Quadrature Modulator Calibration Using Baseband Signal Processing
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Solution Overview
Problem
Modern wireless telecommunication systems face challenges in efficiently compensating for gain and phase errors in quadrature modulators, which affect system performance and out-of-band emission requirements, particularly in homodyne architectures like OFDM systems, where existing calibration methods are complex or impractical.
Innovation Solution
A calibration method involving pairs of test signals in quadrature with equal energy, generating modulated signals, calculating transformed signals in the frequency domain, and determining partial error indicators to estimate and compensate for modulation errors, thereby reducing the negative effects of phase and gain imbalances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a closed feedback measure loop is used to compensate gain and phase errors, then compensation accuracy is improved, but the method requires exact knowledge of loop delay which is usually not available
Solution Approach 1:
The patent extracts the delay dependency from the measurement process by using a different approach that doesn't require loop delay knowledge. Instead of measuring the RF signal after it passes through the feedback loop, the method uses baseband test signals and processes them in a way that eliminates the need for delay information.
Solution Approach 2:
The patent changes the measurement parameters from RF domain measurements requiring loop delay knowledge to baseband signal processing using autocorrelation and cross-correlation functions. This parameter change transforms the measurement approach to one that is independent of loop delay.
2Extent of automation
If a measure loop with switches is implemented for calibration, then automatic calibration is achieved, but the switches need to be perfectly insulated which increases hardware complexity
Solution Approach 1:
The patent implements a self-service calibration approach where the system calibrates itself using signal processing techniques rather than requiring complex hardware switches. The method uses mathematical operations on test signals to automatically determine and correct gain and phase errors without manual intervention or complex switching mechanisms.
Solution Approach 2:
The patent replaces the mechanical/electrical switching system with a signal processing-based approach. Instead of using physical switches that require perfect insulation, the method uses digital signal processing operations (autocorrelation, cross-correlation, and matrix operations) to achieve the same calibration function with reduced hardware complexity.
3Measurement precision
If multiple test signals with different phases are used for calibration, then measurement accuracy is improved, but the calibration process time increases
Solution Approach 1:
The patent performs preliminary actions by using multiple test signals with different phases to gather comprehensive measurement data. This preliminary measurement phase enables accurate estimation of gain and phase errors across different signal conditions, which then allows for effective compensation during actual operation.
Solution Approach 2:
The patent uses periodic test signals with different phases systematically to probe the modulator characteristics. By using multiple phased test signals in a structured manner, the method efficiently extracts error information without requiring excessive measurement time, as each test signal provides specific information about the modulator's performance.
Data Source
AI summary
A calibration method for reducing modulation errors in a telecommunication transmitter apparatus includes providing a plurality of pairs of test signals; the test signals of each pair are substantially in quadrature to each other, and the pairs of test signals are at least in part different with all the pairs of test signals that have a substantially equal energy. A plurality of modulated signals are generated, with each modulated signal that is generated by modulating a corresponding pair of test signals. A plurality of transformed signals are calculated, each one corresponding to the square of a corresponding modulated signal in the frequency domain. A plurality of partial error indicators are calculated, each one as a function of the modulus of a corresponding transformed signal. The partial error indicators are indicative of the modulation errors associated with the modulated signals. A compensation for counterbalancing the modulation errors is calculated according to a combination of the partial error indicators and an indication of the compensation is stored for the application of the compensation to each further pair of operative signals during operation of the telecommunication transmitter apparatus.