writing this article many features of Release-16 have not yet been deployed and there are very few devices available in the market.
References
3GPP TS 38.331 NR; Radio Resource Control (RRC); Protocol specification
3GPP TS 38.133 NR; Requirements for support of radio resource management