Method and apparatus for phase reference recovery in high speed fast turnaround modem

In a data communications receiver, of the type comprising a data decoder, a complex adaptive equalizer for providing an equalized baseband data signal in response to a received baseband data signal, and a phase locked loop comprising a loop filter, for receiving and decoding data encoded in said received baseband signal using a signal constellation consisting of a plurality of discrete points, each of said discrete points having a particular amplitude and a particular phase relative to a predetermined reference phase;

• an improved phase reference recovery circuit for detecting said predetermined reference phase in said received signal, comprising in combination:
• a first constellation decision circuit for providing a first detected data signal and a first complex error signal in response to said equalized baseband signal;
• phase shifting means for providing a shifted baseband signal in response to said equalized baseband signal, said shifted baseband signal having a phase which differs from the phase of said equalized baseband signal by a predetermined phase increment;
• a second constellation decision circuit for providing a second detected data signal and a second complex error signal in response to said shifted baseband signal;
• means for providing an angular error signal in response to a selected one of said received baseband signal and said shifted baseband signal;
• means for providing said angular error signal to said loop filter;
• means for providing a first magnitude error signal in response to said first complex error signal and a second magnitude error signal in response to said second complex error signal;
• comparator means for comparing said first and second magnitude error signals and for providing a selection signal in response to the relative values of said first and second magnitude error signals;
• and
• selection means connected to said comparator means and to said first and second constellation decision circuits for providing one of said first and second detected data signals to said data decoder and one of said first and second complex error signals to said adaptive equalizer in response to said selection signal.

A method of locking a data communications receiver onto a predetermined reference phase of received phase modulated signal of the type using a signal constellation consisting of a plurality of discrete points, each of said discrete points having a particular amplitude and a particular phase relative to said predetermined reference phase, comprising the steps of:

• providing a phase locked loop with a loop filter;
• providing a first constellation decision circuit which produces a first detected data signal and a first magnitude error signal in response to said received phase modulated signal;
• providing a shifted phase modulated signal in response to said received phase modulated signal, said shifted phase modulated signal having a phase which differs from the phase of said received phase modulated signal by a predetermined phase increment;
• providing a second constellation decision circuit which produces a second detected data signal and a second magnitude error signal in response to said shifted phase modulated signal;
• providing an angular error signal in response to a selected one of said received phase modulated signal and said shifted phase modulated signal;
• providing said angular error signal to said loop filter;
• comparing said first and second magnitude error signals;
• providing a selection signal in response to the relative values of said first and second magnitude error signals;
• and
• selecting one of said first and second detected data signals and providing same to a data decoder in response to said selection signal.