Quadrature Receiver IQ Error Correction for Image Rejection
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Solution Overview
Problem
Quadrature receivers experience signal-to-noise ratio (SNR) reduction due to quadrature error (QE) imbalances, particularly in wideband multi-carrier direct conversion receivers, where in-phase and quadrature branches are not perfectly orthogonal, leading to undesirable image formation and reduced receiver sensitivity.
Innovation Solution
A system and method for detecting and correcting quadrature errors in a quadrature receiver, involving the use of error correction coefficients applied in reverse sequence to correct pre-demodulation, local oscillator, and baseband errors, utilizing test tones to identify and compensate for phase and gain mismatches between I and Q channels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If quadrature receivers are used to achieve high spectral efficiency, then signal processing capability is improved, but IQ imbalance causes signal leakage and reduces SNR
Solution Approach 1:
The patent segments the IQ imbalance correction into multiple sequential error sources (pre-demodulation error, LO error, baseband error), each corrected independently in reverse order. This segmentation allows targeted correction of specific imbalance components while maintaining the overall QAM modulation performance.
Solution Approach 2:
The patent applies correction coefficients in reverse order of the error sources (baseband first, then LO, then pre-demodulation). This inversion approach ensures that corrections are applied from the signal processing stage closest to the output working backward, preventing corrected signals from being re-contaminated by subsequent processing stages.
2Productivity
If I and Q channels are used for QAM modulation, then spectral efficiency is improved, but channel imbalance creates frequency images that interfere with signal reception
Solution Approach 1:
The patent extracts and identifies specific error sources (pre-demodulation error, LO error, baseband error) that generate frequency images, separating them from the overall signal processing chain. By isolating these error-generating stages, the system can apply targeted corrections to eliminate image formation while preserving the desired signal.
Solution Approach 2:
The patent implements a feedback mechanism where test tones are injected through the receiver chain, the output is measured, and correction coefficients are calculated based on the measured errors. These coefficients are then applied to correct the imbalance, and the process can be iterated to achieve precise image rejection while maintaining spectral efficiency.
3Measurement precision
If correction coefficients are applied to multiple error sources, then IQ imbalance is accurately corrected, but system complexity increases
Solution Approach 1:
The patent divides the complex correction task into three distinct error sources (pre-demodulation, LO, baseband), each handled by separate correction coefficients. This segmentation makes the correction process more manageable and systematic, allowing each coefficient to be calculated and applied independently rather than requiring a single complex correction mechanism.
Solution Approach 2:
The patent performs preliminary error characterization using test tones before actual signal reception. By pre-calculating correction coefficients for each error source during a calibration phase, the system simplifies real-time operation where only the pre-determined coefficients need to be applied, reducing the computational complexity during active signal processing.
Data Source
AI summary
In an example, there is disclosed a system and method for detecting and correcting error in a quadrature receiver (QR). The QR may include a receiver channel operable to divide a received RF signal into I and Q channels. The receiver channel may include error sources, such as (in sequence) pre-demodulation (PD) error, LO mixer error, and baseband (BB) error. Test tones may be driven on the receiver channel at a plurality of test frequencies, and a quadrature error corrector may be provided to detect error from each source. Upon receiving an RF signal, the quadrature error corrector may apply correction coefficients to correct each source of error in reverse sequence (BB, LO, PD).