AAS Transmitter Clipping Control for Beamformed EVM Stability
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Solution Overview
Problem
Active Antenna Systems (AAS) face challenges with transmitter-induced distortion due to clipping, which causes spatial fluctuations in signal quality, leading to poor EVM (Error Vector Magnitude) in some areas and increased implementation costs, especially when beamforming is involved.
Innovation Solution
Adjusting the amount of clipping and/or clipping threshold individually for each radio transmitter in an AAS system to ensure the ratio of clipping noise to the wanted signal is consistent or below a threshold, thereby matching the spatial pattern of clipping noise with the wanted signal, and optimizing weight tapering to maximize SINR and minimize EVM.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If clipping is applied to limit peak power at each radio transmitter, then peak to average power ratio is reduced and power amplifier design becomes more economic, but transmitter induced distortion increases and spatial fluctuation in signal quality occurs
Solution Approach 1:
The patent applies different clipping thresholds to different radio transmitters based on their individual signal characteristics and beamforming weights. Each transmitter's clipping level is locally optimized rather than using a uniform threshold across all transmitters, which reduces spatial fluctuation in distortion while maintaining peak power control benefits
Solution Approach 2:
The clipping thresholds are adjusted dynamically based on the beamforming weights and signal conditions. The system adapts the clipping level for each transmitter according to its contribution to the overall beamformed signal, making the distortion control responsive to changing spatial and signal conditions
2Device complexity
If uniform clipping threshold is applied to all radio transmitters, then implementation is simplified, but spatial characteristics of clipping noise differ from wanted signal causing poor EVM in some areas
Solution Approach 1:
Instead of uniform clipping thresholds, the patent implements location-specific clipping thresholds for each radio transmitter based on its beamforming weight magnitude. Transmitters with higher weights apply less clipping while those with lower weights apply more clipping, ensuring that the spatial pattern of clipping noise matches the spatial pattern of the wanted signal
Solution Approach 2:
The system uses feedback from beamforming weight calculations to dynamically adjust clipping thresholds. The clipping control is informed by the beamforming processing results, creating a closed-loop system where distortion control adapts to the actual signal distribution and spatial characteristics
3Reliability
If stringent EVM requirements are imposed at each antenna connector, then overall signal quality is improved, but costs and implementation complexity increase significantly
Solution Approach 1:
The patent changes the clipping threshold parameter for each transmitter based on its beamforming weight rather than imposing uniform stringent EVM requirements. This parameter adaptation allows acceptable overall EVM performance with relaxed individual transmitter requirements, reducing implementation cost and complexity
Solution Approach 2:
The overall EVM requirement is segmented and distributed across multiple transmitters with different clipping thresholds. Instead of requiring each transmitter to meet the full stringent requirement, the system divides the distortion control task among transmitters based on their individual contributions to the beamformed signal
Data Source
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AI summary
The invention provides a method for processing signals in a radio transmission apparatus (60) and a radio transmission apparatus (60) comprising a plurality of radio transmitters (61, 62), wherein clipping is applied to a signal in at least one of the plurality of radio transmitters, wherein the amount of clipping or/and a clipping threshold is adjusted individually for each of the plurality of radio transmitters.