Pre-Distortion LUT Calibration for Nonlinear Power Amplifiers
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Solution Overview
Problem
Current communication transmission systems face challenges in achieving reliable and efficient linearly-corrected modulation, especially with non-constant envelope modulation schemes like WCDMA, HSUPA, and LTE, due to the limitations of open-loop LUT correction and closed-loop error-based correction methods, which struggle with nonlinear power amplifiers and synchronization requirements.
Innovation Solution
A communication transmitter system that includes a pre-distortion lookup table and an auto-calibration module to generate pre-distorted baseband I/Q signals, using a digital feedback path for closed-loop auto-calibration and open-loop operation to compensate for nonlinearities in power amplifiers, ensuring efficient and synchronized modulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If nonlinear power amplifiers operate in saturation mode to improve power efficiency, then power consumption is reduced, but waveform quality is degraded
Solution Approach 1:
The system applies pre-distortion to the modulation signal before it enters the nonlinear power amplifier. This preliminary action compensates for the anticipated nonlinear distortion, allowing the amplifier to operate in saturation mode while maintaining waveform quality. The pre-distorted signal is designed such that after passing through the nonlinear amplifier, the output waveform matches the desired linear modulation signal.
Solution Approach 2:
The system implements a feedback mechanism where the output of the power amplifier is monitored and used to adjust the pre-distortion parameters. This closed-loop feedback ensures that the pre-distortion compensation remains accurate even as the amplifier's nonlinear characteristics change with temperature, aging, or operating conditions, thereby maintaining waveform quality while operating in high-efficiency saturation mode.
2Device complexity
If open-loop LUT correction is used to simplify the system, then device complexity is reduced, but manufacturing calibration complexity increases and performance is lost when nonlinearity varies
Solution Approach 1:
The system transitions from a static pre-distortion approach to a dynamic one by implementing adaptive pre-distortion that can adjust to changes in amplifier nonlinearity. The feedback mechanism continuously monitors the amplifier's actual nonlinear characteristics and updates the pre-distortion parameters accordingly, allowing the system to adapt to temperature variations, aging effects, and manufacturing tolerances without requiring complex recalibration procedures.
3Reliability
If closed-loop error based correction is used to improve correction accuracy, then waveform quality is maintained, but the system fails with severe nonlinearity and large delay
Solution Approach 1:
The system applies pre-distortion to the modulation signal before it enters the nonlinear power amplifier. This preliminary action compensates for the anticipated nonlinear distortion, allowing the amplifier to operate in saturation mode while maintaining waveform quality. The pre-distorted signal is designed such that after passing through the nonlinear amplifier, the output waveform matches the desired linear modulation signal.
Solution Approach 2:
The system implements a feedback mechanism where the output of the power amplifier is monitored and used to adjust the pre-distortion parameters. This closed-loop feedback ensures that the pre-distortion compensation remains accurate even as the amplifier's nonlinear characteristics change with temperature, aging, or operating conditions, thereby maintaining waveform quality while operating in high-efficiency saturation mode.
4Reliability
If hybrid polar modulation is used to maintain synchronization for GSM/EDGE, then timing misalignment of 20 ns is tolerated, but HSUPA requires 2 ns synchronization which needs additional hardware
Solution Approach 1:
The feedback mechanism serves multiple functions: it corrects pre-distortion parameters for waveform quality maintenance and simultaneously provides synchronization alignment for the LUT. This multi-functionality allows the system to achieve the stringent 2 ns synchronization required by HSUPA without requiring dedicated synchronization hardware, as the same feedback path used for pre-distortion optimization also enables precise timing alignment.
Data Source
AI summary
A system and method for modulating and amplifying an input signal are provided in a communication transmitter. The transmitter includes a pre-distortion lookup table containing pre-distortion lookup data. An auto-calibration module is in communication with the pre-distortion lookup table. The auto-calibration module is configured to operate during a closed-loop auto-calibration interval and to calibrate the pre-distortion lookup data to compensate for a non-linearity in the transmitter. The pre-distortion is based on the pre-distortion lookup data and is used during an open-loop operational interval.