Low latency analog QAM coherent demodulation algorithm

• US 20090022245A1
• Filed: 05/24/2006
• Published: 01/22/2009
• Est. Priority Date: 05/24/2006
• Status: Active Grant

• Current Assignee: General Dynamics Mission Systems Inc. (General Dynamics Corporation)
• Original Assignee: General Dynamics Advanced Information Systems Incorporated (General Dynamics Corporation)
• Inventors: Binkley, Michael G.
• Granted Patent: US 7,801,250 B2
• Time in Patent Office: Days
• US Class Current: 375/319
• CPC Class Codes: H04L 27/3818 using coherent demodulation, i.e. using one or more nominally phase synchronous carriers H04L27/227 and H04L27/389 take precedence

Abstract

A digital demodulation method for a quadrature amplitude modulated signal uses a phase locked loop to generate a local carrier signal. The phase locked loop uses a feedback signal derived from one or more demodulated signals of interest. The loop has a filter characteristic with a stop band within the information bandwidth(s) of the information signal(s). The preferred method generates an error signal from DC components of in-phase and quadrature-phase baseband signals. DC components are preferably isolated using a low-latency, AC rejection filter.

Claims

1. A digital demodulation method for a QAM signal carrying an information signal of interest comprising:

downconverting the information signal of interest from the QAM signal using a local carrier signal;

using a phase-locked loop to generate the local carrier signal in response to a feedback signal derived from the downconverted signal of interest, said phase-locked loop having a filter characteristic with a stop band within the information bandwidth of the information signal of interest.

2. The method of claim 1 wherein the step of using a phase-locked loop comprises a step of estimating a phase error between the QAM carrier and the local carrier using a DC component of the demodulated signal of interest.

3. The method of claim 1 wherein the filter characteristic has a pass band between about zero hertz and about tens of hertz in the information signal bandwidth.

4. The method of claim 1 wherein the filter characteristic is a low pass filter with a cut-off between about zero hertz and about ones of hertz in the information signal bandwidth.

5. The method of claim 1 wherein the filter characteristic is a low pass filter with a cut-off between about zero hertz and about two hertz in the information signal bandwidth.

6. The method of claim 1 wherein the phase-locked loop includes an AC rejection filter having a stop band within the information bandwidth of the information signal of interest.

7. The method of claim 1 wherein the phase-locked loop filter includes a low-pass filter having a stop band within the information bandwidth of the information signal of interest.

8. The method of claim 1 wherein the filter characteristic is implemented with a latency of less than about thousands of samples.

9. The method of claim 1 wherein the filter characteristic is implemented with a latency of less than about hundreds of samples.

10. The method of claim 1 wherein the filter characteristic is implemented with a latency of less than about tens of samples.

11. A digital method of demodulating a QAM signal having first and second information signals modulated on in-phase and quadrature phase channels of a carrier, the method comprising:

forming a first information signal by downconverting the QAM signal using a first locally-generated carrier signal;

forming a second information signal by downconverting the QAM signal using a second locally-generated carrier signal that is out of phase with first locally-generated carrier signal;

isolating DC components of the first and second downconverted signals, said DC components excluding at least some frequency components in the information bandwidths of the information signals;

deriving an error signal from the isolated DC components; and

adjusting a phase of a locally-generated carrier signal according to the error signal.

12. The method of claim 11 wherein the step of isolating DC components includes a step of excluding frequency components above about tens of hertz in an information signal bandwidth.

13. The method of claim 11 wherein the step of isolating DC components includes a step of excluding frequency components above about ones of hertz in an information signal bandwidth.

14. The method of claim 11 wherein the step of isolating DC components includes a step of excluding frequency components above about two hertz in an information signal bandwidth.

15. The method of claim 11 wherein the step of isolating DC components is implemented with a latency of less than about thousands of samples.

16. The method of claim 11 wherein the step of isolating DC components is implemented with a latency of less than about hundreds of samples.

17. The method of claim 11 wherein the step of isolating DC components is implemented with a latency of less than about tens of samples.

18. The method of claim 11 wherein the step of isolating DC components is implemented with an AC rejection filter.

19. A demodulator for a digitized QAM signal carrying first and second information signals, the demodulator comprising:

a first downconverter generating a first downconverted information signal in response to the QAM signal and a local carrier signal of a first phase;

a second downconverter generating a second downconverted information signal in response to the QAM signal and a local carrier signal of a second phase and;

an oscillator generating the local carrier signal in response to an error signal;

wherein the error signal is derived from components of the first and second downconverted information signals, said components including DC components but attenuating AC components in the information signal information bandwidths.

20. The demodulator of claim 19 wherein the error signal is derived by isolating DC components from AC components using an AC rejection filter;

21. The demodulator of claim 19 wherein the error signal is derived by isolating DC components from AC components at a cutoff above about tens of hertz in the information signal bandwidth.

22. The demodulator of claim 19 wherein the error signal is derived by isolating DC components from AC components at a cutoff above about ones of hertz in the information signal bandwidth.

23. The demodulator of claim 19 wherein the error signal is derived by isolating DC components from AC components at a cutoff above about two hertz in the information signal bandwidth.

24. The demodulator of claim 19 wherein the DC components are derived from the downconverted information signals with a latency of less than about thousands of samples.

25. The demodulator of claim 19 wherein the DC components are derived from the downconverted information signals with a latency of less than about tens of samples.