Power Amplifier Predistortion With Autocalibration for AM-PM Distortion
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Solution Overview
Problem
Variable gain amplifiers in communication devices introduce significant distortion, particularly AM and PM distortion, which can degrade signal quality and fail to meet system specifications, especially when transitioning from constant envelope GMSK to 3π/8 PSK modulation in GSM EDGE systems.
Innovation Solution
The implementation of amplifier AM and PM predistortion and autocalibration methods, where amplifier characteristics are determined and stored in look-up tables, allowing for the predistortion of signals to correct for distortion, using a predistortion module that converts signals to polar format and applies inverse distortion characteristics to generate a distortion-compensated signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If variable gain amplifier is used to set output power and maintain amplitude range, then amplifier gain control is improved, but amplifier introduces nonlinear distortion and harmonics that degrade signal quality
Solution Approach 1:
The system performs preliminary characterization of the amplifier's distortion characteristics (AM-AM and AM-PM) and stores them in lookup tables before actual signal transmission. During operation, predistortion values are retrieved and applied to the signal in advance, compensating for the amplifier's nonlinearities before the signal reaches the amplifier stage.
Solution Approach 2:
The system implements a feedback mechanism where the amplifier's output is monitored and compared against expected characteristics. The measured distortion characteristics are used to update and refine the predistortion lookup tables, creating a closed-loop system that continuously optimizes distortion compensation based on actual amplifier performance.
2Adaptability or versatility
If amplifier is designed for constant envelope GMSK modulation, then performance for GMSK is optimized, but performance degrades when used with 3π/8 PSK modulation that has varying amplitude envelope
Solution Approach 1:
The system dynamically adapts to different modulation schemes by implementing separate predistortion lookup tables for different operating conditions. When transitioning between GMSK and 3π/8 PSK modulation, the system can switch between different characterization data sets, allowing the amplifier to maintain optimal performance across multiple modulation types rather than being fixed for a single mode.
Solution Approach 2:
The system changes operational parameters by storing multiple sets of distortion characteristics corresponding to different modulation schemes and power levels. The appropriate parameter set is selected based on the current modulation type and operating conditions, enabling the amplifier to adapt its distortion compensation strategy to match the specific requirements of each modulation scheme.
3Reliability
If predistortion module with lookup tables is implemented to compensate distortion, then signal quality is improved, but device complexity increases
Solution Approach 1:
Instead of implementing complex real-time mathematical models of the amplifier's nonlinear behavior, the system creates simplified lookup tables that store pre-computed predistortion values. These tables act as copies of the ideal compensation characteristics, allowing the system to achieve accurate distortion compensation through simple table lookups and interpolation rather than complex calculations.
Solution Approach 2:
The predistortion compensation is segmented into separate components: AM-AM distortion compensation and AM-PM distortion compensation. Each component has its own lookup table and processing path. This segmentation allows the system to handle different types of distortion independently, simplifying the overall architecture compared to a monolithic compensation approach.
4Manufacturing precision
If autocalibration technique is used to determine amplifier characteristics, then manufacturing variations are compensated, but calibration time and process complexity increase
Solution Approach 1:
The system implements self-service calibration where the amplifier characterizes itself through built-in test signal generation and measurement capabilities. During manufacturing or initialization, the amplifier automatically performs self-calibration by measuring its own distortion characteristics and populating its lookup tables, eliminating the need for external calibration equipment or manual adjustment procedures.
Solution Approach 2:
The autocalibration process is performed preliminarily during manufacturing or system initialization, so that the distortion compensation characteristics are established before the amplifier enters normal operation. This preliminary characterization ensures that manufacturing variations are compensated upfront, and the lookup tables are ready for immediate use without adding calibration time to the production workflow.
Data Source
AI summary
Methods and apparatus for amplifier AM and PM predistortion and autocalibration. AM and PM amplifier distortion can be corrected using predistortion. The AM and PM distortion characteristics of the amplifier are determined using an autocalibration technique. The amplifier characteristics can be stored in distinct look up tables. Alternatively, the inverse of the amplifier characteristics can be stored in distinct look up tables. Signals that are to be amplified are characterized in polar format having a phase component with a normalized magnitude and a magnitude component. The phase component can be predistorted by applying the inverse of the PM distortion characteristics to the signal. Similarly, the magnitude component can be predistorted by applying the inverse of the AM distortion characteristics to the signal. The predistorted phase component can be amplified using the previously characterized amplifier. The predistorted magnitude component can be used to set the gain of the previously characterized amplifier.