Baseband Predistortion LUTs for Wideband Memory Effect Compensation
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Solution Overview
Problem
Existing baseband predistortion methods struggle to effectively compensate for both RF frequency response memory effects and bias-induced or thermal memory effects in multi-channel wideband wireless transmitters, leading to inefficiencies and distortions due to high computational complexity and numerical instability.
Innovation Solution
A piecewise pre-equalized lookup table (LUT) based digital predistortion system that uses a pre-equalized LUT to compensate for memory effects, reducing computational load and numerical instability while achieving comparable linearity to memory polynomial PD systems.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If Volterra filter structure is used to compensate memory effects, then memory effect compensation is achieved, but the number of optimization coefficients becomes very large making implementation extremely difficult
Solution Approach 1:
The patent segments the predistortion function into multiple independent lookup tables, each handling specific memory effects (electrical, thermal, long-term) separately. This segmentation reduces the complexity by avoiding the need for a large number of optimization coefficients required by Volterra filters, while still achieving comprehensive memory effect compensation through the coordinated operation of multiple simplified LUTs.
2Device complexity
If memory polynomial structure is used to reduce coefficients, then computational load is reduced, but numerical instability occurs when higher order polynomial terms are included
Solution Approach 1:
The patent uses lookup tables that store pre-computed predistortion values instead of performing real-time polynomial calculations. This copying approach replaces higher order polynomial terms with stored data, reducing computational load while avoiding the numerical instability associated with matrix inversion required for estimating polynomial coefficients.
3Device complexity
If Hammerstein predistorter is used to reduce complexity, then implementation complexity is reduced, but performance deteriorates because it assumes Wiener model structure
Solution Approach 1:
The patent merges multiple specialized lookup tables (electrical memory LUT, thermal memory LUT, long-term memory LUT) into a unified predistortion system. Each LUT addresses specific memory effects, and their combined output achieves comprehensive compensation without requiring the restrictive Wiener model structure assumed by Hammerstein predistorters, thus maintaining high linearity performance.
4Reliability
If conventional feedforward linearization is used, then linearity is improved, but system complexity increases compared to predistortion
Solution Approach 1:
The patent applies predistortion as a preliminary action before the signal reaches the power amplifier. By pre-compensating for nonlinearities and memory effects in the digital domain using lookup tables, the system achieves linearity improvement without requiring the complex analog feedforward structure with additional amplifiers and combiners, thus reducing overall system complexity.
Data Source
AI summary
An efficient baseband predistortion linearization method for reducing the spectral regrowth and compensating memory effects in wideband communication systems using effective multiplexing modulation technique such as wideband code division multiple access and orthogonal frequency division multiplexing is disclosed. The present invention is based on the method of piecewise pre-equalized lookup table based predistortion, which is a cascade of a lookup table predistortion and piecewise pre-equalizers.