UE Protocol Stack Fallback After DAPS Handover Failure
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Solution Overview
Problem
The challenge of implementing the Dual Active Protocol Stack (DAPS) mechanism in LTE and NR systems is exacerbated by radio link failures (RLF) occurring after successful handovers, leading to inconsistent UE and network states and potential RRC reconfiguration failures.
Innovation Solution
A method for user equipment (UE) to autonomously or network-instructed fallback from a DAPS state to a single active protocol stack when RLF occurs, involving protocol stack and configuration releases for the source base station.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If the DAPS mechanism is implemented to maintain connection to both source and target base stations simultaneously, then handover interruption time is reduced, but system complexity increases and radio link failure risk increases
Solution Approach 1:
The patent segments the protocol stack management by introducing separate state machines (handover state machine and radio link failure state machine) to handle different operational phases independently. This allows the system to maintain dual connections during handover while having dedicated mechanisms to manage the complexity of simultaneous source and target base station connections.
Solution Approach 2:
The patent implements preliminary actions by pre-configuring state machines and failure detection mechanisms before handover occurs. The radio link failure state machine is prepared in advance to detect and respond to link failures, and the handover state machine is pre-established to manage the transition process, reducing the complexity burden during actual handover execution.
2Reliability
If the UE maintains dual active protocol stacks during handover, then service continuity is improved, but the risk of radio link failure increases
Solution Approach 1:
The patent implements feedback mechanisms through the radio link failure state machine that continuously monitors the radio link status during DAPS handover. When link quality deteriorates or failure conditions are detected, the state machine provides feedback to trigger appropriate recovery actions, such as falling back to single active protocol stack or initiating re-establishment procedures, thereby managing the risk of radio link failure while maintaining service continuity.
Solution Approach 2:
The patent applies beforehand cushioning by preparing failure recovery mechanisms in advance. The radio link failure state machine and associated recovery procedures are established before handover occurs, creating a safety buffer that can absorb potential link failures during the dual-active period without causing service interruption.
3Speed
If the UE autonomously falls back to single active protocol stack upon detecting RLF, then recovery speed is improved, but state consistency between UE and network may be compromised
Solution Approach 1:
The patent implements self-service through the autonomous fallback mechanism where the UE's radio link failure state machine independently detects RLF conditions and triggers the fallback to single active protocol stack without waiting for network instruction. This self-service approach enables rapid recovery by immediately switching to a more robust single-connection mode when failure is detected.
Solution Approach 2:
The patent uses signaling messages as intermediaries to maintain state consistency between the autonomous UE action and the network. When the UE autonomously falls back, it sends appropriate signaling to the network to inform of the state change, ensuring both sides have consistent understanding of the current protocol stack configuration.
Data Source
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AI summary
The present disclosure provides a method for link failure recovery during a handover and corresponding user equipment. The method performed in the user equipment (UE) comprises: after a successful handover procedure of the UE configured with a dual active protocol stack, if a radio link failure (RLF) of a master cell group occurs at the UE, then in a radio resource control re-establishment procedure for connection recovery, or when an RLF occurs on a source link, performing, by the UE, an operation of falling back from the dual active protocol stack to a single active protocol stack, releasing a protocol stack and a configuration associated with a source base station; when the link quality of the source base station is good, releasing, by the UE, a protocol stack and a configuration associated with a target base station, resuming a single connectivity state to the source base station, and reporting the failure information to the base station.