HARQ-ACK Rescheduling on Unlicensed Spectrum Under LBT Failure
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Solution Overview
Problem
HARQ-ACK feedback transmission on unlicensed spectrum in NR-U is prone to LBT failures and hidden node interference, leading to unnecessary DL retransmissions and performance degradation due to failed reception at the gNB.
Innovation Solution
A method and apparatus for triggering HARQ-ACK feedback by using DCI to indicate suspension or rescheduling of HARQ-ACK transmission, including indicators for HARQ timing, PUCCH resources, and LBT types, allowing for efficient HARQ-ACK codebook management.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If HARQ-ACK feedback is transmitted on unlicensed carrier, then NR-U deployment requirements are met, but LBT failure and hidden node interference cause transmission reliability to deteriorate
Solution Approach 1:
Instead of having the UE autonomously determine whether to transmit HARQ-ACK feedback based on LBT outcome, the invention inverts the control by having the gNB explicitly indicate via DCI whether the UE should suspend or transmit the HARQ-ACK feedback. This centralized control resolves the reliability issue by preventing transmissions that would fail due to LBT or hidden node problems.
Solution Approach 2:
The gNB provides feedback to the UE through DCI indicators (suspend indicator, counter DAI, total DAI) that inform the UE whether to transmit or suspend HARQ-ACK feedback. This feedback mechanism allows the gNB to adapt to channel conditions and prevent unnecessary transmissions that would waste resources or fail due to LBT/interference issues.
2Loss of information
If UE transmits HARQ-ACK feedback on unlicensed carrier, then feedback is provided to gNB, but unnecessary DL retransmissions occur due to LBT failure or hidden node problem
Solution Approach 1:
The gNB performs preliminary action by sending DCI with suspend indicator before the UE transmits HARQ-ACK feedback. This allows the gNB to preemptively prevent transmissions that would fail due to LBT or hidden node interference, avoiding the waste of both uplink feedback and downlink retransmission resources.
Solution Approach 2:
The DCI mechanism provides feedback from gNB to UE about whether transmission should occur. By using counter DAI and total DAI, the gNB can precisely control which HARQ-ACK feedback should be transmitted, preventing unnecessary retransmissions while ensuring needed feedback is delivered.
3Reliability
If gNB assumes NACK when HARQ-ACK feedback is not detected, then decoding failure is handled, but DL performance degrades due to unnecessary retransmission
Solution Approach 1:
The DCI mechanism provides explicit feedback from gNB to UE about transmission status. When gNB sends DCI with suspend indicator, the UE knows not to transmit, preventing the gNB from having to assume NACK and retransmit. This eliminates unnecessary retransmissions while maintaining decoding reliability.
Solution Approach 2:
The system uses self-service by having the gNB proactively manage the HARQ-ACK feedback transmission process through DCI indicators. Instead of waiting for potential transmission failures and then retransmitting, the gNB actively prevents failures by controlling when UE should transmit, improving overall system efficiency.
Data Source
AI summary
Methods and apparatuses for triggering HARQ-ACK feedback. A method comprising: receiving a first Downlink Control Information (DCI) from a base unit for scheduling a Downlink (DL) transmission, wherein the first DCI includes a first indicator indicating to suspend transmission of a Hybrid Automatic Repeat Request Acknowledgement (HARQ-ACK) feedback corresponding to the DL transmission; receiving a second DCI from the base unit for scheduling the transmission of the HARQ-ACK feedback, wherein the second DCI indicates a resource for the transmission of the HARQ-ACK feedback; and transmitting the HARQ-ACK feedback to the base unit on the resource indicated by the second DCI.