US 5,930,296 A · Low-complexity bidirectional equalizer · patentgrep

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Abstract

Disclosed are bidirectional equalizer arrangements in which the outputs of the forward and reverse equalizers are combined using low complexity processing to yield a single output sequence. In each disclosed arrangement of the invention, the low complexity processing utilizes the equalizer error outputs of the forward and reverse equalizers, which are then zero-delay lowpass filtered to provide relative reliability estimates. In one arrangement of the invention, these reliability estimates are used to select, on a symbol-by-symbol basis, either the forward or the reverse equalizer output. In another arrangement of the invention, a single linear lowpass filter is used to filter the difference of the equalizer error signals. In another arrangement of the invention, the relative reliability estimates are used to form a weighted average of the forward and reverse equalizer pre-decision signals. This weighted-average complex-valued signal then acts as a new equalized signal, and is fed to a constellation decision device to form a symbol decision. The arrangements of the invention can provide the majority of the gain offered by schemes that optimally (in a maximum-likelihood sense) consider all possible sequences formed from the bidirectional equalizers, while avoiding the optimal scheme's complexity which exhibits an exponential dependence on the duration of the channel impulse response.

Claims17

Dependent claims

A bidirectional equalizer comprising:

first and second equalizers, said first and second equalizers receiving and processing the same sequence of digital signal samples, said sequence of digital signal samples being representative of data symbols to be decoded by said bidirectional equalizer, said first equalizer sequentially processing all of said sequence of digital signal samples in a forward direction relative to the time at which digital signal samples of said sequence are received by said bidirectional equalizer to provide a corresponding first sequence of output signals, said second equalizer sequentially processing all of said sequence of digital signal samples in a reverse direction relative to the time at which said digital signal samples of said sequence are received by said bidirectional equalizer to provide a corresponding second sequence of output signals, said first equalizer supplying a first error signal representative of equalizer error for each output signal of said first sequence of output signals, said second equalizer supplying a second error signal representative of equalizer error for each output signal of said second sequence of output signals;
filter means responsive to and processing said first and second error signals, said filter means exhibiting a lowpass filter function to supply at least one lowpass filtered output signal;
and
signal selection means responsive to said at least one lowpass filtered output signal, said signal selection means operating in synchronism with the first and second sequences of output signals to select an output signal from said first sequence of output signals or from said second sequence of output signals as an output signal of said bidirectional equalizer.

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A bidirectional equalizer comprising:

first and second equalizers, said first and second equalizers being decision feedback equalizers, said first and second equalizers receiving and processing the same sequence of digital signal samples, said sequence of digital signal samples being representative of data symbols to be decoded by said bidirectional equalizer, said first equalizer sequentially processing said sequence of digital signal samples in a forward direction relative to the time at which digital signal samples of said sequence are received by said bidirectional equalizer to provide a corresponding first sequence of output signals, said second equalizer sequentially processing said sequence of digital signal samples in a reverse direction relative to the time at which said digital signal samples of said sequence are received by said bidirectional equalizer to provide a corresponding second sequence of output signals, said first equalizer supplying a first error signal representative of equalizer error for each output signal of said first sequence of output signals, said second equalizer supplying a second error signal representative of equalizer error for each output signal of said second sequence of output signals;
filter means responsive to and processing said first and second error signals, said filter means exhibiting a lowpass filter function to supply at least one lowpass filtered output signal;
and
signal selection means responsive to said at least one lowpass filtered output signal, said signal selection means operating in synchronism with the first and second sequences of output signals to select an output signal from said first sequence of output signals or from said second sequence of output signals as the output signal of said bidirectional equalizer;
wherein said filter means is a linear lowpass filter with a zero-phase response characteristic and wherein said bidirectional equalizer further comprises signal summing means responsive to said first and second error signals for supplying an input error difference signal representative of the difference between said first and second error signals, said difference signal being supplied to said zero-phase lowpass filter, said zero-phase lowpass filter supplying said at least one lowpass filtered output signal that is indicative of said first and second error signals to said signal selection means, said signal selection means including means for selecting the output signal of said first sequence of output signals when said signal supplied by said zero-phase lowpass filter indicates that lowpass filtered content of said first error signal is less than lowpass filtered content of said second error signal and for selecting the output signal of said second sequence of output signals when said signal supplied by said zero-phase lowpass filter indicates that lowpass filtered content of said second error signal is less than lowpass filtered content of said first error signal.

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A bidirectional equalizer comprising:

