DAPS Handover: Release-16 Call flow

IN 5G-NR UE’s operating in RRC_Connected mode can be categorized into two types of mobilities Cell level mobility and Beam Level Mobility which are network controlled procedures.

In Release-15 3GPP has introduced the inter-gNB handover procedure which is similar to the Legacy handover procedure, which is break before make procedure.

However, in Release-16 3GPP introduced two new types of Handover procedures

1. Dual Active Protocol Stack (DAPS) handover
2. Conditional Handover (CHO)

 For the scope of this article, DAPS handover is discussed in detail.

• 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;

In step#1: The source gNB initiates handover and issues a HANDOVER REQUEST over the Xn interface.

In Step#2: The target gNB performs admission control and provides the new RRC configuration as part of the HANDOVER REQUEST ACKNOWLEDGE.

In Step#3: The source gNB provides the RRC configuration to the UE by forwarding the RRCReconfiguration message received in the HANDOVER REQUEST ACKNOWLEDGE.

The RRCReconfiguration message includes at least a cell ID and all information required to access the target cell so that the UE can access the target cell without reading system information. The Network will also include if DAPS is supported both by the UE and the gNB. RRCReconfiguration message also includes DAPS configuration.

When DAPS handover is configured, the UE continues DL/UL data transmission/reception from the source gNB until RACH is successful on the target gNB. After Successful successful random access procedure on the target gNB, the UE releases the source cell.

When Carrier Aggregation is configured on the Source cell and Handover is triggered, the network releases the Scells and only Pcell is kept during DAPS Handover.

During  Normal handover, the RRC layer requires the UE to reset the MAC entity and re-establish RLC, but in the case of DAPS Handover MAC is not reset but UE performs the below actions

• Creates a MAC entity for target cell
• Establishes the RLC entity and an associated DTCH logical channel for the target cell for each DRB configured with DAPS
• For each DRB configured with DAPS, the UE
  • reconfigures the PDCP entity with separate security and
  • reconfigures ROHC functions for the source and target cells and associates them with the RLC entities configured by source and target respectively.
• Retains the Source cell connection and configuration until the cell is released.
• If the UE has multiple DRB’s and certain DRB’s are not configured with DAPS then the UE handles RLC and PDCP for DRBs same as in normal handover i.e re-establish RLC.

In Step#4: The UE moves the RRC connection to the target gNB and replies with the RRCReconfigurationComplete.

The Scope of this article procedure related to and pertaining to DAPS handover is mentioned and below is the Call flow for Intra-AMF/UPF Handover with DAPS.The procedure is similar to Normal Handover procedures but there are certain differences during DAPS handover as mentioned below.

In Step#3 The Source gNB will also include a request for DAPS Handover to the target gNB for one or more DRB’s in the Handover request message.

In Step#5: The target gNB prepares the handover with L1/L2 and sends the HANDOVER REQUEST ACKNOWLEDGE to the source gNB, which includes a transparent container to be sent to the UE as an RRC message to perform the handover. The target gNB also indicates if a DAPS handover is accepted. For DRBs configured with DAPS, downlink PDCP SDUs are forwarded with SN assigned by the source gNB, until SN assignment is handed over to the target gNB in step 8b

In Step#6: The source gNB triggers the Uu handover by sending an RRCReconfiguration message to the UE, the RRC Reconfiguration will also include DAPS configuration if it is configured by the network. For DRBs configured with DAPS, the source gNB does not stop transmitting downlink packets until it receives the HANDOVER SUCCESS message from the target gNB in step 8a.

• The network cannot configure Conditional Handover (CHO) and DAPS Handover simultaneously.

In Step#7a: For DRBs configured with DAPS, the source gNB sends the EARLY STATUS TRANSFER message. The DL COUNT value conveyed in the EARLY STATUS TRANSFER message indicates PDCP SN and HFN of the first PDCP SDU that the source gNB forwards to the target gNB. The source gNB does not stop assigning SNs to downlink PDCP SDUs until it sends the SN STATUS TRANSFER message to the target gNB in step 8b

In Step#7: For DRBs not configured with DAPS, the source gNB sends the SN STATUS TRANSFER message to the target gNB to convey the uplink PDCP SN receiver status and the downlink PDCP SN transmitter status of DRBs for which PDCP status preservation applies (i.e. for RLC AM)

• In case of DAPS handover, the uplink PDCP SN receiver status and the downlink PDCP SN transmitter status for a DRB with RLC-AM and not configured with DAPS may be transferred by the SN STATUS TRANSFER message in step 8b instead of step 7.

• For DRBs configured with DAPS, the source gNB may additionally send the EARLY STATUS TRANSFER message(s) between step 7 and step 8b, to inform discarding of already forwarded PDCP SDUs. The target gNB does not transmit forwarded downlink PDCP SDUs to the UE, whose COUNT is less than the conveyed DL COUNT value and discards them if the transmission has not been attempted already

In Step#8: The UE synchronizes to the target cell and completes RACH and the RRC handover procedure by sending RRCReconfigurationComplete message to target gNB. When DAPS handover is configured, the UE does not detach from the source cell upon receiving the RRCReconfiguration message.

• The UE releases the source SRB resources, security configuration of the source cell and stops DL/UL reception/transmission with the source upon receiving an explicit release from the target node.
• From the RAN point of view, the DAPS handover is considered to only be completed after the UE has released the source cell as explicitly requested from the target node.

Step#8a/b: In the case of DAPS handover, the target gNB sends the HANDOVER SUCCESS message to the source gNB to inform that the UE has successfully accessed the target cell. In return, the source gNB sends the SN STATUS TRANSFER message for DRBs configured with DAPS.

The rest of the Handover completion process is similar to the Normal Handover process related to user data path switch in the core network.

References :

NR; NR and NG-RAN Overall description; Stage-2 3GPP TS 38.300