Digital Predistortion for RF Amplifier Linearization
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Solution Overview
Problem
High efficiency RF amplifiers exhibit significant nonlinear behavior, leading to distortion and interference in wide bandwidth communication systems, which existing linearization techniques such as feedback and feed forward methods fail to adequately address due to efficiency losses and bandwidth limitations.
Innovation Solution
A digital predistortion system using a combination of finite impulse response and infinite impulse response filters, along with adaptive polynomial modeling, is employed to compensate for both static and dynamic nonlinearities, providing a predistorted signal to the amplifier to minimize distortion without severe efficiency degradation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If high efficiency amplifiers are used, then DC-to-RF power conversion efficiency is improved, but output distortion increases significantly
Solution Approach 1:
The patent applies predistortion linearization by injecting a compensatory distortion component at the input of the amplifier before the signal is amplified. This preliminary action pre-compensates for the expected nonlinear distortion, allowing the amplifier to operate in its highly nonlinear region while still producing linear output. The compensatory signal is processed through polynomial-based predistortion circuits that generate the appropriate correction terms.
Solution Approach 2:
The patent changes the operational parameters of the amplifier by operating it at high output back off (OBO) levels where high efficiency is achieved, while simultaneously applying predistortion to compensate for the resulting nonlinearities. This allows the system to operate in a parameter regime (high OBO) that would normally produce severe distortion but becomes acceptable when combined with predistortion compensation.
2Object-generated harmful factors
If output back off is increased to reduce distortion, then linearity is improved, but RF output power and efficiency deteriorate severely
Solution Approach 1:
Instead of reducing power to achieve linearity, the patent applies preliminary predistortion compensation that enables the amplifier to maintain high RF output power while achieving linearity through digital signal processing. The predistorter pre-compensates for nonlinearities, allowing operation at high power levels without sacrificing linearity.
3Object-generated harmful factors
If feedback linearization is used, then distortion correction is achieved, but bandwidth correction capabilities are limited due to stability restrictions
Solution Approach 1:
The patent replaces the analog feedback mechanism with a digital predistortion system. Instead of using continuous analog feedback loops that are constrained by stability and bandwidth limitations, the system uses digital polynomial-based predistortion circuits that can process wide bandwidth signals without stability issues. This substitution of digital processing for analog feedback eliminates the bandwidth restrictions inherent in feedback systems.
4Adaptability or versatility
If feed forward linearization is used, then wide bandwidth distortion correction is achieved, but efficiency deteriorates due to DC power consumption of error amplifier and lossy delay elements
Solution Approach 1:
The patent extracts and eliminates the error amplifier and lossy delay elements from the linearization system. By using direct digital predistortion without feedforward error correction, the system removes the components responsible for high DC power consumption while maintaining wide bandwidth correction capabilities through polynomial-based digital signal processing.
5Measurement precision
If polynomial-based predistortion with multiple filters is used, then distortion correction accuracy is improved, but computational and memory complexity increases
Solution Approach 1:
The patent segments the predistortion function into separate polynomial-based circuits handling different aspects of distortion compensation. By dividing the complex predistortion task into manageable polynomial terms and using dedicated circuits for each, the system achieves high accuracy while keeping individual circuit blocks relatively simple and efficient.
Data Source
AI summary
A system for digitally linearizing the nonlinear behaviour of RF high efficiency amplifiers employing baseband predistortion techniques is disclosed. The system provides additive or multiplicative predistortion of the digital quadrature (I/Q) input signal in order to minimize distortion at the output of the amplifier. The predistorter uses a discrete-time polynomial kernel to model the inverse transfer characteristic of the amplifier, providing separate and simultaneous compensation for nonlinear static distortion, linear dynamic distortion and nonlinear dynamic effects including reactive electrical memory effects. Compensation for higher order reactive and thermal memory effects is embedded in the nonlinear dynamic compensation operation of the predistorter in an IIR filter bank. The digital predistortion system of the invention may provide broadband linearization of highly nonlinear and highly efficient RF amplification circuits including, but not limited to, dynamic load modulation amplifiers.