HARQ-ACK Feedback Transmission in NR-U Unlicensed Spectrum
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Solution Overview
Problem
The challenge in 5G New Radio (NR) systems operating in unlicensed spectrum is the uncertainty in transmitting Hybrid Automatic Repeat Request (HARQ) acknowledgement (ACK) feedback due to the listen-before-talk (LBT) mechanism and potential interference from hidden nodes, leading to unreliable HARQ-ACK feedback and unnecessary retransmissions.
Innovation Solution
The solution involves enhancing the HARQ timing and scheduling procedures by using a modified Downlink Control Indicator (DCI) format with additional fields such as a bitmap of triggered HARQ processes and a PUCCH resource indicator to ensure reliable HARQ-ACK feedback transmission, even in the absence of scheduled downlink data, and incorporating a unique RNTI masked with cyclic redundancy check (CRC) for identification.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If LBT mechanism is used in unlicensed spectrum, then channel access is achieved, but HARQ-ACK feedback reliability deteriorates due to uncertainty and hidden node interference
Solution Approach 1:
The gNB performs preliminary actions by acquiring COT and sending triggering DCI in advance to indicate pending HARQ feedback before actual downlink data transmission occurs. This allows the UE to prepare for HARQ-ACK feedback transmission proactively, reducing uncertainty caused by LBT mechanisms and hidden node interference.
Solution Approach 2:
The patent implements a feedback mechanism where the gNB sends triggering DCI with bitmap indicating pending HARQ processes, and the UE responds with HARQ-ACK feedback in subsequent COT. This closed-loop feedback system ensures reliable HARQ-ACK transmission despite LBT uncertainty by continuously monitoring and adjusting based on channel conditions.
2Reliability
If separate COT is acquired for HARQ-ACK transmission, then feedback reliability is improved, but time delay increases
Solution Approach 1:
The patent merges the HARQ-ACK feedback transmission with the gNB's COT structure. Instead of requiring completely separate COT acquisition, the UE transmits HARQ-ACK feedback within the gNB's acquired COT window, reducing time delay while maintaining reliability through coordinated resource allocation indicated by triggering DCI.
Solution Approach 2:
The system dynamically adjusts HARQ-ACK feedback timing based on COT duration and channel conditions. The triggering DCI provides flexible timing indications allowing the UE to transmit feedback at optimal moments within the COT, balancing reliability requirements with time delay constraints according to real-time channel state.
3Measurement precision
If triggering DCI with bitmap is used to indicate pending HARQ feedback, then feedback accuracy is improved, but signaling overhead increases
Solution Approach 1:
The triggering DCI uses segmented bitmap representation where each bit corresponds to a specific HARQ process. This segmentation allows precise indication of pending feedback for individual processes while keeping the overall signaling overhead manageable by only transmitting bits for processes that require feedback, rather than transmitting complete HARQ state information for all processes.
Data Source
AI summary
An approach is described for a user equipment (UE) for transmission of hybrid automatic repeat request-acknowledgement (HARQ-ACK) feedback in communication with a next-generation NodeB (gNB) using new radio unlicensed (NR-U) in an unlicensed spectrum. The UE receives downlink (DL) data in a first channel occupancy time (COT). The UE also receives a triggering downlink control indicator (DCI) during a second COT, the triggering DCI providing an indication of a HARQ feedback that is pending for a corresponding HARQ process, and the second COT carrying no scheduled DL data. In addition, the UE receives a triggering downlink control indicator (DCI) during a second COT, the triggering DCI providing an indication of a HARQ feedback that is pending for a corresponding HARQ process, and the second COT carrying no scheduled DL data. The UE transmits the HARQ feedback based on the triggering DCI.