Frequency domain decision feedback equalizer

• US 7,212,569 B1
• Filed: 04/11/2003
• Issued: 05/01/2007
• Est. Priority Date: 06/28/2002
• Status: Expired due to Fees

• Current Assignee: AT&T Corporation (AT&T, Inc.)
• Original Assignee: AT&T Corporation (AT&T, Inc.)
• Inventors: Clark, Martin Vivian
• Primary Examiner(s): Patel; Jay K.
• Assistant Examiner(s): Lugo; David B.
• Application Number:
  US10/411,684
• Time in Patent Office: 1,481 Days
• Field of Search: 375/229, 375/230, 375/232, 375/233, 375/346, 375/350
• US Class Current: 375/233
• CPC Class Codes: H04L 25/022 of frequency response
• CPC Class Codes: H04L 25/03057 with a recursive structure H04L25/03031 takes precedence
• CPC Class Codes: H04L 25/03159 operating in the frequency domain H04L25/03165, H04L25/03178 take precedence

Abstract

The feedback structure of a frequency domain decision feedback equalizer is implemented in the frequency domain, rather than in the time domain. Advantageously, this permits the synthesis of long feedback filters with a much smaller increase in receiver complexity than is the case when the feedback structure is illustratively implemented in the time domain. Adaptation of the characteristics of the feedback structure of the frequency domain equalizer (as well as its feedforward structure) is carried out based on tentative symbol decisions that are fed back on a block basis. Although possibly rendering the receiver more susceptible to channel estimation errors, this approach renders the receiver capable not only of the post-cursor intersymbol interference cancellation afforded by a standard DFE, but pre-cursor intersymbol interference cancellation, as well, helping to offset any impact of increased channel estimation errors.

Claims

1. A method for processing a received time domain signal representing a sequence of channel symbols, the method comprising

matched-channel filtering a frequency domain representation R(f) of the received signal to generate a frequency domain representation X_MF(f),

linear equalizing said frequency domain representation X_MF(f) to generate a linear equalized frequency domain representation X_LE(f),

combining said frequency domain representation X_MF(f) of the received signal with a frequency domain estimate ISI(f) of intersymbol interference in said representation X_MF(f) to generate an ISI-mitigated frequency-domain representation X_DFE(f) of said received signal, said estimate ISI(f) being a function of a frequency domain representation {circumflex over (X)}_LE(f) of tentative decisions as to the channel symbols, said tentative decisions being formed as a function of said representation X_LE(f), and

generating final decisions as to the channel symbols as a function of said representation X_DFE(f).

2. The method of claim 1 further comprising

generating an estimate of the characteristics of a channel from which said received signal was received, said estimate being generated in response to a frequency domain representation {circumflex over (X)}_DFE(f) of said final decisions and a frequency domain representation of said received signal, and

using said estimate to update parameters used to perform at least one of said matched-channel filtering, said linear equalizing and the generating of said intersymbol interference estimate ISI(f).

3. The method of claim 2 further comprising

generating an estimate {tilde over (β

)} of the in-band noise power β

in said received signal, said estimate {tilde over (β

)} being generated in response to said frequency domain representation {circumflex over (X)}_DFE(f) of said final decisions and a frequency domain representation of said received signal, and

using said estimate {tilde over (β

)} to update at least one of said parameters used to perform said linear equalizing.

4. The method of claim 1 wherein said received time domain signal includes at least two time domain signals representing said sequence of channel symbols and received over different channels.

5. A method comprising

receiving at least first and second time domain signals over respective channels, said first and second time domain signals representing the same sequence of channel symbols,

matched-channel filtering respective frequency domain representations of the first and second received time domain signals to generate first and second matched-channel-filtered signals,

generating a frequency domain representation X_MF(f) that is a function of the first and second matched-channel-filtered signals,

linear equalizing said frequency domain representation X_MF(f) to generate a linear equalized frequency domain representation X_LE(f),

combining said frequency domain representation X_MF(f) of the received signal with a frequency domain estimate ISI(f) of intersymbol interference in said representation X_MF(f) to generate an ISI-mitigated frequency-domain representation X_DFE(f) of said received signal, said estimate ISI(f) being a function of a frequency domain representation {circumflex over (X)}_LE(f) of tentative decisions as to the channel symbols, said tentative decisions being formed as a function of said representation X_LE(f), and

generating final decisions as to the channel symbols as a function of said representation X_DFE(f).

6. The method of claim 5 further comprising

generating estimates of the characteristics of the channels from which said received signals were received, each of said estimates being generated as a function of a frequency domain representation {circumflex over (X)}_DFE(f) of said final decisions and of frequency domain representation of a respective one of the received signals, and

using said estimates to update parameters used to perform at least one of said matched-channel filtering, said linear equalizing and the generating of said intersymbol interference estimate ISI(f).

7. The method of claim 6 further comprising

generating an estimate {tilde over (β

)} of the in-band noise power β

in said received signals, said estimate {tilde over (β

)} being generated in response to said frequency domain representation {circumflex over (X)}_DFE(f) of said final decisions, and

using said estimate {tilde over (β

)} to update at least one of said parameters used to perform said linear equalizing.

8. Apparatus for processing a time block of a signal received from a channel, said received signal block representing a sequence of channel symbols, the apparatus comprising

means for filtering said received signal block with a filter response that is matched to said channel to generate a filtered version of said received signal block,

means for generating in the frequency domain a linear equalized version of said filtered version of said received signal block,

means for forming tentative decisions as to the channel symbols based on said linear equalized version of said received signal block, and

means for generating in the frequency domain a decision-feedback-equalized version of said received signal block that is a function of said received signal block and of a frequency domain estimate of the intersymbol interference in said received signal block, said estimate being formed in response to a frequency domain representation of said tentative decisions.

9. The apparatus of claim 8 wherein said filter response is such as would maximize the signal-to-noise ratio of the received signal block in the absence of intersymbol interference.

10. The apparatus of claim 8 further comprising

means for forming final decisions as to said channel symbols based on the decision-feedback-equalized version of said received signal block.

11. The apparatus of claim 10 further comprising means for updating said filter response as a function of an estimate of the characteristics of said channel, said channel estimate being formed as a function of a frequency domain representation of said final decisions and a frequency domain representation of said received signal block.

12. The apparatus of claim 10 further comprising means for updating parameters used to generate at least one of said linear equalized version of said received signal block and said decision-feedback-equalized version of said received signal block as a function of an estimate of the characteristics of said channel, said channel estimate being formed as a function of a frequency domain representation of said final decisions and a frequency domain representation of said received signal block.