DAPS Handover: Release-16 Call Flow

DAPS Handover: Release-16 Call Flow

In legacy mobile systems (up to 5G Release 15), cell handover followed a "break-before-make" architecture. When a handover command was received, the User Equipment (UE) disconnected from the source cell before establishing a connection to the target cell. This resulted in an interruption time (typically 30–50 ms) that degraded the user experience, particularly for real-time applications like gaming, voice over NR (VoNR), and industrial automation.

To resolve this, 3GPP introduced the Dual Active Protocol Stack (DAPS) handover in Release 16. It operates on a "make-before-break" principle, maintaining the user plane connection with the source cell until the connection to the target cell is successfully established.

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DAPS Protocol Architecture

To support DAPS, the UE maintains two independent active protocol stacks simultaneously for both the source and target cells during the handover execution phase:

            +---------------------------------------+
            |                  SDAP                 |
            +---------------------------------------+
                                │
            +───────────────────┴───────────────────+
            │                 PDCP                  │
            │       (Source & Target Anchor)        │
            +───────────────────────────────────────+
              /                                   \
  +-----------------------+           +-----------------------+
  |      Source RLC       |           |      Target RLC       |
  +-----------------------+           +-----------------------+
  |      Source MAC       |           |      Target MAC       |
  +-----------------------+           +-----------------------+
  |      Source PHY       |           |      Target PHY       |
  +-----------------------+           +-----------------------+

• SDAP: Remains common across both stacks.
• PDCP: Acts as the anchor, handling packet reordering and duplication across the source and target protocol legs.
• RLC/MAC/PHY: Split into independent entities for the source and target links.

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Detailed DAPS Handover Call Flow

The handover sequence involves three key phases: Preparation, Execution, and Completion.

Phase 1: Preparation

1. The UE monitors neighboring cells and triggers a MeasurementReport when target cell thresholds are met.
2. The source gNB evaluates the report and decides to initiate a DAPS handover. It sends a HANDOVER REQUEST to the target gNB over the Xn interface, indicating DAPS configuration support.
3. The target gNB performs admission control. If approved, it replies with a HANDOVER REQUEST ACKNOWLEDGE containing the DAPS parameters.

Phase 2: Execution

4. The source gNB sends an RRCReconfiguration message to the UE. This acts as the handover command.
5. UE Dual Active Initialization: Instead of tearing down the source connection, the UE spawns a secondary MAC and RLC entity for the target cell while maintaining the source stack active.
6. The source gNB sends an Early Status Transfer message to the target gNB to sync PDCP sequence numbers.
7. During target cell acquisition, the UE continues receiving user plane data from the source gNB.
8. The UE transmits a RACH preamble to the target cell.
9. The target cell responds with a Random Access Response (RAR).
10. The UE completes the target uplink path by sending the RRCReconfigurationComplete message.

Phase 3: Completion

11. Upon verifying the UE's target connection, the target gNB sends a HANDOVER SUCCESS message to the source gNB.
12. The target gNB sends an RRCReconfiguration message to the UE containing the source release instruction.
13. The UE terminates the source physical layer, MAC, and RLC connections, transitioning to a single protocol stack focused on the target cell.

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DAPS Key Benefits

• 0 ms User Plane Interruption: Eliminates call drop risks during high-speed transitions (e.g., high-speed rail mobility).
• Robust Link Integrity: If target cell acquisition fails, the UE immediately falls back to the active source connection without initiating full RRC re-establishment.

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References

• 3GPP TS 38.300: NR and NG-RAN overall description; Stage 2.
• 3GPP TS 38.331: Radio Resource Control (RRC) protocol specification.

Further Reading

• Conditional Handover (CHO) in 5G NR Release 16
• Mobility Robustness Optimization (MRO)