Carrier recovery system using acquisition and tracking modes and automatic carrier-to-noise estimation

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;
• 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.

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:

• 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.

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:

• 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.

Apparatus for estimating a carrier-to-noise ratio (CNR) of symbols recovered from a quadrature amplitude modulated (QAM) signal comprising:

• 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.

A method of estimating a carrier-to-noise ratio (CNR) of symbols recovered from a quadrature amplitude modulated (QAM) signal comprising the steps of:

• 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.