WLAN HARQ Retransmission Using Adaptive MCS Combining
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Solution Overview
Problem
Existing Hybrid Automatic Repeat Request (HARQ) schemes in Wireless Local-Area Networks (WLANs) often fail to provide performance gains when retransmissions use the same Modulation and Coding Scheme (MCS) as the original transmission, especially in scenarios with inaccurate channel knowledge or changing conditions, leading to high likelihood of retransmission failures and degraded system performance.
Innovation Solution
The method involves generating retransmissions using a lower Modulation and Coding Scheme (MCS) with better error performance than the original transmission, combining soft-decoding metrics from both transmissions, and applying Forward Error Correction (FEC) decoding to reconstruct data, which increases robustness and reduces the need for additional diversity measures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the same MCS is used for retransmissions as the original transmission, then spectral efficiency is maintained, but the likelihood of decoding failure increases when channel conditions change or initial MCS selection is inaccurate
Solution Approach 1:
The system dynamically adapts the MCS for retransmissions based on channel conditions and initial transmission outcomes. When the original transmission fails to decode, the receiver requests a retransmission with a more robust (lower order) MCS, allowing the system to adjust its parameters in response to actual channel performance rather than using a static MCS for all transmissions.
Solution Approach 2:
The invention changes the MCS parameters (modulation order and coding rate) for retransmissions compared to the original transmission. Specifically, it uses a lower order modulation scheme (e.g., QPSK instead of 16-QAM) and potentially a lower coding rate for retransmissions, which improves error performance and decoding reliability when the initial transmission fails.
2Reliability
If a more robust MCS is used for retransmissions, then error performance improves and decoding success rate increases, but bandwidth overhead increases
Solution Approach 1:
The system applies a more robust MCS selectively - only when the original transmission fails to decode. The receiver monitors decoding success and only requests retransmissions with more robust MCS when needed, rather than always using the more robust scheme. This partial application of the robust MCS minimizes bandwidth overhead while still improving reliability when required.
Solution Approach 2:
The invention implements a feedback mechanism where the receiver monitors the success of the original transmission decoding and provides feedback (ACK/NACK) to the transmitter. Based on this feedback, the transmitter decides whether to retransmit with the same MCS or with a more robust MCS, allowing the system to adapt to actual channel conditions and avoid unnecessary bandwidth overhead.
Data Source
AI summary
A method for receiving a WLAN data transmission includes receiving, at a first WLAN device from a remote WLAN device, a WLAN transmission including first bits that are modulated and encoded using a first MCS associated with a first modulation scheme and a first FEC coding rate. First soft-decoding metrics are computed for the first bits. A WLAN retransmission corresponding to the WLAN transmission is received. The WLAN retransmission includes second bits that are modulated and encoded using a second MCS associated with a second modulation scheme and a second FEC coding rate, the second MCS having better error performance than the first MCS. Second soft-decoding metrics are computed for the second bits. The first soft-decoding metrics and the second soft-decoding metrics are combined, to produce combined soft-decoding metrics. Data carried by the WLAN transmission is reconstructed by applying FEC decoding to the combined soft-decoding metrics.