Adaptive I/Q Imbalance Estimation in RF Loopback Transceivers
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Solution Overview
Problem
Direct conversion transceivers face I/Q imbalance issues due to hardware limitations and require lengthy calibration processes, often relying on external hardware like envelope detectors, which are not available for regular data transmission.
Innovation Solution
A communication apparatus with a phase-locked loop and RF loopback path uses adaptive filtering to estimate and compensate combined TX/RX frequency-dependent I/Q imbalance without external hardware, employing pseudo-noise or OFDM signals and FIR filters for efficient compensation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If existing methods use additional external hardware (e.g., envelope detector) for I/Q imbalance estimation, then measurement precision is improved, but device complexity and ease of operation deteriorate due to unavailable hardware for regular data transmission
Solution Approach 1:
The communication apparatus uses its own internal resources (transmitter, receiver, phase-locked loop, and existing signal paths) to perform I/Q imbalance estimation without requiring external envelope detectors. The system serves itself by utilizing the RF loopback path and existing quadrature mixing circuits to generate and process calibration signals, eliminating dependency on unavailable external hardware.
Solution Approach 2:
The phase-locked loop is configured to perform dual functions: generating LO signals for normal communication operation and providing controlled phase shifts for I/Q imbalance calibration. The RF loopback path serves both as a signal transmission path for data communication and as a calibration path for I/Q estimation, making the hardware universally applicable to both regular operation and calibration modes.
2Manufacturing precision
If existing methods perform I/Q estimation and compensation in two steps (TX path first, then RX path), then manufacturing precision is improved, but loss of time increases due to lengthy calibration processes
Solution Approach 1:
The method combines TX and RX I/Q imbalance estimation into a single integrated process. By using the RF loopback path to transmit calibration signals from TX to RX and processing them through the adaptive filter in one continuous operation, the system simultaneously characterizes both transmitter and receiver I/Q imbalances, eliminating the need for separate calibration steps.
Solution Approach 2:
The system performs preliminary phase shifting of the RX LO signal before the calibration measurement to create distinct signal conditions. By pre-configuring the PLL to provide specific phase shifts (including 90-degree shifts) before the actual I/Q estimation, the system prepares the measurement conditions in advance, enabling faster single-step calibration without iterative adjustments.
3Measurement precision
If phase-locked loop provides different phase shifts for calibration, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The phase-locked loop acts as an intermediary device that automatically provides the required phase shifts between TX and RX LO signals. Instead of requiring complex external phase shifters or manual adjustment mechanisms, the PLL internally generates the appropriate phase relationships through its standard operation, simplifying the overall system while maintaining measurement precision.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables efficient estimation and compensation of I/Q imbalance in a single step, reducing calibration time and eliminating the need for external hardware, while maintaining accuracy and flexibility across different duplexing modes.
Implementation Method 1
a phase-locked loop (PLL), and a RF loopback path. The PLL is configured to generate a TX local oscillator (LO) signal and a RX LO signal, and to successively provide a first phase shift and a different second phase shift between the TX LO signal and the RX LO signal
Implementation Method 2
The TX is configured to generate a reference signal and to obtain a TX RF signal by quadrature mixing of the reference signal with the TX LO signal. The RX is configured to obtain a representation of the reference signal by quadrature mixing of the RX RF signal with the RX LO signal
Implementation Method 3
The TX comprises an adaptive filter configured to: based on the reference signal and the representation of the reference signal in case of the first phase shift between the TX LO signal and the RX LO signal, obtain a first estimate of a combined TX/RX frequency-dependent I/Q imbalance in a time domain
Implementation Method 4
The RF loopback path is configured to output the TX RF signal to the RX as a RX RF signal
Data Source
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AI summary
The present disclosure relates to a method for In-phase and Quadrature (l/Q) imbalance estimation and a corresponding communication apparatus. The method involves using an adaptive filter in a loopback architecture of the communication apparatus to estimate and compensate a combined TX/RX frequency-dependent l/Q imbalance. The combined TX/RX frequency-dependent l/Q imbalance is estimated by using a reference signal and two different phase shifts between a TX local oscillator (LO) signal and a RX LO signal. The resulting two estimates of the combined TX/RX frequency-dependent l/Q imbalance are used to calculate separately contributions made by a TX and a RX in the combined TX/RX frequency-dependent l/Q imbalance. By so doing, it is possible to estimate the combined TX/RX frequency- dependent l/Q imbalance without having to use additional external hardware. The calculated contributions may be then used to compensate the combined TX/RX frequency-dependent l/Q imbalance in a time domain.