FFT-Based RFI Cancellation for Multi-Carrier Receivers
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Solution Overview
Problem
Existing RFI cancellation methods in multi-carrier modulation systems face high computational complexity and inaccurate results due to the need for per-tone reciprocal computation and the assumption of a trapezoidal RFI envelope, leading to performance degradation with non-trapezoidal envelopes and small or large δ values.
Innovation Solution
The method represents RFI spectral components as a Taylor series of 1/(m−1/2) and computes coefficients using linear equations from FFT output at chosen frequencies, allowing for pre-computation and storage of coefficients for simple multiplications and additions, reducing computational complexity and improving accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If per-tone reciprocal computation is used for RFI cancellation, then RFI cancellation accuracy is improved, but computational complexity increases
Solution Approach 1:
The patent pre-calculates and stores reciprocal values in a lookup table before RFI cancellation operations. During actual RFI cancellation, the system simply retrieves pre-computed reciprocal values instead of performing real-time division operations, thereby maintaining cancellation accuracy while dramatically reducing computational complexity.
Solution Approach 2:
The patent creates a lookup table that stores pre-computed reciprocal values as copies of the mathematical relationships. This allows the system to use simple table lookups and multiplications instead of complex division operations, preserving accuracy through pre-computed values while reducing runtime computational burden.
2Device complexity
If trapezoidal RFI envelope assumption is made, then computational complexity is reduced, but measurement precision deteriorates for non-trapezoidal envelopes
Solution Approach 1:
The patent transitions from a static assumption of trapezoidal RFI envelope to a dynamic approach that adapts to actual envelope shapes. By using measured RFI spectral components from the signal and applying polynomial fitting, the system dynamically adjusts to match the actual RFI envelope characteristics regardless of shape, thereby maintaining accuracy without excessive complexity.
Solution Approach 2:
The patent changes the approach from assuming a fixed envelope shape (trapezoidal) to using measured spectral data and polynomial parameters that can represent various envelope shapes. By fitting polynomials to the measured RFI spectral components, the system adapts its parameters to match the actual RFI characteristics, improving accuracy while keeping computational complexity manageable.
3Adaptability or versatility
If polynomial fitting is used to match RFI envelope, then adaptability to different envelope shapes is improved, but computational complexity increases
Solution Approach 1:
The patent uses a limited-degree polynomial (e.g., quadratic or cubic) rather than high-degree polynomials that would provide even better fitting. This partial action approach provides sufficient adaptability to handle various RFI envelope shapes while keeping the computational complexity manageable through restricted polynomial degrees and efficient least-squares fitting algorithms.
4Speed
If RFI components are estimated using conventional methods, then processing speed is maintained, but measurement precision deteriorates for small or large δ values
Solution Approach 1:
The patent replaces conventional RFI estimation methods with a polynomial fitting approach based on measured spectral components. This substitution maintains processing speed by using efficient polynomial fitting algorithms while improving accuracy across the full range of δ values by adapting to the actual measured RFI characteristics rather than relying on assumptions that fail at extreme values.
Data Source
AI summary
A method and system of canceling radio frequency interference (RFI) sourced from a narrow-band radio frequency transmitter to interfere with the operation of a multi-carrier modulation system. The spectral components of the RFI can be represented as a Taylor series of 1/(m-½) where m is an integer and (m-½) is a normalized approximate distance between the frequency of interest and the center frequency of the RFI. The coefficients of the Taylor series are obtained by solving a set of linear equations using the Fourier output at several properly chosen frequencies where the REI components dominate. The coefficients for the Taylor series are formulated as linear combinations of the FFT output at the chosen frequencies and are computed and stored beforehand. Simple multiplications and additions are needed for obtaining the Taylor series coefficients from the FFT output at the chosen frequencies. The coefficients of the Taylor series can be obtained for any frequency, and the spectral component of the RFI can be estimated and removed from the MCM signals.