Constrained clipping for peak-to-average power ratio (crest factor) reduction in multicarrier transmission systems

• US 20080019453A1
• Filed: 12/11/2006
• Published: 01/24/2008
• Est. Priority Date: 10/27/2005
• Status: Active Grant

• Current Assignee: Georgia Tech Research Corporation (University System of Georgia)
• Original Assignee: Georgia Tech Research Corporation (University System of Georgia)
• Inventors: Baxley, Robert, Zhou, Guotong, Zhao, Chunming
• Granted Patent: US 7,944,991 B2
• Time in Patent Office: Days
• US Class Current: 375/260
• CPC Class Codes: H04L 27/2623 Reduction thereof by clipping

Abstract

Disclosed is a constrained clipping technique for reducing the peak-to-average power ratio (PAR) or crest factor of a multicarrier communications signal. This is a transmitter-side processing technique that does not impose any modification at the receiver. Constrained clipping achieves PAR reduction while simultaneously satisfying spectral mask and error vector magnitude (EVM) constraints that are specified by most modern communications standards. The constrained clipping technique includes two independent processing units, one to satisfy an in-band EVM constraint and the other to satisfy an out-of-band spectral constraint. Achievable PAR reduction results vary depending on a particular standard'"'"'s requirements, but by using constrained clipping on a QPSK WiMax signal with 256 subcarriers, for example, a 4.5 dB PAR reduction at the 10⁻² complementary cumulative distribution function (CCDF) level can be obtained.

Claims

1. Signal transmission apparatus, comprising:

a clipping module for clipping a multicarrier time-domain signal at a predetermined clipping level to produce a clipped multicarrier signal;

a transforming module for Fourier transforming the clipped multicarrier signal into the frequency domain to produce a frequency domain clipped multicarrier signal;

an in-band processing module for processing the frequency domain clipped multicarrier signal, if required, to produce a signal whose error vector magnitude is below a predetermined threshold and whose peak-to-average power ratio (PAR) is at a relatively low level;

an out-of-band processing module for processing the frequency domain clipped multicarrier signal to satisfy an out-of-band spectral power mask; and

an inverse transforming module for inverse Fourier transforming the combined in-band and out-of-band processed signal to produce a low-PAR time-domain signal.

2. The apparatus recited in claim 1, wherein the in-band module modifies the error vector and adds the modified error vector back to the clipped signal so that the error vector magnitude is reduced.

3. The apparatus recited in claim 1, wherein a sorting algorithm is applied to determine the set of in-band error vectors that are to be modified.

4. The apparatus recited in claim 1, wherein the frequency domain signal after the in-band processing can be obtained by modifying the smallest set of in-band error vectors to meet the desired threshold.

5. The apparatus recited in claim 1, wherein the out-of-band module spectrally clips the out-of-band components of the frequency domain clipped multicarrier signal.

6. The apparatus recited in claim 1, wherein the out-of-band portion of the frequency domain transmitted signal can be obtained by setting the power of each out-of-band subcarrier that exceeds a given threshold power to the threshold power.

7. The apparatus recited in claim 1, wherein the out-of-band portion of the frequency domain transmitted signal can be obtained by setting the power of each out-of-band subcarrier that exceeds a given threshold power to the threshold power while retaining the subcarrier phase.

8. The apparatus recited in claim 1, wherein an initial clipping level determination procedure is applied to achieve a target PAR reduction result.

9. A method comprising:

clipping a multicarrier time-domain signal at a predetermined clipping level to produce a clipped multicarrier signal;

Fourier transforming the clipped multicarrier signal into the frequency domain to produce a frequency domain clipped multicarrier signal;

in-band processing the frequency domain clipped multicarrier signal, if required, to produce a signal whose error vector magnitude is below a predetermined threshold and whose peak-to-average power ratio (PAR) is at a relatively low level;

out-of-band processing module for processing the frequency domain clipped multicarrier signal to satisfy an out-of-band spectral power mask; and

inverse Fourier transforming the combined in-band and out-of-band processed signal to produce a low-PAR time-domain signal.

10. The method recited in claim 9, wherein the in-band module modifies the error vector and adds the modified error vector back to the clipped signal so that the error vector magnitude is reduced.

11. The method recited in claim 9, wherein a sorting algorithm is applied to determine the set of in-band error vectors that are to be modified.

12. The method recited in claim 9, wherein the frequency domain signal after the in-band processing can be obtained by modifying the smallest set of in-band error vectors to meet the desired threshold.

13. The method recited in claim 9, wherein the out-of-band module spectrally clips the out-of-band components of the frequency domain clipped multicarrier signal.

14. The method recited in claim 9, wherein the out-of-band portion of the frequency domain transmitted signal can be obtained by setting the power of each out-of-band subcarrier that exceeds a given threshold power to the threshold power.

15. The method recited in claim 9, wherein the out-of-band portion of the frequency domain transmitted signal can be obtained by setting the power of each out-of-band subcarrier that exceeds a given threshold power to the threshold power while retaining the subcarrier phase.

16. The method recited in claim 9, wherein a clipping level determination procedure is applied to achieve the target PAR reduction result.