MIMO Rank Reduction for Downlink Throughput
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Solution Overview
Problem
Inaccurate Rank Indicators (RI) reported by wireless devices in MIMO systems can lead to reduced downlink throughput due to factors like abrupt channel condition changes, device-specific algorithms, inter-cell interference, and antenna correlation, causing radio access nodes to transmit more data than channel conditions support.
Innovation Solution
Implementing a method for radio access nodes to determine imbalances between parallel channels and high Negative Acknowledgement (NACK) rates, allowing for a fast rank reduction in downlink transmissions, which includes adjusting transmit power and using transmit diversity or precoding matrix indicators to improve throughput.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the radio access node transmits data based on the reported RI, then the downlink throughput is maximized under ideal conditions, but the throughput decreases when the RI is inaccurate due to channel changes, interference, or antenna correlation
Solution Approach 1:
The patent uses NACK rate feedback from the wireless device to detect RI inaccuracy. When the NACK rate exceeds a threshold, the system infers that the reported RI is inaccurate and triggers a rank reduction. This feedback mechanism allows the system to adapt to changing channel conditions and interference levels, resolving the contradiction between maximizing throughput and ensuring reliable transmission.
Solution Approach 2:
The patent dynamically changes the transmission rank parameter based on detected channel conditions. When RI inaccuracy is detected through NACK rate monitoring or channel imbalance detection, the system reduces the rank from the reported value. This parameter change allows the system to maintain high throughput when conditions are good while ensuring reliability when conditions deteriorate.
2Reliability
If the rank is reduced to compensate for inaccurate RI, then the downlink throughput is improved under poor channel conditions, but the throughput is reduced when the channel conditions actually support higher rank transmission
Solution Approach 1:
The patent implements a dynamic rank adaptation mechanism that adjusts the transmission rank based on real-time channel conditions. The system monitors NACK rates and channel imbalances continuously, reducing rank only when evidence of RI inaccuracy is detected. This dynamic approach allows the system to maintain high throughput when conditions support it while ensuring reliability when conditions deteriorate, avoiding the static limitation of always using reduced rank.
Solution Approach 2:
The system uses NACK rate feedback to determine when rank reduction is necessary. By monitoring whether NACK rates exceed thresholds, the system can distinguish between cases where rank reduction is needed for reliability versus cases where high rank should be maintained for throughput. This feedback-based decision-making resolves the contradiction by adapting the rank to actual channel conditions.
3Measurement precision
If RI reporting is done frequently to capture channel changes, then the accuracy of RI is improved, but the uplink resources consumed by reporting increase
Solution Approach 1:
The patent performs preliminary detection of RI inaccuracy using downlink measurements (NACK rates, channel imbalances) before actual transmission occurs. By detecting potential RI inaccuracies in advance through downlink signal quality measurements, the system avoids the need for frequent uplink RI reporting while still maintaining accurate rank selection. This preliminary action in the downlink direction resolves the resource trade-off.
Solution Approach 2:
The system uses NACK rate and channel imbalance measurements as intermediary indicators to detect RI inaccuracy. Instead of relying directly on frequent RI reports, these intermediary measurements serve as proxies that indicate when RI is likely inaccurate. This intermediary approach allows the system to detect RI issues without requiring frequent uplink reporting resources.
Data Source
AI summary
Systems and methods for Multiple-Input and Multiple-Output (MIMO) rank reduction to improve downlink throughput are disclosed. A method of operation of a radio access node includes determining that an imbalance between parallel channels of a spatial multiplexing downlink transmission to a wireless device is greater than an imbalance threshold and/or that a Negative Acknowledgement (NACK) rate over time for the parallel channels of a spatial multiplexing downlink transmission reported by the wireless device is greater than a NACK rate threshold. The method also includes, in response to determining that the imbalance between the parallel channels is greater than the imbalance threshold and/or that the NACK rate is greater than the NACK rate threshold, performing a fast rank reduction for a next downlink transmission whereby a rank is reduced from a rank indicator reported by the wireless device to some lower rank. Reducing the rank may improve downlink throughput.