Methods and systems to discriminate betwen PSK and FSK signals

• US 20100316168A1
• Filed: 06/11/2009
• Published: 12/16/2010
• Est. Priority Date: 06/11/2009
• Status: Active Grant

• Current Assignee: L3harris Technologies Integrated Systems LP (L3Harris Technologies, Inc.)
• Original Assignee: L-3 Communications Integrated Systems LP (L3Harris Technologies, Inc.)
• Inventors: Ha, Stephen T., Mahaffey, Michael L., Boyle, Frank A.
• Granted Patent: US 8,275,073 B2
• Time in Patent Office: Days
• US Class Current: 375/322
• CPC Class Codes: H04L 27/0012 arrangements for identifying the type of modulation
• CPC Class Codes: H04L 27/10 Frequency-modulated carrier systems, i.e. using frequency-shift keying H04L27/32 takes precedence
• CPC Class Codes: H04L 27/18 Phase-modulated carrier systems, i.e. using phase-shift keying H04L27/32 takes precedence

Abstract

Methods and systems are disclosed herein for discriminating phase-shift-key (PSK) modulated signals from frequency-shift-key (FSK) modulated signals in received signal streams. First, a bit frequency analysis is applied to a digital signal stream representing the received signal stream. The bit frequency analysis computes bit frequencies for bit segments within the digital signal stream. These bit frequencies are then summed in an array to form a bit frequency histogram. Second, a bit phase analysis is applied to the digital signal stream. The bit phase analysis computes bit phases for bit segments within the digital signal stream. These bit phases are then summed in an array to form a bit phase histogram. Spikes within the bit frequency and bit phase histograms can be used to determine whether the received signal stream includes FSK or PSK signals. An enhanced bit phase analysis can also used, if needed.

Claims

1. A method for discriminating between phase-shift-key (PSK) modulation and frequency-shift-key (PSK) modulation in a received signal stream, comprising:

receiving an analog signal stream with receive path circuitry;

converting the analog signal stream to a digital signal stream;

analyzing bit frequency for a plurality of bit segments within the digital signal stream to determine for a each bit segment a frequency (f_max) at which a maximum signal energy occurs within the bit segment;

combining the maximum frequencies (f_max) to generate a bit frequency histogram for the bit frequency of the plurality of bit segments;

determining that the digital signal stream includes PSK modulated signals if a number of frequency spikes in the bit frequency histogram is one, that the digital signal stream includes FSK modulated signals if a number of frequency spikes in the bit frequency histogram is greater than one, and that a PSK/FSK determination cannot be made if a number of frequency spikes is not determinable; and

outputting PSK/FSK determination signals representing the results of the determining step.

2. The method of claim 1, wherein the analyzing step comprises applying a Fourier transform to each bit segment to determine the frequency (f_max) at which the maximum signal energy occurs within the bit segment.

3. The method of claim 2, wherein the combining step comprises starting with an empty array and adding a peak value to the array at an index equal to the maximum frequency (f_max) for each bit segment to form the bit frequency histogram.

4. The method of claim 1, further comprising:

estimating a carrier frequency;

analyzing a bit phase for a plurality of bit segments within the digital signal stream to determine for a each bit segment a phase shift relative to the estimated carrier frequency;

combining the phase shifts to generate a bit phase histogram for the bit phase of the plurality of bit segments;

determining that the digital signal stream includes FSK modulated signals if no phase spikes occur in the bit phase histogram, that the digital signal stream includes PSK modulated signals if a phase spikes occur in the bit phase histogram, and that a PSK/FSK determination cannot be made if a number of phase spikes is not determinable;

generating PSK/FSK determination signals representing the results of the bit phase determining step; and

outputting the PSK/FSK determination signals relating to the bit phase determining step.

5. The method of claim 4, wherein the estimating step comprises using a frequency spike from the bit frequency histogram as an estimated carrier frequency.

