RF Transmitter Predistortion Mapping for Wideband Switch-Mode PAs
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Solution Overview
Problem
Conventional RF transmitters face inefficiencies due to the use of linear power amplifiers and are limited to narrowband modulation, while digitally-assisted transmitters struggle with high clock frequencies and noise shaping requirements, making them impractical for wideband applications.
Innovation Solution
A radio frequency transmitter architecture that employs a digital signal processing module for two-dimensional non-uniform mapping of complex input signals to in-phase and quadrature digital pre-distortion control words, driving an array of switch-mode power cells to generate analogue RF signals, thereby compensating for non-linearity and extending digital domain efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If linear power amplifiers are used in conventional RF transmitters, then the transmitter can operate with simple architecture, but the power efficiency is very low
Solution Approach 1:
The patent replaces the conventional linear power amplifier (analog/direct amplification system) with a digital polar transmitter architecture that uses digital signal processing and switching components. The digital domain processing substitutes for the analog linear amplification, enabling the use of highly efficient switch-mode power amplifiers while maintaining signal integrity through digital predistortion and polar coordinate transformation.
Solution Approach 2:
The patent transforms the signal representation from Cartesian coordinates (I/Q components) to polar coordinates (amplitude and phase), fundamentally changing the parameter domain. This parameter transformation enables the use of switch-mode power amplifiers by separating the amplitude modulation (handled digitally) from the phase modulation (handled by the switch-mode PA), thereby achieving high power efficiency.
2Loss of energy
If digital polar transmitter design is used, then power efficiency is high and silicon area is small, but the transmitter is only suitable for narrowband modulated signals due to bandwidth expansion characteristics
Solution Approach 1:
The patent extends the digital domain into the power amplifier stage, creating a digital-to-analog conversion capability within the polar architecture. By implementing digital predistortion and amplitude control in the digital domain before the switch-mode PA, the system can handle wideband signals without the bandwidth expansion limitations of conventional digital polar transmitters, effectively adding a dimensional capability for wideband operation.
3Adaptability or versatility
If hybrid polar transmitter designs use two-dimensional modulation for wideband phase modulation, then wideband capability is achieved, but the transmitter suffers from both amplitude and phase quantization noise requiring significant noise shaping
Solution Approach 1:
The patent applies digital predistortion in advance before the signal enters the switch-mode power amplifier. By pre-compensating for the non-linearities and quantization effects in the digital domain, the system eliminates the need for complex noise shaping circuits that would be required if the same corrections were attempted after amplification. This preliminary digital correction simplifies the overall architecture while maintaining wideband performance.
4Loss of energy
If switch-mode power amplifiers are used, then power efficiency is very high, but the amplifier exhibits highly non-linear input-output relationship
Solution Approach 1:
The patent converts the harmful non-linearity of the switch-mode power amplifier into a beneficial characteristic by using digital predistortion. The known non-linear transfer function of the switch-mode PA is deliberately exploited and pre-compensated in the digital domain, transforming what would be a distortion source into a predictable, controllable element that can be precisely managed through digital signal processing.
Data Source
AI summary
A radio frequency (RF) transmitter has at least one digital signal processing module and at least one power amplifier module. The digital signal processing module includes at least one digital pre-distortion component arranged to receive at least one complex input signal, perform two-dimensional non-uniform mapping of the complex input signal to a first, in-phase, digital pre-distortion control word and a further, quadrature, digital pre-distortion control word, and output the in-phase and quadrature pre-distortion digital control words. The power amplifier module includes a first, in-phase, array of switch-mode power cells and at least one further, quadrature, array of switch-mode power cells. The two-dimensional non-uniform mapping has a pre-distortion profile at least partly based on an input/output relationship for the power amplifier module arranged to generate an analog RF signal based at least partly on the in-phase and quadrature digital pre-distortion control words.