LBT Contention Window Adjustment Using HARQ Feedback
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Solution Overview
Problem
In unlicensed frequency bands, the hidden node problem occurs due to interference from other nodes that are out of sensing range, leading to inefficient channel access and contention window size adjustments that do not accurately reflect transmission success or failure.
Innovation Solution
Adjusting the contention window size based on HARQ-ACK feedback, specifically considering the ratio of TB and CBG NACKs, to improve channel access efficiency and mitigate interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a device randomly selects a backoff value from a contention window for channel access, then channel access fairness is improved, but channel access efficiency deteriorates due to excessive backoff delays when the contention window is large
Solution Approach 1:
The contention window is divided into multiple sub-contention windows, each with its own backoff value. Instead of selecting one backoff value from a single large contention window, the device selects a sub-contention window and then selects a backoff value from that smaller subset. This segmentation reduces the average backoff delay while maintaining fairness through random selection within each segment.
Solution Approach 2:
The contention window size is dynamically adjusted based on channel conditions and traffic requirements. The system can switch between different contention window sizes or configurations, allowing it to adapt to changing conditions. This dynamic adjustment enables the system to optimize between fairness and efficiency depending on the current operational context.
2Reliability
If a device waits for a random backoff value before accessing the channel, then collision avoidance is improved, but access delay increases
Solution Approach 1:
By segmenting the contention window into multiple sub-contention windows with smaller sizes, the random backoff delay is reduced while still providing collision avoidance. The segmentation creates multiple smaller pools from which backoff values are drawn, reducing the expected wait time while maintaining the randomization benefit for collision prevention.
Solution Approach 2:
The system applies partial backoff by using sub-contention windows that represent only a portion of the full contention window range. This partial action provides sufficient collision avoidance for practical purposes while avoiding the excessive delays that would result from using the full contention window size in all conditions.
3Reliability
If a large contention window is used for channel access, then collision probability is reduced, but channel access efficiency deteriorates due to increased backoff delays
Solution Approach 1:
The large contention window is segmented into multiple sub-contention windows. Each sub-contention window maintains a smaller size that provides adequate collision protection for its subset of devices, while the collection of segments effectively covers the full range needed for fairness. This allows the system to achieve collision avoidance benefits without the efficiency penalty of a single large window.
Solution Approach 2:
Different sub-contention windows can be tailored to different local conditions or device groups. Each segment can be optimized for its specific context, providing appropriate collision protection locally while maintaining overall system efficiency. This local optimization allows the system to avoid the one-size-fits-all inefficiency of a uniformly large contention window.
Data Source
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AI summary
Method and apparatuses for communication over an unlicensed band are provided herein. A method may comprise transmitting transport blocks in each slot of a plurality of slots and determining a Hybrid Automatic Repeat Request (HARQ) status for each of the transmitted transport blocks. The method may further comprise determining a weighting coefficient for each of the determined HARQ statuses and calculating, based on the determined HARQ statuses and weighting coefficients, a weighted average of the HARQ statuses for the transmitted transport blocks. The method may further comprise adjusting a Listen-Before-Talk (LBT) contention window size based on the calculated weighted average of the HARQ statuses.