first and second equalizers, said first and second equalizers being decision feedback equalizers, said first and second equalizers receiving and processing the same sequence of digital signal samples, said sequence of digital signal samples being representative of data symbols to be decoded by said bidirectional equalizer, said first equalizer sequentially processing said sequence of digital signal samples in a forward direction relative to the time at which digital signal samples of said sequence are received by said bidirectional equalizer to provide a corresponding first sequence of output signals, said second equalizer sequentially processing said sequence of digital signal samples in a reverse direction relative to the time at which said digital signal samples of said sequence are received by said bidirectional equalizer to provide a corresponding second sequence of output signals, said first equalizer supplying a first error signal representative of equalizer error for each output signal of said first sequence of output signals, said second equalizer supplying a second error signal representative of equalizer error for each output signal of said second sequence of output signals;
filter means responsive to and processing said first and second error signals, said filter means exhibiting a lowpass filter function to supply at least one lowpass filtered output signal;
and
signal selection means responsive to said at least one lowpass filtered output signal, said signal selection means operating in synchronism with the first and second sequences of output signals to select an output signal from said first sequence of output signals or from said second sequence of output signals as the output signal of said bidirectional equalizer;
wherein:
said filter means comprises first and second zero-delay lowpass filters, said first zero-delay lowpass filter receiving and processing said first error signal to supply a first lowpass filtered error signal, said second zero-delay filter receiving and processing said second error signal to supply a second lowpass filtered error signal;
and
said signal selection means includes a signal comparator responsive to said first and second lowpass filtered error signals, said signal comparator supplying a switch control signal representative of said first and second lowpass filtered error signals, said signal selection means further including switching means responsive to said switch control signal supplied by said signal comparator, said switching means being activated by said switch control signal to supply the output signal of said first sequence of output signals as the output of said bidirectional equalizer when said first filtered error signal is less than said second filtered error signal, said switch signal supplied by said signal comparator activating said switching means to supply the output signal of said second sequence of output signals as the output signal of said bidirectional equalizer when said first filtered error signal exceeds said second filtered error signal.

5 more elements

A bidirectional equalizer comprising:

first and second equalizers, said first and second equalizers receiving and processing the same sequence of digital signal samples, said sequence of digital signal samples being representative of data symbols to be decoded by said bidirectional equalizer, said first equalizer sequentially processing all of said sequence of digital signal samples in a forward direction to the time at which digital signal samples of said sequence are received by said bidirectional equalizer to provide a corresponding first sequence of equalized signals and a corresponding first sequence of output signals, said second equalizer sequentially processing all of said sequence of digital signal samples in a reverse direction relative to the time at which said digital signal samples of said sequence are received by said bidirectional equalizer to provide a corresponding second sequence of equalized signals, and a corresponding second sequence of output signals, said first equalizer supplying a first error signal representative of a difference between each output signal and the corresponding equalized signal of said first sequence of output signals, said second equalizer supplying a second error signal representative of a difference between each output signal and the corresponding equalized signal of said second sequence of output signals;
first and second zero-delay lowpass filters, said first zero-delay lowpass filter receiving and processing said first error signal to supply a first lowpass filtered error signal, said second zero-delay filter receiving and processing said second error signal to supply a second lowpass filtered error signal, and
signal combining means for receiving and processing said first and second sequences of equalized signals and said first and second lowpass filtered error signals, said signal combining means producing a sequence of equalized signal that is the weighted average of said first and second equalized signals.

A bidirectional equalizer comprising:

first and second equalizers, said first and second equalizers receiving and processing the same sequence of digital signal samples, said sequence of digital signal samples being representative of data symbols to be decoded by said bidirectional equalizer, said first equalizer sequentially processing said sequence of digital signal samples in a forward direction to the time at which digital signal samples of said sequence are received by said bidirectional equalizer to provide a corresponding first sequence of equalized signals and a corresponding first sequence of output signals, said second equalizer sequentially processing said sequence of digital signal samples in a reverse direction relative to the time at which said digital signal samples of said sequence are received by said bidirectional equalizer to provide a corresponding second sequence of equalized signals, and a corresponding second sequence of output signals, said first equalizer supplying a first error signal representative of a difference between each output signal and the corresponding equalized signal of said first sequence of output signals, said second equalizer supplying a second error signal representative of a difference between each output signal and the corresponding equalized signal of said second sequence of output signals;
first and second zero-delay lowpass filters, said first zero-delay lowpass filter receiving and processing said first error signal to supply a first lowpass filtered error signal, said second zero-delay filter receiving and processing said second error signal to supply a second lowpass filtered error signal, and
signal combining means for receiving and processing said first and second sequences of equalized signals and said first and second lowpass filtered error signals, said signal combining means producing an equalized signal that is the weighted average of said first and second sequence of equalized signals;
wherein said signal combining means includes means for determining a relative reliability term that is based on said first and second lowpass filtered error signals and wherein said relative reliability term determines said weighted average of said first and second sequence of equalized signals, wherein said relative reliability term is of the form e.sub.f (k)/(e.sub.f (k)+e.sub.r (k)), where e.sub.f (k) represents the kth signal of said first lowpass filtered error signals and e.sub.r (k) represents the kth signal of said second lowpass filtered error signals.

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A method for bidirectional equalization of a received sequence of digital signal samples comprising the steps of:

sequentially processing said sequence of digital signal samples in a forward time direction to supply a first sequence of output signals and a corresponding first sequence of equalization error signals;
sequentially processing said received sequence of digital sequence samples in a reverse time direction to supply a second sequence of output signals and a corresponding second sequence of equalization error signals;
lowpass filtering said first and second sequence of error signals to supply at least one lowpass filtered estimated error signal;
generating an estimated reliability signal based upon said at least one lowpass filtered estimated error signal;
selecting for output of said bidirectional equalization method the first sequence of output signals or the second sequence of output signals, with said step of selecting being based upon said estimated reliability signal;
wherein said step of lowpass filtering includes the steps of separately filtering said first and second equalization error signals to provide first and second lowpass filtered estimated error signals;
comparing said first and second lowpass filtered estimated error signals;
selecting the output signal of said first sequence of output signals when said first lowpass filtered estimated error signal is less than said second lowpass filtered estimated error signal;
and, selecting the output signal of said second sequence of output signals when said first lowpass filtered estimated error signal is greater than said second lowpass filtered estimated error signal.

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Dependencies are inferred from the claim text.