Carrier recovery system using acquisition and tracking modes and automatic carrier-to-noise estimation
First Claim
1. In a communications receiver, apparatus for recovering a carrier signal from a received signal and for producing de-rotated symbols therefrom, said apparatus comprising:
- means, responsive to an incoming stream of symbols obtained from said received signal and to a reference carrier signal, for producing a stream of de-rotated symbols;
tracking slicing means for converting each one of said de-rotated symbols into a corresponding first sliced value so as to define a stream of first sliced values, wherein each of said first sliced values defines a corresponding symbol in a pre-selected constellation;
acquisition slicing means for converting ones of said de-rotated symbols into corresponding second sliced values, wherein each one of said second sliced values is specified by a pre-defined attribute of a corresponding one of said de-rotated symbols;
means, operative in response to a mode select signal, for comparing said each one of said de-rotated symbols to a corresponding one of selectively either said first or second sliced values in order to generate a phase error signal;
means, operative in response to said stream of first sliced values and to said stream of de-rotated symbols, for producing a mode error signal;
means, responsive to said mode error signal and a switching threshold value, for producing said mode select signal; and
means, responsive to said phase error signal, for generating said reference carrier.
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Accused Products
Abstract
A carrier recovery system for use in illustratively a passband QAM (quadrature amplitude modulation) demodulator (10) and specifically such a system, including apparatus and accompanying methods, that employs separate acquisition and tracking modes and automatic carrier-to-noise estimation. In particular, a carrier recovery system (200) is operated in two modes: an acquisition mode to first attain an initial carrier lock, during which simplified reduced constellation slicing is used to determine a quadrant of each incoming symbol; and a tracking (or lock) mode, during which full slicing is used, to accurately track variations in frequency and phase that may occur to a carrier while the system remains locked. The mode changes based upon whether, e.g., the average error energy associated with fully sliced symbols is greater than or less than a pre-defined switching threshold value. The value of the switching threshold changes in a pre-determined manner with changes in noise content, particularly a carrier-to-noise ratio (CNR), of the received symbols. Two appropriately spaced switching threshold values, one for use in each mode, can be used to provide enhanced operational stability. An implementation of a QAM CNR estimator circuit (800) is also provided.
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Citations
60 Claims
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1. In a communications receiver, apparatus for recovering a carrier signal from a received signal and for producing de-rotated symbols therefrom, said apparatus comprising:
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means, responsive to an incoming stream of symbols obtained from said received signal and to a reference carrier signal, for producing a stream of de-rotated symbols; tracking slicing means for converting each one of said de-rotated symbols into a corresponding first sliced value so as to define a stream of first sliced values, wherein each of said first sliced values defines a corresponding symbol in a pre-selected constellation; acquisition slicing means for converting ones of said de-rotated symbols into corresponding second sliced values, wherein each one of said second sliced values is specified by a pre-defined attribute of a corresponding one of said de-rotated symbols; means, operative in response to a mode select signal, for comparing said each one of said de-rotated symbols to a corresponding one of selectively either said first or second sliced values in order to generate a phase error signal; means, operative in response to said stream of first sliced values and to said stream of de-rotated symbols, for producing a mode error signal; means, responsive to said mode error signal and a switching threshold value, for producing said mode select signal; and means, responsive to said phase error signal, for generating said reference carrier. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18)
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19. In a communications receiver, apparatus for recovering a carrier signal from a received signal and for producing de-rotated symbols therefrom, said apparatus comprising:
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means, responsive to an incoming stream of symbols obtained from said received signal and to a reference carrier signal, for producing a stream of de-rotated symbols; tracking slicing means for converting each one of said de-rotated symbols into a corresponding first sliced value so as to define a stream of first sliced values, wherein each of said first sliced values defines a corresponding symbol in a pre-selected constellation; acquisition slicing means for converting ones of said de-rotated symbols into corresponding second sliced values, wherein each one of said second sliced values is specified by a pre-defined attribute of a corresponding one of said de-rotated symbols; means, operative in response to a mode select signal, for comparing said each one of said de-rotated symbols to a corresponding one of selectively either said first or second sliced values in order to generate a phase error signal; means, operative in response to said stream of first sliced values and to said stream of de-rotated symbols, for producing a mode error signal; means, responsive to said mode error signal and a switching threshold value, for producing said mode select signal; means for varying said switching threshold value in response to variations in said de-rotated symbols; and means for generating said reference carrier in response to said phase error signal. - View Dependent Claims (20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31)
