Spectrally constrained impulse shortening filter for a discrete multi-tone receiver
View Patent ↗A channel in a multiple carrier communication system is equalized by computing a target spectral response, shortening the impulse response of the channel so that a significant part of an energy of the impulse response is confined to a region that is shorter than a target length and filtering the signal based on the target spectral response.
1. A spectrally constrained impulse shortening filter for a multiple carrier communication system, the system being configured to receive a signal and includes a channel that has an impulse response, the filter comprising:
a digital filter structure configured to apply a frequency characteristic to the signal, the frequency characteristic being determined by filter coefficients; and taps coupled to the digital filter structure, wherein the filter coefficients being selected to shorten the impulse response of the channel so that a significant part of an energy of the impulse response is confined to a region that is shorter than a length of a cyclic prefix of the signal.
2. The filter of claim 1 wherein the filter coefficients are selected to apply a frequency characteristic to the signal based on a target spectral response having a magnitude constraint that is based on measured noise power spectral density.
3. The filter of claim 2 , wherein the noise power spectral density is measured at an output of a discrete Fourier transform coupled to receive an output of the spectrally constrained impulse shortening filter.
4. A receiver for receiving a multiple carrier signal from a communication channel having an impulse response, the receiver comprising:
an analog-to-digital converter configured to receive the signal from the communication channel; and
a spectrally constrained impulse shortening filter coupled to the analog-to-digital converter and configured to shorten the impulse response of the channel so that a significant part of an energy of the impulse response is confined to a region that is shorter than a length of a cyclic prefix of the received signal.
5. The receiver of claim 4 comprising:
a discrete Fourier transform coupled to receive an output of the spectrally constrained impulse shortening filter; and
a decoder coupled to receive an output of the discrete Fourier transform.
6. The receiver of claim 5 wherein the spectrally constrained impulse shortening filter is configured to apply a frequency characteristic to the signal based on a target spectral response having a magnitude constraint that is based on measured noise power spectral density to reduce noise bleeding from the channel to adjacent channels.
7. The receiver of claim 6 wherein the target spectral response is computed from measured noise power spectral density.
8. The receiver of claim 7 wherein the target spectral response is approximated by the inverse of the measured noise power spectral density.
9. The receiver of claim 6 wherein the noise power spectral density is measured at an output of a discrete Fourier transform.
10. The receiver of claim 6 wherein the applied frequency characteristic causes attenuation of noise and signal components in the channel substantially equally.
11. A modem comprising:
an encoder arranged to receive digital data and output a constellation of complex values;
an inverse discrete Fourier transform arranged to receive the constellation;
a digital-to-analog converter coupled to the inverse discrete Fourier transform and arranged to output a signal to a communication channel;
an analog-to-digital converter arranged to receive the signal; and
a spectrally constrained impulse shortening filter coupled to the analog-to-digital converter and arranged to shorten an impulse response of the channel so that a significant part of an energy of the impulse response is confined to a region that is shorter than a length of a cyclic prefix of the received signal.
12. The modem of claim 11 comprising:
a discrete Fourier transform coupled to receive an output of the spectrally constrained impulse shortening filter; and
a decoder coupled to the discrete Fourier transform and arranged to output digital data.
13. The modem of claim 12 wherein the spectrally constrained impulse shortening filter is arranged to filter the signal based on a target spectral response having a magnitude constraint that is based on measured noise power spectral density.
14. The modem of claim 13 wherein the noise power spectral density is measured at an output of the discrete Fourier transform.
15. A receiver for receiving a multiple carrier signal from a communication channel having an impulse response, the receiver comprising:
an analog-to-digital converter coupled to receive the signal from the communication channel; and
a spectrally constrained impulse shortening filter coupled to the analog-to-digital converter and configured to (1) measure received noise power spectral density, (2) compute a cost function using the noise power, the cost function being dependent on the impulse response, (3) reduce the dimensionality of a space over which the cost function is defined and (4) minimize the cost function.
16. A receiver for receiving a multiple carrier signal from a communication channel having an impulse response, the receiver comprising:
an analog-to-digital converter configured to receive the signal from the communication channel; and
a spectrally constrained impulse shortening filter coupled to receive the signal from the analog-to-digital converter and configured to (1) measure received noise power spectral density, (2) compute a target spectral response having a: magnitude constraint that is based on the measured noise power spectral density and (3) select a frequency response of the spectrally constrained impulse shortening filter based on the target spectral response.
17. The receiver of claim 16 wherein the target spectral response is computed from measured noise power spectral density.
18. The receiver of claim 16 , wherein the target spectral response is approximated by the inverse of the measured noise power spectral density.
19. A modem comprising:
a digital-to-analog converter arranged to output a signal to a communication channel;
an analog-to-digital converter arranged to receive the signal; and
a spectrally constrained impulse shortening filter coupled to the analog-to-digital converter and configured to (1) measure received noise power spectral density, (2) compute a cost function using the noise power, the cost function being dependent on the impulse response, (3) reduce the dimensionality of a space over which the cost function is defined and (4) minimize the cost function.
20. The modem of claim 19 comprising:
an encoder arranged to receive digital data and output a constellation of complex values; and
an inverse discrete Fourier transform arranged to receive the constellation;
wherein the inverse discrete Fourier transform is coupled to the analog-to-digital converter.
21. The modem of claim 19 comprising:
a discrete Fourier transform arranged to receive an output of the spectrally constrained impulse shortening filter; and
a decoder coupled to the discrete Fourier transform and arranged to output digital data.
22. The modem of claim 21 wherein the noise power spectral density is measured at an output of the discrete Fourier transform.
23. A modem comprising:
an encoder arranged to receive digital data and output a constellation of complex values;
an inverse discrete Fourier transform coupled to receive the constellation;
a digital-to-analog converter coupled to the inverse discrete Fourier transform and arranged to output a signal to a communication channel;
an analog-to-digital converter arranged to receive the signal; and
a spectrally constrained impulse shortening filter couple to the analog-to-digital converter and configured to (1) measure received noise power spectral density, (2) compute a target spectral response having a magnitude constraint that is based on the measured noise power spectral density and (3) select a frequency response of the spectrally constrained impulse shortening filter based on the target spectral response.
24. A receiver for receiving a multiple carrier signal from a communication channel having an impulse response, the receiver comprising a spectrally constrained impulse shortening filter coupled to an analog-to-digital converter and configured to shorten the impulse response of the channel so that a significant part of an energy of the impulse response is confined to a region that is shorter than a length of a cyclic prefix of the signal.