Application of crest factor reduction to a signal
View Patent ↗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.
1. An apparatus for applying a crest factor reduction to a signal, comprising:
a peak detector module configured to identify peaks of an oversampled digital signal that exceed a crest factor reduction threshold;
a correction waveform generator configured to generate a correction waveform corresponding to a sequence of correction pulses for respective peaks of the oversampled digital signal;
a delta-sigma modulator configured to receive the oversampled digital signal at a signal input and the correction waveform at an error input and to apply the correction waveform to the oversampled digital signal to generate an oversampled digital output signal with peaks below the crest factor reduction threshold, wherein the delta-signal 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 signal;
a low pass filter configured to filter the oversampled digital output signal 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; and
a down-sampling module configured to perform down sampling on the oversampled digital output signal to produce a digital output signal with a lower sampling rate than the oversampled digital output signal.
2. The apparatus of claim 1 , wherein the oversampled digital signal is oversampled by a factor of at least two relative to a Nyquist sampling rate of the oversampled digital signal.
3. The apparatus of claim 1 , wherein amplitudes of individual correction pulses in the sequence are scaled in accordance with amplitudes of corresponding peaks of the oversampled digital signal.
4. The apparatus of claim 1 , further comprising:
an up-sampling module configured to perform up sampling on a digital input signal to produce an up-sampled digital input signal sampled at the oversampling rate; and
an interpolation filter configured to generate the oversampled digital signal from the up-sampled digital input signal.
5. The apparatus of claim 1 , wherein the oversampled digital signal represents an Orthogonal Frequency Division Multiplexing (OFDM) signal.
6. The apparatus of claim 1 , wherein the delta-sigma modulator is a first order delta-sigma modulator and the correction waveform has an integral relationship to the sequence of correction pulses.
7. The apparatus of claim 1 , wherein the delta-sigma modulator is a second order delta-sigma modulator and the correction waveform has a double integral relationship to the sequence of correction pulses.
8. A method of applying crest factor reduction to a signal, the method comprising:
identifying, via a peak detector, peaks of an oversampled digital signal that exceed a crest factor reduction threshold;
generating a correction waveform corresponding to a sequence of correction pulses for respective peaks of the oversampled digital signal;
applying the correction waveform to the oversampled digital signal via a delta-sigma modulator to generate an oversampled digital output signal with peaks below the crest factor reduction threshold, wherein the delta-signal 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 signal;
low pass filtering the oversampled digital output signal 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; and
performing down sampling on the oversampled digital output signal to produce a digital output signal with a lower sampling rate than the oversampled digital output signal.
9. The method of claim 8 , wherein the oversampled digital signal is oversampled by a factor of at least two relative to a Nyquist sampling rate of the oversampled digital signal.
10. The method of claim 8 , wherein generating the correction waveform includes scaling amplitudes of individual correction pulses in the sequence in accordance with amplitudes of corresponding peaks of the oversampled digital signal.
11. The method of claim 8 , further comprising:
performing up sampling on a digital input signal to produce an up-sampled digital input signal sampled at the oversampling rate; and
interpolation filtering the up-sampled digital input signal to generate the oversampled digital signal.
12. The method of claim 8 , wherein the oversampled digital signal represents an Orthogonal Frequency Division Multiplexing (OFDM) signal.
13. The method of claim 8 , wherein applying the correction waveform to the oversampled digital signal is performed with a first order delta-sigma modulator and the correction waveform has an integral relationship to the sequence of correction pulses.
14. The method of claim 8 , wherein applying the correction waveform to the oversampled digital signal is performed with a second order delta-sigma modulator and the correction waveform has a double integral relationship to the sequence of correction pulses.
15. A non-transitory computer readable medium encoded with software comprising processor-executable instructions that, when executed by a processor, cause the processor to perform functions of:
identifying peaks of an oversampled digital signal that exceed a crest factor reduction threshold;
generating a correction waveform corresponding to a sequence of correction pulses for respective peaks of the oversampled digital signal;
applying the correction waveform to the oversampled digital signal via delta-sigma modulation to generate an oversampled digital output signal with peaks below the crest factor reduction threshold, wherein the delta-signal modulation 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 signal;
low pass filtering the oversampled digital output signal 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; and
performing down sampling on the oversampled digital output signal to produce a digital output signal with a lower sampling rate than the oversampled digital output signal.
16. The computer readable medium of claim 15 , wherein the instructions for identifying peaks include instructions for identifying peaks of the oversampled digital signal that is oversampled by a factor of at least two relative to a Nyquist sampling rate of the oversampled digital signal.
17. The computer readable medium of claim 15 , wherein the instructions for generating a correction waveform include instructions for scaling amplitudes of individual correction pulses in the sequence in accordance with amplitudes of corresponding peaks of the oversampled digital signal.
18. The computer readable medium of claim 15 , encoded with software comprising further processor-executable instructions that, when executed by a processor, cause the processor to perform the further functions of:
performing up sampling on a digital input signal to produce an up-sampled digital input signal sampled at the oversampling rate; and
interpolation filtering the up-sampled digital input signal to generate the oversampled digital signal.
19. The computer readable medium of claim 15 , wherein the instructions for identifying peaks include instructions for identifying peaks of the oversampled digital signal which represents an Orthogonal Frequency Division Multiplexing (OFDM) signal.
20. The computer readable medium of claim 15 , wherein the instructions for applying the correction waveform to the oversampled digital signal include instructions for performing first order delta-sigma modulation, wherein the correction waveform has an integral relationship to the sequence of correction pulses.