Transmit Amplifier Predistortion Using On-Chip Receiver Feedback
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Solution Overview
Problem
Existing linearization techniques for RF power amplifiers in complex modulation schemes, such as EDGE and WCDMA, are not suitable for low-cost GSM/EDGE/3G cellular market due to high test equipment costs and time requirements, and are not amenable to ultra-low cost factory testing or built-in self-test techniques.
Innovation Solution
A novel apparatus and method for linearization of a digitally controlled pre-power amplifier (DPA) and RF power amplifier (PA) using a predistortion look-up table (LUT) that compensates for nonlinearities, utilizing on-chip receiver resources for calibration and updating predistortion tables during power ramp, without requiring extra analog hardware.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional linearization techniques are used for RF power amplifiers, then linearity performance is improved, but test equipment costs and calibration time increase significantly
Solution Approach 1:
The system uses its own receiver chain to measure and characterize the power amplifier's nonlinearities during normal operation. The receiver demodulates the transmitted signal and provides feedback to the baseband processor, which automatically calculates and updates predistortion correction values without external test equipment.
Solution Approach 2:
A feedback loop is established where the receiver measures the actual transmitted signal characteristics, compares them with expected values, and uses the error information to continuously update the predistortion lookup tables. This closed-loop approach enables automatic adaptation to amplifier drift and environmental changes.
2Measurement precision
If external test equipment is used for linearization calibration, then measurement accuracy is improved, but cost and time requirements increase
Solution Approach 1:
The linearization calibration is performed continuously during normal transmitter operation rather than requiring separate calibration sessions. The system measures amplifier characteristics and updates predistortion tables in real-time during each transmission burst, eliminating idle calibration time.
Solution Approach 2:
The receiver chain serves dual purposes: it functions as both the normal reception pathway and as the measurement instrument for linearization calibration. The baseband processor also handles both communication protocol processing and distortion analysis, eliminating the need for dedicated measurement equipment.
3Reliability
If complex linearization schemes are implemented, then transmitter linearity is improved, but hardware complexity increases
Solution Approach 1:
A predistortion lookup table acts as an intermediary component that pre-compensates nonlinearities before signal amplification. The lookup table stores correction values that are applied to the baseband signal, effectively linearizing the overall transmitter response without modifying the power amplifier hardware.
Solution Approach 2:
The patent replaces complex analog linearization circuits with digital signal processing techniques. Instead of using analog components to compensate for nonlinearities, the system uses digital predistortion algorithms and lookup tables implemented in the baseband processor, simplifying the RF hardware while maintaining linearity performance.
Data Source
AI summary
A novel apparatus and method of linearization of a digitally controlled pre-power amplifier (DPA) and RF power amplifier (PA). The mechanism is operative to perform predistortion calibration to compensate for nonlinearities in the DPA and PA circuits. A predistortion look up table (LUT) stores measured distortion compensation data that is applied to the TX data before being input to the digital to frequency converter (DFC), DPA and PA. The mechanism of the invention takes advantage of the on-chip receiver, which is normally inactive during the TX burst in a half-duplex operation, to demodulate the RF PA output and use the digital I/Q RX outputs to perform calibration of the TX pre-distortion tables. Controlled RF coupling is used to provide a sample of the RF output signal that to the receiver chain. The contents of the predistortion LUT are typically updated during the PA power up or down ramp. While the digitally-controlled PA (DPA) code is increasing (or decreasing), the amplitude and phase of the recovered I/Q samples are used to determine the instantaneous value of the AM/AM and AM/PM pre-distortion from which an update to the predistortion tables may be computed.