Pulse or digital communications – Spread spectrum
Reexamination Certificate
1998-06-15
2001-04-17
Pham, Chi (Department: 2631)
Pulse or digital communications
Spread spectrum
C375S144000
Reexamination Certificate
active
06219373
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to the filtering of interference from a spread-spectrum signal, and more particularly to a wavelet-based method of filtering interference from low-power spread-spectrum signals such as those output from GPS or GLONASS satellites.
BACKGROUND OF THE INVENTION
The Global Positioning System (GPS) constellation of satellites transmits a 10-Watt signal from space. The power level of the signal on earth is −166 dBW or about 20 dB below the radio frequency (RF) L-band noise floor of a typical earth environment. Since the GPS signals are spread-spectrum modulated, the process gain of 43 dB (for L2 band) is sufficient to recapture the low-power signal. However, GPS bands are subject to interference from cell phones, television broadcast harmonics as well as the intentional jamming thereof. Because of the widespread adoption of GPS for military and commercial navigation, inadvertent interference as well as jamming from hostile parties represents a potentially serious problem.
Several antenna concepts have been developed to mitigate interference for GPS receivers. For example, controlled-radiation pattern antennas (CRPA) are currently being considered in several weapons systems as an anti-jam element. The CRPA operates using spatial filtering techniques. More specifically, assuming a stable platform and constant radiation, a CRPA drives an antenna null in a direction of received interference. However, drawbacks associated with CRPA's include it's cost, variable convergence delay, aerodynamic penalty, radar cross-section and performance limits.
As an alternative to antenna-based anti-jam systems and methods, classical digital signal processing (DSP) can be used to discriminate signals in the electromagnetic spectral frequency. Classical DSP (e.g., uniform scale Fourier Transform) is best applied to stationary signals or those signals having constant statistical properties. However, if the interference is dynamic (i.e., non-stationary), neither classical DSP techniques nor CRPA's perform well.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method of filtering interference from a spread-spectrum signal.
Another object of the present invention is to provide a method of filtering interference from low-power spread-spectrum signals occurring below the receiving environment's noise floor.
Still another object of the present invention is to provide a method of filtering dynamic types of interference from GPS signals.
Yet another object of the present invention is to provide a method of filtering interference from GPS signals utilizing existing GPS receiver hardware and systems.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
In accordance with the present invention, a method is provided for filtering out interference from a spread-spectrum signal such as GPS signals. A wavelet packet decomposition scheme is applied to a spread-spectrum signal having a known noise floor to generate a first plurality of coefficient sets. Each coefficient set corresponds to a unique wavelet decomposition level. An entropy measurement is determined for each wavelet decomposition level using a corresponding one of the first plurality of coefficient sets. Portions of the spread-spectrum signal occurring above the noise floor for the wavelet decomposition level having the lowest entropy measurement are then filtered out so that a revised coefficient set is generated therefor. As a result, a second plurality of coefficient sets can be formed such that the first plurality of coefficient sets differs from the second plurality of coefficient sets only by the revised coefficient set. A new signal is formed by applying a wavelet packet reconstruction scheme to the second plurality of coefficient sets.
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Hansen Grant A.
Kuo Wei
Lee David A.
Bayard Emmanuel
Kalmbaugh David
Pham Chi
The United States of America as represented by the Secretary of
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