LTM Timer Supervised Cell Switch for 5G NR Mobility
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Solution Overview
Problem
Current 5G NR network handovers during UE mobility are inefficient, leading to longer latency, higher overhead, and longer interruption times due to complete L2 and L3 resets, and there is a need to balance efficiency and success rate in LTM handover procedures.
Innovation Solution
Implementing an LTM timer to control the handover procedure, allowing UE to switch between cells using L1/L2 signaling, with explicit or implicit success indications and a recovery procedure triggered when the timer expires, including RRC re-establishment and random access attempts to candidate cells.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional L3-based handover with complete L2/L1 resets is performed, then handover reliability is improved, but latency and interruption time increase
Solution Approach 1:
The handover procedure is segmented into two distinct phases: LTM handover (L1/L2-based) for fast execution, and a separate recovery procedure (RRC re-establishment) for reliability assurance. This segmentation allows the system to benefit from both fast handover and reliable connection restoration without the trade-off present in conventional approaches.
Solution Approach 2:
The recovery procedure is prepared in advance with pre-configured parameters and candidate cells. When LTM handover fails, the UE can immediately execute the pre-prepared recovery procedure without additional signaling delays, thus reducing overall handover latency while maintaining reliability.
2Loss of time
If L1/L2-based inter-cell mobility with beam management is implemented, then latency and overhead are reduced, but handover success rate may be compromised
Solution Approach 1:
The system implements feedback mechanisms where the UE monitors LTM handover execution and provides success/failure indications to the network. This feedback enables the network to detect failures and trigger appropriate recovery actions, ensuring high success rate despite using fast L1/L2-based handover.
Solution Approach 2:
The system prepares recovery procedures in advance as a cushion against potential LTM handover failures. By pre-configuring recovery parameters and candidate cells before handover execution, the system ensures that failure recovery can proceed smoothly without compromising the speed advantage of L1/L2-based handover.
3Reliability
If complete L2 and L1 resets are performed during serving cell change, then connection reliability is improved, but device complexity and processing overhead increase
Solution Approach 1:
The patent extracts the reset operation from the mandatory handover procedure. Instead of always performing complete L2/L1 resets, the system selectively applies resets only when needed (i.e., when LTM handover fails and recovery is required), thereby reducing processing overhead while maintaining connection reliability.
Solution Approach 2:
The system applies partial action by performing L1/L2-based handover without complete resets when conditions permit, and only performs full recovery procedures with necessary resets when handover failure is detected. This partial application of reset operations reduces overall processing overhead while maintaining reliability where needed.
Data Source
AI summary
Apparatus and methods are provided for supervised L1/L2-triggered mobility (LTM) cell switch. In one novel aspect, supervised LTM is performed by the UEs. In one embodiment, an LTM timer is preconfigured for the supervised LTM procedure. The UE starts the LTM timer upon receiving the cell switch command and monitors the process with the LTM timer. When the LTM timer expires and the UE fails to switch to a target cell, a recovery procedure is triggered. In one embodiment, the UE sends LTM procedure success indication explicitly through a new RRC message, a MAC CE, or DCI; or implicitly indicated by a HARQ ACK for a first downlink transmission, a first PUSCH transmission, or a first UL transmission through a PUSCH/PUCCH. In one embodiment, the recovery procedure performs an RRC re-establishment, or performs a failure recovery before the RRC re-establishment.