Crest Factor Reduction Using Delta-Sigma Noise Shaping
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Solution Overview
Problem
Conventional crest factor reduction techniques introduce significant in-band noise when applied to non-constant envelope signals, such as OFDM signals, which is undesirable for efficient amplification.
Innovation Solution
The technique involves oversampling a digital input signal to identify peaks exceeding a crest factor reduction threshold, generating a correction waveform with scaled pulses, and applying it via a delta-sigma modulator to reduce peaks while shaping the noise outside the signal band, thereby minimizing in-band noise through low-pass filtering.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If conventional crest factor reduction techniques are applied to non-constant envelope signals, then the peak-to-average power ratio is reduced, but significant in-band noise is introduced to the signal
Solution Approach 1:
The patent applies oversampling to expand the frequency domain, allowing noise shaping to push quantization noise to higher frequency bands outside the signal bandwidth. This dimensional expansion in the frequency domain enables separation of signal and noise, resolving the contradiction between peak reduction and noise introduction.
Solution Approach 2:
The delta-sigma modulator serves as an intermediary that shapes the spectral distribution of quantization noise. It acts as a mediator between the peak reduction requirement and the noise floor, redistributing noise energy to out-of-band frequencies while maintaining signal integrity in the passband.
2Productivity
If crest factor reduction is applied to improve amplification efficiency, then power amplifier performance is enhanced, but signal quality deteriorates due to added in-band noise
Solution Approach 1:
By transitioning to an oversampled representation, the patent creates additional frequency space where noise can be relocated. This allows the system to achieve both high amplification efficiency through peak reduction and maintain signal quality by confining noise to out-of-band regions that are subsequently filtered.
Solution Approach 2:
The patent converts the harmful quantization noise inherent in digital processing into a beneficial out-of-band noise component. Through noise shaping, the previously harmful in-band noise is transformed into out-of-band noise that can be easily removed by filtering, thereby converting a disadvantage into an advantage.
3Manufacturing precision
If correction waveform energy is concentrated to reduce peaks effectively, then crest factor reduction is achieved, but in-band noise increases
Solution Approach 1:
The patent utilizes the oversampled frequency domain to redistribute correction waveform energy. Instead of concentrating correction energy in the baseband where it would increase in-band noise, the correction is applied in the oversampled domain where energy can be directed to higher frequency components that fall outside the signal band after decimation.
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
This approach effectively reduces the peak-to-average power ratio of signals without introducing substantial in-band noise, enhancing amplification efficiency by ensuring most of the correction waveform's energy falls outside the signal's frequency band.
Implementation Method 1
The delta-sigma modulator causes a majority of the energy of the correction waveform in the oversampled digital output signal to fall outside a frequency band of the oversampled digital input signal such that in-band noise introduced into the oversampled digital output signal by the correction waveform is greatly reduced
Implementation Method 2
The oversampled digital output signal is low pass filtered to remove energy outside the frequency band of the oversampled digital signal
Data Source
AI summary
A technique for applying crest factor reduction to a signal involves identifying peaks of an oversampled digital signal that exceed a threshold and generating a correction waveform corresponding to a sequence of correction pulses for respective peaks. The correction waveform is applied to the oversampled digital signal via a delta-sigma modulator to generate an oversampled digital output signal with reduced peaks. The delta-sigma modulator causes most of the energy of the correction waveform in the oversampled digital output signal to fall outside a frequency band of the oversampled digital signal. The oversampled digital output signal is low pass filtered to remove energy outside the frequency band of the oversampled digital signal such that noise introduced into the oversampled digital output signal by the correction waveform is reduced. The oversampled digital output signal is down sampled to produce a digital output signal with a lower sampling rate.