Dual Predistortion Transmitter for Saturated Amplifier Linearity
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Solution Overview
Problem
Existing predistortion techniques for digital communication systems are sensitive to amplifier saturation and fall-back, leading to significant distortion and performance issues, especially in satellite communications where spectral regrowth and high-order system constellations pose challenges.
Innovation Solution
A transmitter device with a first predistortion circuit that includes a correction stage to emulate the non-linear transmission link for both amplitude levels before and beyond saturation, using a non-decreasing function for the AM/AM characteristic beyond saturation, and a second predistortion circuit capable of performing clipping and low-pass filtering to reduce peak-to-average power ratio and mitigate distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If the amplifier is used at or close to its saturation point to optimize relay resources usage, then power efficiency is improved, but signal distortion increases
Solution Approach 1:
The patent applies predistortion circuits that pre-compensate the signal before it enters the saturated amplifier. The first predistortion circuit adjusts digital symbols before modulation, and the second predistortion circuit adjusts the modulated signal before amplification, thereby counteracting the amplifier's non-linear distortion in advance and enabling operation at saturation without signal degradation
Solution Approach 2:
The patent employs feedback mechanisms where the predistortion circuits continuously adapt their compensation based on the actual amplifier output characteristics. This feedback loop allows the system to maintain optimal predistortion parameters even as amplifier conditions change, ensuring consistent performance at saturation
2Productivity
If existing predistortion techniques are used with high-order system constellations, then data rate is improved, but sensitivity to amplifier saturation and fall-back increases
Solution Approach 1:
The patent divides the predistortion process into two separate circuits: the first predistortion circuit operates on digital symbols before modulation, and the second predistortion circuit operates on the modulated signal before amplification. This segmentation allows each circuit to be optimized for its specific stage, reducing overall sensitivity to saturation effects while maintaining high-order constellation performance
Solution Approach 2:
The patent dynamically adjusts predistortion parameters based on the operating conditions and amplifier characteristics. By changing the predistortion parameters in response to saturation and fall-back conditions, the system maintains robust performance with high-order constellations without excessive sensitivity to amplifier non-linearities
3Reliability
If a non-decreasing function is used for the AM/AM characteristic beyond saturation, then the AM/AM curve remains flat after saturation, but the complexity of the predistortion circuit increases
Solution Approach 1:
The patent uses a simplified model of the amplifier's AM/AM characteristic that copies the essential behavior (flat response beyond saturation) without requiring complex mathematical functions. This approximate modeling approach achieves the desired distortion reduction while keeping the predistortion circuit complexity manageable
Data Source
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AI summary
The present invention relates to a transmitter device arranged for receiving one or more 5 source signals, each containing a sequence of source data symbols. The transmitter device comprises a first predistortion circuit and a second predistortion circuit. The first predistortion circuit comprises at least one correction stage provided with a correction path for determining a correction term. The correction term for each source signal is a function of a corresponding symbol of a predefined constellation for each source signal 10 and of the output of a model. This model comprises a circuit to emulate modulation means and the second predistortion circuit, a circuit to emulate a non-linear transmission link over which a predistorted version of an aggregate pulse-shaped signal is to be transmitted for amplitude levels before saturation of an AM/AM characteristic of the transmission link occurs and to emulate the non-linear transmission link for amplitude levels beyond saturation of the 15 AM/AM characteristic and a circuit to emulate demodulation means. The first predistortion circuit is arranged for adding, for each source signal, the correction term to a digital symbol applied to the first predistortion circuit or to a digital symbol output by a preceding correction stage of the first predistortion circuit.