CU-DU Signaling for Reliable L1/L2 Inter-Cell Mobility
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Solution Overview
Problem
The current 3GPP specifications for L1/L2-based inter-cell mobility in Release 18 are unclear regarding preparation procedures, reliability enhancements, and internal UE behavior, particularly in inter-DU scenarios, and lack detailed signaling mechanisms for intra-DU and inter-DU transitions.
Innovation Solution
The proposed solution involves a central unit (CU) and distributed units (DUs) exchanging messages to prepare for L1/L2-based inter-cell mobility, utilizing multi-TRP operations to enhance reliability, and defining the internal behavior of the UE, including RRC and lower layers' interactions during cell transitions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If L1/L2-based inter-cell mobility is implemented without clear preparation procedures, then mobility speed is improved, but reliability deteriorates due to uncertainty in inter-DU scenarios
Solution Approach 1:
The patent implements preliminary preparation procedures where the source DU sends a first message to the target DU before actual mobility execution. This advance notification allows the target DU to prepare resources and configure parameters, ensuring reliable mobility execution while maintaining fast switching capability. The preparation phase separates resource allocation from execution, resolving the contradiction between speed and reliability.
Solution Approach 2:
The patent establishes a feedback mechanism where the target DU sends a second message back to the source DU confirming preparation status. This feedback loop ensures both DUs have synchronized information about mobility parameters, TRP configurations, and resource allocations, thereby improving reliability without sacrificing the fast mobility speed enabled by L1/L2-based procedures.
2Reliability
If detailed signaling mechanisms are added for intra-DU and inter-DU transitions, then reliability is improved, but device complexity increases
Solution Approach 1:
The patent segments the mobility signaling into distinct phases: preparation phase (first message from source DU to target DU) and execution phase (actual cell transition). This segmentation allows detailed signaling to be concentrated where needed for reliability, while keeping the overall procedure simple and efficient. The first message carries necessary preparation information, and the second message confirms readiness, providing reliability without requiring complex continuous signaling.
3Reliability
If multi-TRP operations are utilized for reliability enhancement, then mobility reliability is improved, but overhead increases
Solution Approach 1:
The patent makes the multi-TRP operation universal by enabling the source DU to indicate in the first message whether multi-TRP should be used during mobility. This single indication parameter controls the activation of multi-TRP operations, allowing reliability enhancement only when needed. The target DU then configures appropriate TRP resources based on this indication, providing reliability improvement without requiring continuous multi-TRP overhead in all mobility scenarios.
Data Source
AI summary
A central unit (CU) (301) receives from a first distributed unit (DU) (302) a first message related to preparation for Layer-1 or Layer-2 (L1/L2) based inter-cell mobility of a UE. The L1/L2-based inter-cell mobility is intra-DU L1/L2-based inter-cell mobility within the first DU (302) or inter-DU L1/L2-based inter-cell mobility from the first DU (302) to a second DU. After receiving the first message, the CU (301) sends to the first DU (302) a second message related to the preparation for the L1/L2-based inter-cell mobility. This helps, for example, to enable the source distributed unit (DU) and the central unit (CU) to prepare for L1/L2 based inter-cell mobility.