Homodyne Transmitter Distortion Characterization via Model Fitting
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Solution Overview
Problem
Existing homodyne systems, such as transmitters and receivers, suffer from signal distortions due to imperfections like amplitude imbalance and phase shifts, which current methods struggle to accurately characterize and correct, especially in analog components and multi-tone signals across broad frequency spectra.
Innovation Solution
A method and system that measure and correct for distortions in homodyne systems by analyzing the frequency spectra of input and output signals using a model fitting approach, employing vector network analyzers or similar instruments to determine parameters for linear and non-linear distortions, and applying these to pre-distort input signals for improved output quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If demodulation-based methods are used to characterize IQ-imbalance, then the measurement can be performed, but errors are introduced due to noise and nonlinear distortions in the demodulation process
Solution Approach 1:
The patent extracts the distortion characterization from the demodulation process by using a modulation-independent method. Instead of relying on demodulation to reveal IQ-imbalance, the system directly measures the distorted constellation points and fits a model to extract imbalance parameters, thereby eliminating demodulation errors caused by noise and nonlinearities
Solution Approach 2:
The patent creates a mathematical model (copy) of the distorted signal that represents the relationship between ideal and actual constellation points. This model, expressed as s̃ = αs + βs*, allows accurate characterization of IQ-imbalance without requiring actual demodulation of the signal
2Adaptability or versatility
If demodulation-based methods are used, then IQ-imbalance can be estimated, but the method cannot characterize distortion in analog IQ-modulators or IQ-demodulators separately
Solution Approach 1:
The patent segments the transmitter or receiver system into distinct functional blocks (analog IQ-modulator, digital signal processor, analog IQ-demodulator) and applies the modulation-independent characterization method to each segment separately. This allows distortion sources to be identified and characterized at specific stages rather than treating the entire system as a black box
Solution Approach 2:
The patent introduces known test signals as intermediaries to probe specific components. By injecting known signals at intermediate points in the signal chain and measuring the output, the distortion characteristics of individual analog components can be isolated and characterized without analyzing the entire complex system
3Ease of operation
If single-tone signals are used for characterization, then the measurement process is simple, but the method cannot capture broad frequency spectrum distortions and multi-tone signal behavior
Solution Approach 1:
The patent uses periodic multi-tone test signals instead of single-tone signals. These periodic signals excite multiple frequency components simultaneously, allowing the system to measure distortion across the broad frequency spectrum while maintaining a relatively simple measurement procedure through Fourier analysis of the periodic response
Solution Approach 2:
The patent changes the frequency parameters of the test signal from a single frequency to multiple frequencies spanning the operational bandwidth. By sweeping through different frequency combinations and analyzing the constellation distortion at each, the system captures frequency-dependent distortion characteristics while extending beyond simple single-tone measurements
Data Source
AI summary
A system of measuring and correcting for distortions in homodyne systems and a method for operating a data processing system to provide an estimate of distortions in homodyne systems are disclosed. The method for operating a data processing system to provide an estimate of a distortion introduced by a homodyne system when the homodyne system processes a time a multi-tone time domain input signal, x(t), to obtain a time domain output signal, y(t) includes receiving a frequency spectrum, X(f), of the multi-tone time domain input signal, x(t) and measuring an output frequency spectrum, Y(f), when the homodyne system operates on x(t). A plurality of parameters of a model that represents a linear frequency response of the homodyne system when operating on X(f) to arrive at Y(f) by fitting the model to Y(f) and X(f) is determined, and the model is applied to X(f) and Y(f) to estimate the distortions.