Carrier phase offset correction for a QAM system
Systems and methods are described that may be used to detect and correct carrier phase offset in a signal. A phase offset corrector receives an equalized signal representative of a quadrature amplitude modulated signal and derives a phase-corrected signal from the equalized signal. The equalized signal is sliced to obtain real and imaginary sequences and a frame synchronizer performs a correlation of the real and imaginary sequences with corresponding parts of a stored frame-sync pseudo-random sequence. Phase correction is based on the maximum real and imaginary values of the correlation. The signal is typically quadrature amplitude modulated signal is modulated using punctured trellis codes. Quadrature phase shift keying modulation, 16-QAM, 64-QAM and other QAM schemes may be used.
1. A system embodied in a modem, the system comprising:
a phase offset corrector that receives an equalized signal representative of a quadrature amplitude modulated signal and derives a phase-corrected signal from the equalized signal;
a two-level slicer that slices the equalized signal to obtain real and imaginary sequences;
a frame synchronizer that performs a correlation of the real and imaginary sequences with corresponding parts of a stored frame-sync pseudo-random sequence; and
a phase correction signal provided by the frame synchronizer to the phase offset corrector, wherein the phase correction signal is based on the maximum real and imaginary values of the correlation, and wherein the real and imaginary parts of the frame sync symbols of the quadrature amplitude modulated signal have the same sign, and the signs of the maximum real and imaginary values of the correlation are indicative of phase rotation in the equalized signal.
2. The system of claim 1 , wherein the frame synchronizer performs continuous cross-correlation on incoming sliced quadrature amplitude modulated symbols.
3. The system of claim 2 , wherein the continuous cross-correlation is performed separately for the real and imaginary sequences with a stored copy of a binary frame-sync pseudo-random noise sequence.
4. The system of claim 1 , wherein the quadrature amplitude modulated signal is modulated using punctured trellis codes.
5. The system of claim 1 , wherein the quadrature amplitude modulated signal is modulated using quadrature phase shift keying modulation.
6. The system of claim 1 , wherein the quadrature amplitude modulated (QAM) signal is modulated using 16-QAM.
7. The system of claim 1 , wherein the quadrature amplitude modulated (QAM) signal is modulated using 64-QAM.
8. The system of claim 1 , wherein the phase correction signal provided by the frame synchronizer comprises the signs of the maximum real and imaginary values of the correlation.
9. The system of claim 1 , wherein the phase offset corrector derives the phase-corrected signal by indexing a lookup table with the signs of the maximum real and imaginary values of the correlation to determine a phase correction value.
10. A method for correcting carrier phase offset in a quadrature amplitude modulated signal in a receiver, the method comprising:
equalizing the signal;
slicing the equalized signal, thereby obtaining real and imaginary sequences from the equalized signal;
identifying a frame synchronization sequence in the real and imaginary sequences, wherein identifying the frame synchronization sequence includes correlating a stored pseudo-random sequence with the real and imaginary sequences, and determining a start of a frame from maximum correlation values associated with the real and imaginary sequences; and
correcting a phase error in the equalized signal based on the maximum correlation values,
wherein the correlating step includes performing continuous cross-correlation on a stored copy of the frame synchronization sequence separately with the real and imaginary sequences, and
wherein real and imaginary parts of frame sync symbols in the frame synchronization sequence have the same sign.
11. The method of claim 10 , wherein the correlating step includes performing continuous cross-correlation on a series of sliced quadrature amplitude modulated symbols with a stored copy of a binary frame-sync pseudo-random noise sequence.
12. The method of claim 10 , wherein correcting a phase error includes determining phase rotation in the equalized signal based on the signs of the maximum correlation values.
13. The system of claim 12 , wherein correcting a phase error in the equalized signal includes indexing a lookup table with the signs of the real and imaginary maximum correlation values.
14. A method for correcting carrier phase offset in a quadrature amplitude modulated signal, wherein the method is implemented in a system comprising one or more processors configured to execute instructions, the method comprising:
executing, on the one or more processors, instructions configured to equalize the signal;
executing, on the one or more processors, instructions configured to slice the equalized signal, thereby obtaining real and imaginary sequences from the equalized signal;
executing, on the one or more processors, instructions configured to identify a frame synchronization sequence in the real and imaginary sequences, wherein identifying the frame synchronization sequence includes performing continuous cross-correlation on a stored copy of the frame synchronization sequence separately with the real and imaginary sequences, and determining a start of a frame from maximum correlation values associated with the real and imaginary sequences; and
executing, on the one or more processors, instructions configured to correct a phase error in the equalized signal based on the maximum correlation values, wherein the real and imaginary parts of the frame sync symbols of the frame synchronization sequence have the same sign, and wherein correcting a phase error includes determining phase rotation in the equalized signal based on the signs of the maximum correlation values.
15. A non-transitory computer-readable medium comprising code for:
equalizing the signal;
slicing the equalized signal, thereby obtaining real and imaginary sequences from the equalized signal;
identifying a frame synchronization sequence in the real and imaginary sequences, wherein identifying the frame synchronization sequence includes performing continuous cross-correlation on a stored copy of the frame synchronization sequence separately with the real and imaginary sequences, and determining a start of a frame from maximum correlation values associated with the real and imaginary sequences; and
correcting a phase error in the equalized signal based on the maximum correlation values, wherein the real and imaginary parts of the frame sync symbols of the frame synchronization sequence have the same sign, and wherein correcting a phase error includes determining phase rotation in the equalized signal based on the signs of the maximum correlation values.