Receiver Feedback Predistortion for Saturated Power Amplifiers
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Solution Overview
Problem
Higher order modulations in bandwidth-limited wireless channels, such as QAM systems, are sensitive to nonlinear distortions caused by high-power amplifiers (HPAs), which can be mitigated through pre-distortion, but existing open-loop methods lack feedback and are inefficient, reducing output signal levels.
Innovation Solution
A closed-loop communication system that calculates and transmits pre-distortion coefficients from a receiver to a transmitter, using techniques like least squares estimation or least mean squares, to modify signals before amplification, thereby counteracting nonlinear distortions in HPAs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If higher order modulations are used to increase data throughput, then productivity is improved, but sensitivity to nonlinear distortions increases causing reliability to deteriorate
Solution Approach 1:
The system applies pre-distortion to the transmitted signal before amplification to pre-compensate for the nonlinearities that will be introduced by the HPA. This preliminary action counteracts the expected distortion, allowing higher order modulations to be used without sacrificing signal quality.
Solution Approach 2:
The system implements a closed-loop feedback mechanism where the receiver measures the actual signal quality and sends feedback to the transmitter. The transmitter uses this feedback to adjust the pre-distortion coefficients, continuously optimizing the compensation for HPA nonlinearities to maintain reliable higher order modulation.
2Power
If HPA operates at saturation power to maximize output signal level, then power efficiency is improved, but nonlinear distortions increase causing reliability to deteriorate
Solution Approach 1:
The transmitter applies pre-distortion coefficients to the signal before amplification, pre-compensating for the nonlinearities that will occur when the HPA operates at saturation. This allows the HPA to run at full power while the pre-distorted signal counteracts the resulting distortions.
Solution Approach 2:
The closed-loop system continuously monitors signal quality and adjusts pre-distortion coefficients based on feedback from the receiver. This dynamic adjustment maintains signal linearity even as the HPA operates at saturation power, resolving the contradiction between power efficiency and linearity.
3Reliability
If output back-off is increased to reduce HPA nonlinearities, then reliability is improved, but output signal level decreases causing productivity to deteriorate
Solution Approach 1:
Instead of reducing the input power to avoid nonlinearities, the system applies pre-distortion to the signal before amplification. This allows the HPA to operate at full power while the pre-distorted signal compensates for the nonlinearities, maintaining both high output signal level and signal linearity.
Solution Approach 2:
The system changes the parameters of the transmitted signal by applying pre-distortion coefficients that modify the signal characteristics before amplification. This transformation allows the signal to withstand the nonlinearities of saturation operation, eliminating the need for output back-off.
4Reliability
If sample-by-sample pre-distortion is used to improve performance, then reliability is improved, but device complexity increases due to requirement of HPA characteristics knowledge
Solution Approach 1:
The closed-loop system uses feedback from the receiver to automatically determine the optimal pre-distortion coefficients. The receiver measures the actual signal quality and sends this information back to the transmitter, which then adjusts its pre-distortion parameters. This eliminates the need for complex prior knowledge or modeling of HPA characteristics.
Solution Approach 2:
The system performs self-calibration through the feedback loop, where the receiver's measurements directly inform the transmitter's pre-distortion adjustments. The system automatically adapts to the actual HPA characteristics without requiring external characterization or complex modeling, reducing overall system complexity.
Data Source
AI summary
Methods and systems for calculating pre-distortion coefficients in a closed-loop communication system are presented. Transmission terminals that include high power amplifiers are difficult to operate at or near the saturation point without transmitting signals with nonlinear distortions. By pre-distorting the signal prior to amplification the transmitted nonlinear distortions may be decreased. A closed-looped pre-distortion system may include a receiver that calculates the pre-distortion coefficients and transmits these coefficients back to the transmitter. These coefficients may be stored in a pre-distortion coefficient lookup table and may be updated by the receiver terminal.