RF-Sampled Digital Predistortion for PA Linearity and Efficiency
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Solution Overview
Problem
Power amplifiers in base transceiver stations face challenges in achieving high efficiency and linearity over a large amplitude range, leading to undesirable signal effects and adjacent channel leakage, particularly due to their non-linearity and memory effects, which are difficult to correct.
Innovation Solution
A digital pre-distortion system that uses an RF analog-to-digital converter to sample the output signal of the power amplifier at or above the Nyquist rate, eliminating the need for down mixing and anti-aliasing modules, and employs a digital signal processing module with adaptive algorithms to correct non-linearity, either by directly sampling or undersampling, allowing for reduced complexity and cost while maintaining effectiveness across single-band and multi-band scenarios.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If the PA operates at high input power to achieve high efficiency, then efficiency is improved, but linearity deteriorates causing non-linearity and adjacent channel leakage
Solution Approach 1:
The system applies digital pre-distortion to the input signal before it reaches the power amplifier. The DPD module pre-compensates for the expected non-linear distortion by applying an inverse distortion function, so that when the signal passes through the non-linear PA, the combined effect produces a linear output. This preliminary correction action prevents distortion rather than correcting it afterward.
Solution Approach 2:
The system implements a feedback loop where the output signal from the power amplifier is captured by the observing path, converted to digital form by the ADC, and fed back to the digital signal processing module. The DPD module continuously compares the pre-distorted signal with the actual output signal, measures the residual non-linearity, and adapts the pre-distortion parameters to minimize distortion. This closed-loop feedback enables real-time correction of PA non-linearity.
2Measurement precision
If traditional down mixing and anti-aliasing modules are used in the observing path, then measurement accuracy is improved, but device complexity increases
Solution Approach 1:
The system extracts and removes the down mixing and anti-aliasing filter modules from the traditional observing path. Instead of using complex RF signal processing hardware, the invention directly samples the PA output signal using an ADC operating at or above the Nyquist rate (fs ≥ 2fH). This extraction of unnecessary components simplifies the observing path while maintaining measurement accuracy through direct digital sampling.
Solution Approach 2:
The system replaces the mechanical/RF signal processing approach (down mixing, analog filtering) with a digital sampling approach. By using an ADC to directly convert the RF output signal to digital form at sufficient sampling rates, the system substitutes complex analog signal processing hardware with straightforward digital conversion, reducing component count and system complexity while preserving measurement capability.
3Object-generated harmful factors
If the PA operates at low input power to maintain linearity, then linearity is improved, but efficiency deteriorates
Solution Approach 1:
The system applies preliminary anti-action by pre-distorting the input signal in the opposite direction of the expected non-linear distortion. The DPD module calculates and applies correction parameters that counteract the PA's non-linear behavior before the signal is amplified. This preliminary counter-action ensures that the combined effect of pre-distortion and PA non-linearity produces a linear output, allowing the PA to operate efficiently without compromising linearity.
Data Source
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Figure 6A~6C
AI summary
Various digital pre-distortion systems for use in transmitters are disclosed. The digital pre-distortion system comprises an observing path, which performs either undersampling or radio frequency sampling of the output of a power amplifier. Undersampling may be performed at a rate, which causes aliasing to occur in the undersampled frequency domain. Both undersampling and radio frequency sampling reduces the complexity of the digital pre-distortion system by removing any down mixing modules or anti-aliasing modules, while maintaining reasonable performance of the digital pre-distortion systems.