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A multiple carrier communication system includes a primary impulse shortening filter that receives an output signal of an analog to digital converter and accepts coefficients. A secondary impulse shortening filter receives the output signal of the analog to digital converter, outputs an output signa

A multiple carrier communication system includes a primary impulse shortening filter that receives an output signal of an analog to digital converter and accepts coefficients. A secondary impulse shortening filter receives the output signal of the analog to digital converter, outputs an output signal, and passes coefficients to the primary impulse shortening filter. A reference signal generator outputs a reference signal. A comparator compares the output signal and the reference signal and outputs a resulting error signal. An adaptive processor computes coefficients for the secondary impulse shortening filter based on the error signal.

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1. A method for computing coefficients of an impulse shortening filter, the method comprising: comparing a digital signal received by a spectrally constrained impulse shortening filter and a reference signal and generating an error signal based on a difference between the reference signal and the di

1. A method for computing coefficients of an impulse shortening filter, the method comprising: comparing a digital signal received by a spectrally constrained impulse shortening filter and a reference signal and generating an error signal based on a difference between the reference signal and the digital signal; andupdating coefficients of the spectrally constrained impulse shortening filter based on the error signal. 2. The method of claim 1 wherein the reference signal is a scaled reference signal and further comprising generating the scaled reference signal by scaling a reference signal using a plurality of scaling factors determined based on a desired spectral response. 3. The method of claim 1 further including: calculating a noise power spectral density in the digital signal received by the spectrally constrained impulse shortening filter; anddetermining the desired spectral response based on the noise power spectral density. 4. A system for computing coefficients of an impulse shortening filter, the system comprising: a comparator configured to compare a digital signal received by a spectrally constrained impulse shortening filter and a reference signal and generate an error signal based on a difference between the reference signal and the digital signal; andan updater configured to determine coefficients for the spectrally constrained impulse shortening filter based on the error signal. 5. The system of claim 4 wherein the reference signal is a scaled reference signal and further comprising a scaling filter configured to generate the reference signal by scaling a reference signal using a plurality of scaling factors determined based on a desired spectral response. 6. The system of claim 4 further including: a calculator configured to calculate a noise power spectral density in the digital signal received by the spectrally constrained impulse shortening filter; anda processor configured to determine the spectral response based on the noise power spectral density. 7. A method for adapting an impulse shortening filter, the method comprising: computing a reference signal by applying a digital signal to a spectrally constrained impulse shortening filter;delaying the digital signal applied to the spectrally constrained impulse shortening filter to produce a delayed digital signal;comparing the reference signal to the delayed digital signal and generating an error signal based on a difference between the reference signal and the delayed digital signal; andupdating coefficients of the spectrally constrained impulse shortening filter in an adaptive processor based on the error signal. 8. The method of claim 7 further including: decoding an output signal obtained from applying the digital data to the spectrally constrained impulse shortening filter; andcomputing the reference signal by encoding output data obtained from the output signal. 9. The method of claim 8 wherein computing the reference signal includes applying a discrete Fourier transform to the output signal prior to decoding the output signal. 10. The method of claim 9 wherein computing the reference signal includes applying an inverse discrete Fourier transform to the output data. 11. The method of claim 8 wherein computing the reference signal includes scaling the output data with a plurality of scaling factors based on a desired spectral response. 12. The method of claim 11 further including: determining the scaling factors by measuring a noise power spectral density in the digital signal received by the spectrally constrained impulse shortening filter;computing a desired spectral response based on the noise power spectral density; anddetermining the scaling factors such that the coefficients for the spectrally constrained impulse shortening filter provide the spectrally constrained impulse shortening filter with a spectral response that matches the desired spectral response. 13. The method of claim 12 further including applying a discrete Fourier transform to the output signal prior to decoding the output signal and measuring the noise power spectral density at an output of the discrete Fourier transform. 14. The method of claim 8 further including scaling the encoded output data and applying an inverse discrete Fourier transform to the scaled encoded output data. 15. A system for adapting an impulse shortening filter, the system comprising: a first computer configured to compute a reference signal by applying a digital signal to a spectrally constrained impulse shortening filter;a delayer configured to delay the digital signal applied to the spectrally constrained impulse shortening filter to produce a delayed digital signal;a comparator configured to compare the reference signal to the delayed digital signal and generate an error signal based on a difference between the reference signal and the delayed digital signal; andan updater configured to update coefficients of the spectrally constrained impulse shortening filter in an adaptive processor based on the error signal. 16. The system of claim 15 further including: a decoder arranged to decode an output signal obtained from applying the digital data to the spectrally constrained impulse shortening filter;an encoder arranged to encode output data obtained from the output signal; andwherein the first computer is arranged to compute the reference signal using encoded output data. 17. The system of claim 16 wherein the first computer is arranged to compute the reference signal by applying a discrete Fourier transform to the output signal prior to decoding the output signal. 18. The system of claim 17 wherein the first computer is arranged to compute the reference signal by applying an inverse discrete Fourier transform to the output data. 19. The system of claim 15 wherein the first computer is arranged to compute the reference signal by scaling the output data with a plurality of scaling factors. 20. The system of claim 19 wherein the first computer is arranged to determine the scaling factors by: measuring a noise power spectral density in the digital signal received by the spectrally constrained impulse shortening filter;computing a desired spectral response based on the noise power spectral density; anddetermining the scaling factors such that the coefficients for the spectrally constrained impulse shortening filter provide the spectrally constrained impulse shortening filter with a spectral response that matches the desired spectral response. 21. The system of claim 20 wherein the first computer is arranged to apply a discrete Fourier transform to the output signal prior to decoding the output signal and measure the noise power spectral density at an output of the discrete Fourier transform. 22. The system of claim 16 wherein the first computer is arranged to compute the reference signal by scaling the encoded output data and applying an inverse discrete Fourier transform to the scaled encoded output data.