6. The method of claim 4, wherein if the carrier frequency cannot be accurately estimated, the method further comprises:

unwrapping the bit phase to generate a bit phase array representing bit phase correlation values;

applying a Fourier transform to the bit phase array;

utilizing a most common minimum interval between bit phase correlation values in the Fourier transform as a prevalent repetition interval; and

using the prevalent repetition interval to determine an appropriate estimation for the PSK phases.

7. The method of claim 6, further comprising further processing the signal stream using a Radon transform to determine if a PSK signal is using non-standard phase states.

8. The method of claim 1, wherein the receiving step comprises receiving the analog signal stream using an antenna.

9. The method of claim 8, wherein the analog signal stream includes signals generated from one or more radio frequency (RF) systems that are unknown signal sources.

10. The method of claim 9, wherein the one or more RF systems includes a PSK-based system or a FSK-based system.

11. A receiver system for discriminating between phase-shift-key (PSK) modulation and frequency-shift-key (PSK) modulation in a received signal stream, comprising:

receive path circuitry configured to receive an analog signal stream and to output a digital signal stream;

detection circuitry configured to analyze bit frequency for a plurality of bit segments within the digital signal stream to determine for a each bit segment a frequency (f_max) at which a maximum signal energy occurs within the bit segment and to combine the maximum frequencies (f_max) to generate a bit frequency histogram for the bit frequency of the plurality of bit segments;

wherein the detection circuitry is further configured to determine that the digital signal stream includes PSK modulated signals if a number of frequency spikes in the bit frequency histogram is one, that the digital signal stream includes FSK modulated signals if a number of frequency spikes in the bit frequency histogram is greater than one, and that a PSK/FSK determination cannot be made if a number of frequency spikes is not determinable; and

wherein the detection circuitry is further configured to output PSK/FSK determination signals representing the PSK/FSK determination.

12. The receiver system of claim 11, wherein the detection circuitry is further configured to apply a Fourier transform to each bit segment to determine the frequency (f_max) at which the maximum signal energy occurs within the bit segment.

13. The receiver system of claim 12, wherein the detection circuitry is further configured to utilize an array and to add a peak value to the array at an index equal to the maximum frequency (f_max) for each bit segment to form the bit frequency histogram.

14. The receiver system of claim 11, wherein the detection circuitry is further configured to estimate a carrier frequency;

analyze a bit phase for a plurality of bit segments within the digital signal stream to determine for a each bit segment a phase shift relative to the estimated carrier frequency, and to combine the phase shifts to generate a bit phase histogram for the bit phase of the plurality of bit segments;

wherein the detection circuitry is further configured to determine that the digital signal stream includes FSK modulated signals if no phase spikes occur in the bit phase histogram, that the digital signal stream includes PSK modulated signals if phase spikes occur in the bit phase histogram, and that a PSK/FSK determination cannot be made if a number of phase spikes is not determinable; and

wherein the detection circuitry is further configured to output PSK/FSK determination signals representing the PSK/FSK determination relating to the bit phase histogram.

15. The receiver system of claim 14, wherein the detection circuitry is further configured to a frequency spike from the bit frequency histogram as an estimated carrier frequency.

16. The receiver system of claim 14, wherein the detection circuitry is further configured to unwrap the bit phase histogram to generate a bit phase array representing bit phase correlation values, to apply a Fourier transform to the bit phase array;

to utilize a most common minimum interval between bit phase correlation values in the Fourier transform as a prevalent repetition interval and to use the prevalent repetition interval to determine an appropriate estimation for the PSK phases.

17. The receiver system of claim 16, further comprising additional signal processing circuitry configured to apply a Radon transform to determine if a PSK signal is using non-standard phase states.

18. The receiver system of claim 11, further comprising an antenna configured to receive the analog signal stream and to provide the analog signal stream to the receive path circuitry.

19. The receiver system of claim 18, wherein the analog signal stream includes signals generated from one or more radio frequency (RF) systems that are unknown signal sources.

20. The receiver system of claim 19, wherein the one or more RF systems includes a PSK-based system or a FSK-based system.