Non-DC Handover Interruption Reduction via Frozen Sequence Numbers
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Solution Overview
Problem
Current 3GPP Radio (NR) specifications do not effectively address interruptions in Non-Dual Connectivity (non-DC) based handover (HO) processes, particularly in Radio Link Control Acknowledged Mode (RLC-AM) and downlink (DL) transmissions, due to unspecified timing for Sequence Number (SN) Status Transfer, leading to potential packet delivery interruptions.
Innovation Solution
Implementing mechanisms to enable early acquisition of Sequence Number (SN) or Hyper Frame Number (HFN) information by the target node, either through source node freezing and notification or UE-based solutions, to facilitate immediate processing of pending PDCP PDUs without relying on SN Status Transfer, thereby reducing interruption delays.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If the UE continues UL transmission with the source NodeB until successful first PUSCH transmission toward the target NodeB, then the interruption time for UL transmission is reduced to 0 ms, but the timing for SN Status Transfer remains unspecified causing potential packet delivery interruptions in RLC-AM mode
Solution Approach 1:
The source NodeB is configured to freeze the UL COUNT at a specific point before handover and provide this frozen value to the target NodeB in advance. This preliminary action allows the target to have the necessary sequence information ready before the actual handover occurs, enabling immediate processing of PDCP PDUs without waiting for SN Status Transfer, thus resolving both the time loss and reliability issues
Solution Approach 2:
The frozen UL COUNT value acts as an intermediary information element that bridges the source and target NodeBs during handover. Instead of relying on the SN Status Transfer message which causes interruption, the frozen count value is transmitted separately and used by the target to immediately process packets, eliminating the dependency on timing-critical status transfer messages
2Reliability
If the target NodeB waits for SN Status Transfer from the source NodeB to process pending PDCP PDUs, then packet delivery reliability is maintained, but interruption delays occur in UL and DL transmissions
Solution Approach 1:
The source NodeB freezes the UL COUNT at a predetermined point and provides this frozen value to the target NodeB before the handover is executed. This preliminary provision of sequence information allows the target to immediately process pending PDCP PDUs upon receiving them from the UE, without needing to wait for the SN Status Transfer message, thereby eliminating interruption delays while maintaining reliability
Solution Approach 2:
The handover process is segmented into distinct phases: (1) source NodeB freezes UL COUNT and provides it to target in advance, (2) UE performs handover with continuous transmission, and (3) target NodeB uses the pre-provided frozen count to process packets immediately. This segmentation allows the sequence information transfer to occur independently from the packet data transfer, eliminating the bottleneck caused by waiting for SN Status Transfer
Data Source
AI summary
An apparatus of a Radio Access Network (RAN) node, a system, and a method. The apparatus includes one or more processors to, during a non-dual connectivity (non-DC) radio link control (RLC) acknowledged mode (AM) handover of a user equipment (UE) from a source RAN node to the RAN node: process a message including information on at least one of an uplink (UL) COUNT value, a downlink (DL) COUNT value, or a hyper frame number (HFN) corresponding to UL or DL packet data convergence protocol (PDCP) data units (DUs) communicated between the UE and the source RAN node; determine the at least one of the UL COUNT value, the DL COUNT value or the HFN from the message; and process the PDCP DUs based on the at least one of the UL COUNT value, the DL COUNT value.