METHOD AND APPARATUS FOR CREST FACTOR REDUCTION

• US 20110228872A1
• Filed: 02/22/2011
• Published: 09/22/2011
• Est. Priority Date: 02/22/2010
• Status: Active Grant

• Current Assignee: Kabushiki Kaisha Toshiba (Toshiba Corporation)
• Original Assignee: Kabushiki Kaisha Toshiba (Toshiba Corporation)
• Inventors: SOLER GARRIDO, Josep Vicent
• Granted Patent: US 8,724,721 B2
• Time in Patent Office: Days
• US Class Current: 375/295
• CPC Class Codes: H04L 27/2623 Reduction thereof by clipping

Abstract

According to embodiments there is provided a method for reducing the crest factor of an input signal. The method is performed in a part of a transmitter and comprises creating a clipped signal by clipping the input signal using a predetermined clipping threshold and creating a peak signal from the input signal. The peak signal comprises the difference between the input signal and the clipped signal. At least part of the peak signal is convoluted with an inverse Fourier transform of a filtering mask defined in the frequency domain. In this way a filtered signal is created. A subtraction signal is generated by subtracting the filtered signal from the input signal.

Claims

1. A method for reducing the crest factor of an input signal, the method performed in a part of a transmitter and comprising:

creating a clipped signal by clipping the input signal using a predetermined clipping threshold;

creating a peak signal from the input signal, the peak signal comprising a difference between the input signal and the clipped signal;

convoluting at least a part of the peak signal with the inverse Fourier transform of a filtering mask defined in the frequency domain to create a filtered signal; and

creating a subtraction signal by subtracting the filtered signal from the input signal.

2. A method according to claim 1, wherein only a predetermined number of data points of the peak signal are convoluted, wherein the predetermined number of data points are the predetermined number of data points of the peak signal with the highest magnitudes.

3. A method according to claim 1, wherein the input signal relates to a symbol and wherein an initial peak signal is created prior to receipt of all data points of the input signal relating to the symbol and wherein said convolution is started based on the initial peak signal.

4. A method according to claim 3, wherein further data points are added to the initial peak signal as the further data points are received as part of the input signal relating to the said symbol, thereby creating an updated initial peak signal and wherein said convolution is continued based on the updated initial peak signal.

5. A method according to claim 1, wherein the inverse Fourier transform of the filtering mask is generated once and stored in a memory.

6. A method according to claim 1, further comprising computing the inverse Fourier transform of the filtering mask using a processor of the transmitter, wherein the processor is also adapted to generate the input signal by computing the inverse Fourier transform of signals that are to be transmitted by the transmitter.

7. A method according to claim 1, wherein only the largest values of the inverse Fourier transform of the filtering mask representing at least 80%, at least 90% or at least 99% of the energy of the inverse Fourier transform of the filtering mask are used in convoluting the at least a part of the peak signal.

8. A method according to claim 1, wherein at least 80%, at least 90% or at least 99% of the energy of the inverse Fourier transform of the filtering mask is concentrated within a first number of data points of the inverse Fourier transform of the filtering mask, centred on the origin of the inverse Fourier transform of the filtering mask;

and

wherein the input signal is a part of the inverse Fourier transform of a symbol of an OFDM signal extending from a first data point of the inverse Fourier transform and comprising at least half of the first number of data points.

9. A method according to claim 8, wherein the method is consecutively applied to a plurality of input signals, the plurality of input signals relating to the same symbol of an OFDM signal, wherein the first input signal is part of the inverse Fourier transform of the symbol, the part comprising at least a number of data points that is equal to half of the first number of data points, the first data point of the part being the first data point of the inverse Fourier transform;

and

wherein subsequent input signals are generated by receiving further parts of the inverse Fourier transform of the symbol and appending the received further parts to a previous input signal.

10. A method according to claim 1, further comprising determining the error vector magnitude of the subtraction signal and, if the error vector magnitude exceeds a maximum acceptable error vector magnitude, setting a data point of the peak signal that has the smallest magnitude of the data points of the peak signal to zero.

11. A method for reducing the crest factor of a time domain signal, the method performed in a transmitter and comprising:

creating a clipped signal by clipping the signal using a predetermined clipping threshold;

subtracting the clipped signal from the signal to create a peak signal;

generating a filtered peak signal by transforming at least a part of the peak signal into the frequency domain, creating a filtered signal by applying a frequency domain filtering mask to the transformed peak signal and transforming the filtered signal into the time domain; and

subtracting the transformed filtered signal from the signal.

12. A method according to claim 11, further comprising applying complex weights to the data values in the at least part of the peak signal prior to convolution or transformation.

13. An apparatus comprising an input for receiving a time domain input signal, a processor arranged to create a clipped signal by clipping the input signal using a predetermined clipping threshold, to create a peak signal that comprises a difference between the input signal and the clipped signal, to convolute at least a part of the peak signal with the inverse Fourier transform of a filtering mask defined in the frequency domain to create a filtered signal, and to create a subtraction signal by subtracting the filtered signal from the input signal.

14. An apparatus according to claim 13, wherein the processor comprises a number of multipliers, the processor arranged to input data values of the peak signal into respective multipliers, the processor further arranged to input values of the inverse Fourier transform of the filter mask into the said respective multipliers, each of the input values of the inverse Fourier transform of the filter mask associated with a respective one of the data values of the peak signal.

15. An apparatus according to claim 13, further comprising a memory storing at least part of the inverse Fourier transform of the filtering mask.

16. An apparatus according to claim 15, further comprising an IFFT processor arranged to perform an inverse Fourier transform of a signal to create the input signal, the IFFT processor further arranged to calculate the inverse Fourier transform of the filtering mask during an idle period and store the inverse Fourier transform of the filtering mask in the memory.

17. An apparatus according to claim 13, further comprising a peak sorting arrangement, arranged to sort the peaks of the input signal in order of magnitude and to select a predetermined number of peaks for convolution.

18. An apparatus according to claim 13, further arranged to convolute said peak signal in real time after a predetermined number of peak signal values have become available for convolution.

19. An apparatus according to claim 13, wherein the processor is further arranged to determine whether the error vector magnitude of the subtraction signal is above a predetermined threshold and to set a smallest non-zero value of the peak signal that has been used or that would be used in creating the subtraction signal to zero prior to the convoluting.

20. An apparatus for reducing the crest factor of a time domain signal comprising a signal input for receiving the time domain signal and a processor arranged to:

create a clipped signal by clipping the signal using a predetermined clipping threshold;

create a peak signal that comprises the difference between the input signal and the clipped signal;

transform the at least a part of the peak signal into the frequency domain;

create a filtered signal by applying a frequency domain filtering mask to the transformed peak signal;

transform the filtered signal into the time domain; and

subtract the transformed filtered signal from the signal.

21. An apparatus according to claim 20, the processor further arranged to apply complex weights to the at least a part of the peak signal prior to convolution or transformation.