Downlink Retransmission via CBG ACK/NACK Feedback Indication
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Solution Overview
Problem
In 5G New Radio (NR) designs, the lack of sufficient CRC protection for Code Block Group (CBG) level ACK/NACK feedback leads to errors in receiving feedback at the gNB, resulting in incorrect retransmission of CBGs, which can cause decoding failures due to mismatched Log Likelihood Ratios (LLRs) from incorrect feedback decoding.
Innovation Solution
The gNB sends an indication to the UE regarding the correct receipt of CBG ACK/NACK feedback, allowing the UE to process retransmissions based on this indication, ensuring that only correctly decoded CBGs are combined for decoding, and preventing the use of incorrectly decoded CBGs by comparing the received and transmitted bitmaps.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If CBG level ACK/NACK feedback is used for downlink retransmission, then retransmission efficiency is improved, but feedback reliability deteriorates due to insufficient CRC protection
Solution Approach 1:
The patent applies preliminary action by having the gNB proactively send an indication to the UE about whether the CBG ACK/NACK feedback was correctly received, before the UE processes the retransmission. This allows the UE to prepare appropriately - either to combine LLRs for correct feedback or to discard them for incorrect feedback - thereby resolving the contradiction between maintaining retransmission efficiency and ensuring feedback reliability
Solution Approach 2:
The patent implements a feedback mechanism where the gNB sends back an indication to the UE regarding the correctness of received CBG ACK/NACK feedback. This feedback loop enables the UE to adjust its retransmission processing accordingly, ensuring that retransmission efficiency is maintained when feedback is correct while preventing decoding failures when feedback is incorrect
2Speed
If the gNB assumes correct decoding of CBG ACK/NACK feedback, then processing speed is improved, but decoding accuracy deteriorates due to error events
Solution Approach 1:
The patent applies self-service by enabling the UE to autonomously determine whether to combine or discard LLRs based on the indication received from the gNB. The UE independently assesses the reliability of the feedback and adjusts its decoding process accordingly, eliminating the need for the gNB to assume correct decoding while maintaining processing speed
Solution Approach 2:
The patent changes the parameter of LLR handling based on the feedback reliability indication. When the gNB correctly receives and decodes the CBG ACK/NACK feedback, the UE combines LLRs to improve decoding accuracy. When decoding is incorrect, the UE discards the LLRs to prevent errors. This dynamic parameter change resolves the contradiction between processing speed and decoding accuracy
Data Source
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AI summary
Methods and Apparatus are provided for downlink retransmission of Code Block Groups (CBGs) when CBG level ACK and NACK feedback is unreliable. A User Equipment (UE) transmits to a Base Station (BS) feedback indicating an Acknowledgement (ACK) or a Negative Acknowledgement (NACK) corresponding to each of received set of CBGs transmitted by the BS to the UE. The BS receives and decodes the feedback and transmits back to the UE information regarding a result of the decoding. The UE, based on the received information, determines whether the BS correctly received and decoded the ACK/NACK feedback, and in some cases, ACKs and NACKs corresponding to which CBGs were incorrectly decoded by the BS. The UE processes retransmitted CBGs received from the BS based on this determination.