UE Uplink Time Alignment Timer Management for Small Data Grants
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Solution Overview
Problem
In wireless communication systems, user equipment (UE) operating in RRC_INACTIVE or RRC_IDLE states face challenges in managing uplink time alignment when transmitting small data using configured grants, particularly when receiving a time alignment command (TAC) with a cell network temporary identifier (C-RNTI) or preconfigured uplink resources RNTI, as the MAC layer is unclear about which timer to start or restart.
Innovation Solution
The UE initiates a small data transmission (SDT) procedure, starting a first timer upon initiating SDT, and upon receiving a TAC during this timer's run, starts or restarts a second timer, referred to as SDT-TAT, to manage the use of configured grants for transmitting small data, based on specific conditions such as the type of MAC control element, search space, and random access procedure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the UE uses a C-RNTI to transmit time alignment command, then the base station can identify the UE, but the MAC layer cannot determine which timer to start or restart
Solution Approach 1:
The patent introduces a new intermediary element - a dedicated MAC control element (MAC CE) - that acts as a mediator between the base station's time alignment command and the UE's timer management. This MAC CE explicitly indicates whether the UE should start or restart the SDT-TAT timer, eliminating the ambiguity in timer selection without requiring complex decision logic in the MAC layer.
Solution Approach 2:
The patent segments the time alignment command into distinct components: a dedicated MAC control element that carries the timing alignment information and an explicit indicator field that specifies the timer action. This segmentation allows the UE to process the command systematically by first identifying the MAC CE type and then following the indicated timer management action, simplifying the decision process.
2Reliability
If the UE starts multiple timers for different transmission scenarios, then the UE can manage time alignment accurately, but the UE cannot determine which timer to use when receiving TAC
Solution Approach 1:
The patent implements feedback by having the base station explicitly indicate the intended timer action through the MAC control element. The indicator field in the MAC CE provides direct feedback to the UE about whether to start or restart the SDT-TAT timer, eliminating the need for the UE to guess or complex decision-making. This feedback mechanism ensures reliable timer management while making operation simple and deterministic.
Data Source
AI summary
To maintain uplink time alignment, a UE initiates a small data transmission (SDT) procedure; the starts a first timer in response to the initiating; the UE receives, while the first timer is running, a time alignment command (TAC); and in response to receiving the TAC, the UE starts or restarts a second timer that controls a use of a configured grant for transmitting small data.