Timing Advance Acquisition in Layer 1/2 Mobility
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Solution Overview
Problem
In wireless communications, layer 1/layer 2-triggered mobility (LTM) systems face challenges in acquiring timing advance (TA) information before a user equipment (UE) switches to a candidate target cell without performing a random access channel (RACH) procedure, as the UE to which the TA information corresponds cannot be identified at the candidate target cell.
Innovation Solution
The described techniques involve coordinating between distributed units (DUs) and a central unit (CU) to allocate unique RACH preambles for TA information acquisition, allowing the DU associated with the serving cell to identify and forward the TA information to the UE before switching, or by using UE-specific RACH preambles to uniquely identify the UE at the candidate target cell.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If TA information is acquired before UE switches to candidate target cell to enable switching without RACH procedure, then switching latency is reduced, but identification of which UE TA information corresponds may be unsupported
Solution Approach 1:
The patent applies preliminary action by having the UE transmit a RACH message to the candidate target cell before the actual cell switch occurs. This allows the candidate target cell to measure and determine TA information in advance. The TA information is then forwarded to the serving cell and provided to the UE before switching, enabling the UE to skip the RACH procedure during handover and reducing switching latency.
Solution Approach 2:
The patent uses the serving cell as an intermediary to resolve the identification problem. The serving cell receives TA information from the candidate target cell, associates it with the specific UE based on the RACH message transmission, and then forwards the identified TA information to the UE. This intermediary role ensures that even though the candidate target cell measures the TA, the correct UE-specific TA information is accurately delivered to the right UE.
2Reliability
If RACH procedure is performed during cell switching, then TA information can be acquired, but switching latency increases
Solution Approach 1:
The patent performs the TA information acquisition in advance by having the UE send a RACH message to the candidate target cell before the handover. The candidate target cell measures this message and determines the TA information early. This preliminary acquisition eliminates the need for a RACH procedure during the actual cell switch, thereby reducing switching latency while maintaining reliable TA information acquisition.
3Measurement precision
If UE-specific RACH preambles are allocated for TA information acquisition, then UE identification at candidate target cell is enabled, but coordination complexity between DUs and CU increases
Solution Approach 1:
The patent segments the RACH preamble resources by allocating specific preambles to specific DUs. This segmentation allows the candidate target cell to identify which DU the RACH message originated from, and consequently identify the associated UE. By dividing the preamble space into DU-specific subsets, the system enables precise UE identification while managing coordination complexity through structured resource allocation.
Data Source
AI summary
Methods, systems, and devices for wireless communications are described. A user equipment (UE) may transmit a random access channel (RACH) message to a first distributed unit (DU) via a cell supported by the first DU to support acquisition of timing advance (TA) information prior to switching to the cell. A second DU serving the UE may identify and indicate the TA information to the UE based on the UE transmitting the RACH message indicating a RACH preamble allocated to the second DU by a central unit (CU). The RACH preamble may be included in a set of preambles indicated to the CU by the first DU for TA information acquisition. Alternatively, the UE may be identified by the first DU based on a UE-specific RACH preamble, a RACH preamble, RACH occasion (RO) pair, or a request by the second DU to order the UE to transmit the RACH message.