RACH-Less LTM Execution for Serving Candidate Cells

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Solution Overview

Problem

Existing 3GPP Rel-18 specifications do not provide optimizations for Layer 1/Layer 2 Triggered Mobility (LTM) when the LTM candidate cell is a currently configured serving cell, leading to unclear procedures for Timing Advance (TA) acquisition and potential RACH-based LTM execution, especially in Conditional LTM scenarios.

Innovation Solution

The UE determines to perform a RACH-less LTM execution procedure to an LTM candidate cell that is also a current serving cell, utilizing existing UL synchronization and pre-configured grants to transmit UL control messages and apply the LTM candidate cell configuration without TA acquisition.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If the UE performs RACH-based LTM cell switch when no valid TA value is available, then the LTM execution is more reliable, but the mobility interruption time increases

Engineering Contradiction:

ImproveLTM execution reliabilityVSAvoidmobility interruption time

Core Design Contradiction:

ReliabilityVSLoss of time

Solution Approach 1:

The network performs early TA acquisition for LTM candidate cells before triggering LTM, storing the TA values in advance. When LTM is triggered, if a valid TA value is available from early acquisition, the UE can perform RACH-less LTM, avoiding the time-consuming RACH procedure and reducing mobility interruption time while maintaining reliability

Inventive Principle:

Principle #10Preliminary action

2Loss of time

If the UE performs RACH-less LTM using pre-configured grants, then the mobility interruption time is reduced, but the procedure becomes unclear when LTM candidate cell is a current serving cell

Engineering Contradiction:

Improvemobility interruption timeVSAvoidprocedure clarity

Core Design Contradiction:

Loss of timeVSEase of operation

Solution Approach 1:

The patent introduces a new parameter 'ltm-CandidateCellList' in the LTM MAC CE that explicitly indicates which LTM candidate cells are current serving cells. This clear parameter definition resolves the procedural ambiguity, allowing the UE to correctly identify when to use pre-configured grants for RACH-less LTM even when the target cell is a current serving cell, thereby maintaining fast execution while improving procedure clarity

Inventive Principle:

Principle #35Parameter changes

3Productivity

If early TA acquisition is performed for all LTM candidate cells, then the LTM execution speed is improved, but the network signaling overhead increases

Engineering Contradiction:

ImproveLTM execution speedVSAvoidnetwork signaling overhead

Core Design Contradiction:

ProductivityVSQuantity of substance

Solution Approach 1:

Instead of performing early TA acquisition for all LTM candidate cells, the network performs early TA acquisition only for specific candidate cells that are likely to be selected, identified through the 'l tm-CandidateCellList' parameter. This partial action approach maintains fast LTM execution speed for the most probable target cells while reducing unnecessary TA acquisition and signaling overhead for other candidate cells

Inventive Principle:

Principle #16Partial or excessive action

Data Source

PatentUS20260006504A1RACH-less LTM execution when LTM candidate cell is a current serving cell

Publication Date: 2026.01.01 TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)

AI summary

A radio access network node, a user equipment, UE, and a method of the UE are disclosed. The method which is performed by the UE comprises performing a Random Access Channel, RACH,-less Layer 1/Layer 2 Triggered Mobility, LTM, execution procedure to an LTM candidate cell when the LTM candidate cell is also a current serving cell with which the UE is configured. The performing of the RACH-less LTM execution comprises sending a Scheduling Request, SR, over Physical Uplink Control Channel, PUCCH, on the LTM candidate cell.