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32. In a communications receiver, a method for recovering a carrier signal from a received signal and for producing de-rotated symbols therefrom, said method comprising the steps of:
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producing, in response to an incoming stream of symbols obtained from said received signal and to a reference carrier signal, a stream of de-rotated symbols; first converting each one of said de-rotated symbols into a corresponding first sliced value so as to define a stream of first sliced values, wherein each of said first sliced values defines a corresponding symbol in a pre-selected constellation; second converting ones of said de-rotated symbols into corresponding second sliced values, wherein each one of said second sliced values is specified by a pre-defined attribute of a corresponding one of said de-rotated symbols; comparing, in response to a mode select signal, said each one of said de-rotated symbols to a corresponding one of selectively either said first or second sliced values in order to generate a phase error signal; producing, in response to said stream of first sliced values and to said stream of de-rotated symbols, a mode error signal; producing, in response to said mode error signal and a switching threshold value, said mode select signal; and generating said reference carrier in response to said phase error signal. - View Dependent Claims (33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43)
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44. In a communications receiver, a method for recovering a carrier signal from a received signal and for producing de-rotated symbols therefrom, said method comprising:
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producing, in response to an incoming stream of symbols obtained from said received signal and to a reference carrier signal, a stream of de-rotated symbols; first converting each one of said de-rotated symbols into a corresponding first sliced value so as to define a stream of first sliced values, wherein each of said first sliced values defines a corresponding symbol in a pre-selected constellation; second converting ones of said de-rotated symbols into corresponding second sliced values, wherein each one of said second sliced values is specified by a pre-defined attribute of a corresponding one of said de-rotated symbols; comparing, in response to a mode select signal, said each one of said de-rotated symbols to a corresponding one of selectively either said first or second sliced values in order to generate a phase error signal; producing, in response to said stream of first sliced values and to said stream of de-rotated symbols, a mode error signal; producing, in response to said mode error signal and a switching threshold value, said mode select signal; varying said switching threshold value in response to variations in said de-rotated symbols; and generating said reference carrier in response to said phase error signal. - View Dependent Claims (45, 46, 47, 48, 49, 50, 51, 52, 53, 54)
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55. Apparatus for estimating a carrier-to-noise ratio (CNR) of symbols recovered from a quadrature amplitude modulated (QAM) signal comprising:
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means for ascertaining whether each of said symbols has a symbol magnitude that falls within any one of a group of pre-defined symbol magnitude ranges and, in response thereto, producing an output value indicative of a number of said symbols each of which has a symbol magnitude falling within any one of said ranges, said ranges being defined by corresponding symbol magnitude rings of a QAM constellation containing said symbols, said ascertaining means comprising; means for generating a magnitude squared value of each of said symbols; and a multi-level histogram comparator for classifying whether the magnitude squared value of each of said symbols falls within any one of said pre-defined ranges and for producing the output value; means for filtering the output value so as to generate a filtered value, said filtering means being a infinite impulse response filter, a finite impulse response filter or a periodically reset accumulator; and a pre-defined look-up table for mapping said filtered value into a CNR estimate. - View Dependent Claims (56, 57)
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58. A method of estimating a carrier-to-noise ratio (CNR) of symbols recovered from a quadrature amplitude modulated (QAM) signal comprising the steps of:
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ascertaining whether each of said symbols has a symbol magnitude that falls within any one of a group of pre-defined symbol magnitude ranges and, in response thereto, producing an output value indicative of a number of said symbols each of which has a symbol magnitude falling within any one of said ranges, said ranges being defined by corresponding symbol magnitude rings of a QAM constellation containing said symbols, said ascertaining step comprising the steps of; generating a magnitude squared value of each of said symbols; and classifying, through a multi-level histogram comparator, whether the magnitude squared value of each of said symbols falls within any one of said pre-defined ranges and for producing the output value; filtering the output value, through an infinite impulse response filter, a finite impulse response filter or a periodically reset accumulator, so as to generate a filtered value; and mapping, through a pre-defined look-up table, the filtered value into a CNR estimate. - View Dependent Claims (59, 60)
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Specification