Adaptive Complex Gain Predistortion for Transmitter Linearization
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Solution Overview
Problem
Wireless cellular transmitters face challenges in achieving linearization of amplifiers due to nonlinearities caused by amplitude and phase modulation interactions, which are not adequately addressed by factory-calibrated predistortion lookup tables under varying operational conditions such as temperature and voltage changes, leading to signal distortion and spectral degradation.
Innovation Solution
An adaptive digital predistortion technique using a complex gain lookup table with an amplitude approximation method and adaptive loop gain/phase normalization, which updates in real-time to compensate for temperature variations and maintains accurate open-loop power control, avoiding curve discontinuities and reusing existing hardware for cost-effective implementation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If factory-calibrated predistortion lookup tables are used, then initial linearization is achieved, but performance degrades under varying operational conditions such as temperature and voltage changes
Solution Approach 1:
The patent implements adaptive predistortion that dynamically updates the lookup table entries based on real-time feedback from the power amplifier output. The system continuously adjusts predistortion parameters to compensate for temperature and voltage variations, transforming the static factory-calibrated system into a dynamic adaptive system that maintains linearization performance across varying operational conditions.
Solution Approach 2:
The patent employs a feedback mechanism where the output of the power amplifier is monitored and used to update the predistortion lookup table. The feedback signal is processed to determine corrections to the predistortion parameters, which are then applied to maintain accurate linearization despite environmental changes, resolving the contradiction between initial performance and adaptability.
2Reliability
If highly linear amplifiers are used, then signal quality is improved, but power consumption and cost increase
Solution Approach 1:
The patent introduces a digital predistortion intermediary that processes the input signal before it reaches the power amplifier. This predistortion stage pre-compensates for the amplifier's nonlinearities, allowing the use of simpler, lower-power amplifiers while maintaining signal quality. The predistortion lookup table acts as the intermediary that enables high-fidelity transmission without requiring expensive, power-hungry highly linear amplifiers.
3Ease of manufacture
If simpler amplifiers with nonlinearities are used, then cost and power consumption are reduced, but signal distortion increases
Solution Approach 1:
The patent applies preliminary anti-action by predistorting the input signal in advance to counteract the expected nonlinearities of the simpler power amplifier. The lookup table stores pre-calculated correction values that are applied to the input signal before amplification, effectively canceling out the distortion that would otherwise be generated by the cost-effective but nonlinear amplifier.
Data Source
AI summary
Symbols are transmitted in a Cartesian transmitter by pre-distorting an input signal X having in-phase and quadrature components using a first compensation lookup table operable to hold complex valued entries to carry out in-phase and quadrature compensation pre-distortion with respect to the input signal to form a pre-distorted signal Z. The pre-distorted signal Z is processed to form an output signal Y using a nonlinear element. A complex gain normalization parameter adaptively updated to reflect varying gain of a linear region of the nonlinear element. A normalized feed back signal {tilde over (Y)} is formed using the adaptively updated complex gain normalization parameter. The first compensation lookup table is updated based on the pre-distorted input signal Z and the adaptively normalized feedback signal {tilde over (Y)}.