Compensator for removing nonlinear distortion
The present invention is a computationally-efficient compensator for removing nonlinear distortion. The compensator operates in a digital post-compensation configuration for linearization of devices or systems such as analog-to-digital converters and RF receiver electronics. The compensator also operates in a digital pre-compensation configuration for linearization of devices or systems such as digital-to-analog converters, RF power amplifiers, and RF transmitter electronics. The compensator effectively removes nonlinear distortion in these systems in a computationally efficient hardware or software implementation by using one or more factored multi-rate Volterra filters. Volterra filters are efficiently factored into parallel FIR filters and only the filters with energy above a prescribed threshold are actually implemented, which significantly reduces the complexity while still providing accurate results. For extremely wideband applications, the multi-rate Volterra filters are implemented in a demultiplexed polyphase configuration which performs the filtering in parallel at a significantly reduced data rate. The compensator is calibrated with an algorithm that iteratively subtracts an error signal to converge to an effective compensation signal. The algorithm is repeated for a multiplicity of calibration signals, and the results are used with harmonic probing to accurately estimate the Volterra filter kernels. The compensator improves linearization processing performance while significantly reducing the computational complexity compared to a traditional nonlinear compensator.
1. A multi-rate Volterra compensator for removing nonlinear distortion introduced by an electronic system, the multi-rate Volterra compensator comprising:
a number, k, of processing arms interconnected in a parallel configuration, wherein k is equal to a maximum order of the multi-rate Volterra compensator and is greater than one; and wherein
for each kth order processing arm,
an upsampler for increasing a sampling rate of the multi-rate Volterra compensator's input by a factor of M k , wherein the increased sample rate is commensurate with a bandwidth of the nonlinear distortion;
a first bandpass filter coupled to an output of the upsampler, and
an upsampled Volterra filter coupled to an output of the first bandpass filter, wherein each kth order processing arm removes a different order of nonlinear distortion.
2. The multi-rate Volterra compensator of claim 1 , wherein each kth order processing arm further comprises
a second bandpass filter coupled to an output of the upsampled Volterra filter, and
a downsampler for decreasing the increased sample rate to a sample rate of the multirate Volterra compensator's input.
3. The multi-rate Volterra compensator of claim 2 , wherein for each kth order processing arm, the downsampler and first bandpass filter are implemented as a first parallel polyphase bandpass filter, and the upsampled Volterra filter is implemented as a parallel downsampled Volterra filter.
4. The multi-rate Volterra compensator of claim 3 , wherein for each kth order processing arm, the second bandpass filter and upsampler are implemented as a second parallel polyphase bandpass filter.
5. The multi-rate Volterra compensator of claim 4 , wherein for each kth order processing arm, the first parallel polyphase bandpass filter, the parallel downsampled Volterra filter, and the second parallel polyphase bandpass filter are implemented as an equivalent Volterra filter, wherein the equivalent Volterra filter is a multidimensional convolution of a tensor outer product of the first parallel polyphase bandpass filter, the parallel downsampled Volterra filter, and a multidimensional diagonal matrix containing coefficients of the second parallel polyphase bandpass filter.
6. The multi-rate Volterra compensator of claim 2 , wherein for each kth order processing arm, the upsampler, first bandpass filter, upsampled Volterra filter, second bandpass filter, and downsampler are implemented as a multidimensional bandpass filter convolved with the downsampled Volterra filter.
7. An oversampled Volterra compensator for removing nonlinear distortion introduced by an electronic system, the oversampled Volterra compensator comprising:
a number, k, of processing arms interconnected in a parallel configuration, wherein k is equal to a maximum order of a multi-rate Volterra compensator and is greater than one; and wherein
for each kth order processing arm, an upsampled Volterra filter operating on a bandlimited subband that is a portion of a full Nyquist bandwidth of an electronic system to remove nonlinear distortion within the bandlimited subband.
8. The oversampled Volterra compensator of claim 7 , wherein each kth order processing arm further comprises:
a bandpass filter coupled to an output of the upsampled Volterra filter, and
a downsampler coupled to an output of the bandpass filter.
9. The oversampled Volterra compensator of claim 7 , wherein each kth order processing arm further comprises an analog bandpass filter coupled to an output of the upsampled Volterra filter.
10. A Volterra compensator for removing nonlinear distortion introduced by an electronic system, the Volterra compensator comprising:
a Volterra kernel of order N, wherein N is equal to or greater than two, wherein the Volterra kernel removes nonlinear distortion in a digital signal and is implemented in a processor comprising:
a plurality of exponentiators each operating on differently delayed inputs of the digital signal, and
a plurality of parallel FIR filters.
11. The Volterra compensator of claim 10 , wherein the plurality of exponentiators are second-order exponentiators.
12. The Volterra compensator of claim 10 , wherein the plurality of exponentiators are third-order exponentiators.
13. The Volterra compensator of claim 10 , wherein the plurality of exponentiators are fourth or higher-order exponentiators.
14. The Volterra compensator of claim 10 , wherein only a subset of the plurality of parallel FIR filters with energy above a prescribed threshold are implemented.
15. The Volterra compensator of claim 10 , wherein the plurality of parallel FIR filters is implemented in a polyphase configuration.