Adaptive RF Predistortion for Open-Loop Power Amplifier Drift
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Solution Overview
Problem
Open loop power control systems in RF transmitters are vulnerable to changes in operating conditions such as temperature variations and voltage standing wave ratio (VSWR), leading to transmission failures due to unstable power amplifier characteristics.
Innovation Solution
An adaptive predistortion system that includes phase and amplitude determination elements to generate updated predistortion signals based on real-time transmitter characteristics, comparing them with factory-calibrated curves to adjust power amplifier output and compensate for non-linearities.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If factory-estimated non-linearities are used for pre-distortion in open loop power control, then cost is reduced and device space is saved, but transmission reliability deteriorates when operating conditions change (temperature, VSWR, aging)
Solution Approach 1:
The patent implements dynamic adaptation of predistortion curves by monitoring transmitter output characteristics and automatically updating predistortion parameters when deviations are detected. This transforms the static factory-calibrated system into a dynamic one that adapts to changing operating conditions (temperature, VSWR, aging), thereby maintaining transmission reliability without requiring complex closed-loop hardware
Solution Approach 2:
The system incorporates a feedback mechanism where the transmitter output is monitored and compared against expected characteristics. When deviations exceed a threshold, the system retrieves updated predistortion curves from storage and applies them to compensate for the drift. This feedback loop enables the open-loop system to respond to condition changes, resolving the reliability issue while maintaining simplicity
2Reliability
If closed-loop power control is implemented to maintain stable power amplifier characteristics, then transmission reliability is improved, but device cost and space consumption increase
Solution Approach 1:
The patent uses a memory device to store multiple predistortion curves that can be quickly swapped based on operating conditions. Instead of implementing complex closed-loop control hardware, the system uses inexpensive stored lookup tables that can be retrieved and applied as needed. This approach achieves reliability improvement through software-based adaptation rather than expensive hardware modifications
Solution Approach 2:
The system changes the predistortion parameters dynamically by selecting different stored curves based on monitored transmitter characteristics. When operating conditions drift, the system retrieves pre-calibrated curves with updated parameters from memory and applies them to the power amplifier control. This parameter substitution approach maintains reliability without requiring complex real-time control circuitry
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AI summary
An adaptive predlstortlon system for controlling an open loop power amplifier includes a transmitter, a receiver, a phase and amplitude determination element configured to determine amplitude, and phase characteristics of an output signal generated In the transmitter, an amplitude resampling element to generate an updated AM-AM predlstortlon signal based on the output signal generated In the transmitter, and an amplitude predlstortlon element to compare the updated AM-AM predlstortlon signal with a factory-calibrated AM-A predlstortlon signal and generate an amplitude compensation signal. The adaptive predlslortlon system Includes a phase comparison element configured to compare the signal representing transmitter characteristics with a desired phase signal, a phase resampling lement configured to generate an updated AM-PM predlstortlon signal based on the output signal generated In the transmitter, and a hasθ predistortlon element configured to compare the updated AM-PM predlstortlon signal with a factory-calibrated AM-PM redlstortlon signal and generate a phase compensation signal.