Partial Decoding for Early HARQ Feedback in LTE
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Solution Overview
Problem
LTE HARQ systems face latency issues when transmitting small packets and struggle with link adaptation, especially in critical machine type communication (C-MTC) with extreme latency and reliability requirements, due to rare error events leading to slow outer-loop adjustments.
Innovation Solution
Facilitating early HARQ feedback by allowing radio receivers to decode and provide feedback on a first portion of the transmission before the end of the transmission unit, enabling zero delay retransmissions and improving link adaptation through partial decoding attempts and likelihood-based feedback.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the receiver waits for the complete transmission unit before decoding and providing HARQ feedback, then the feedback is based on the full transmission improving reliability, but the latency increases significantly
Solution Approach 1:
The receiver performs preliminary decoding on a first portion of the transmission before the transmission unit ends, generating early HARQ feedback. This preliminary action allows the feedback to be sent earlier (reducing latency) while still maintaining reliability because the feedback is based on actual decoding of received bits, even if partial
Solution Approach 2:
The transmission is effectively segmented into a first portion (decoded for early feedback) and a second portion (received but not yet decoded). This segmentation allows the system to provide feedback based on the first portion while still receiving the complete transmission, resolving the contradiction between early feedback and full transmission reliability
2Reliability
If the system uses traditional HARQ with waiting for complete transmission before feedback, then outer-loop link adaptation can adjust to radio conditions, but the adjustment is slow due to rare error events at low BLER targets
Solution Approach 1:
By performing preliminary decoding and generating HARQ feedback earlier in the transmission interval, the system creates more frequent feedback opportunities. This increases the number of feedback events available for outer-loop link adaptation, allowing faster adjustment to radio conditions even at low BLER targets like 10^-6 where error events are rare
Solution Approach 2:
The early HARQ feedback based on partial decoding provides additional feedback information to the transmitter. This enhanced feedback mechanism gives the outer-loop link adaptation more data points to work with, improving its ability to accurately track and adjust to radio conditions without waiting for complete transmission failures
3Loss of time
If the receiver decodes only a portion of the transmission before the transmission unit ends, then HARQ feedback is provided earlier reducing latency, but the feedback is based on incomplete data potentially reducing accuracy
Solution Approach 1:
The receiver performs partial decoding on the first portion of the transmission to generate early HARQ feedback. This partial action is sufficient to provide useful feedback information early in the transmission interval, achieving the goal of reduced latency while maintaining acceptable feedback accuracy for HARQ purposes
Data Source
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AI summary
According to some embodiments, a method in a radio network element of providing hybrid automatic repeat request (HARQ) feedback comprises receiving, from a wireless transmitter, a first set of coded bits. The first set of coded bits comprises a partial subset of coded bits to be received during a transmission time interval (TTI). The radio network element decodes the first set of coded bits; determines a first decoding indicator based on the success or failure of the decoding of the first set of coded bits; and communicates the first decoding indicator to the wireless transmitter. Particular embodiments communicate the first decoding indicator to the wireless transmitter within four TTI of receiving the first set of coded bits. Particular embodiments estimate a decoding indicator for the entire TTI based on the success or failure of the decoding of the first set of coded bits.