Telecommunications – Receiver or analog modulated signal frequency converter – With wave collector
Reexamination Certificate
2003-03-27
2004-05-11
Urban, Edward F. (Department: 2685)
Telecommunications
Receiver or analog modulated signal frequency converter
With wave collector
C455S273000, C455S283000, C455S562100
Reexamination Certificate
active
06735427
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to signal acquisition and processing. In particular, the present invention relates to a system that combines Adaptive Antenna Arraying and Adaptive Locally-Optimum Detection signal processing techniques and achieves the capabilities of both techniques. The system automatically distributes interference signals among Adaptive Antenna Arraying and Adaptive Locally-Optimum Detection sub-systems according to which sub-system's algorithms can provide a more optimum suppression of the interference signals. The present invention also relates to a system that combines Adaptive Filtering and Adaptive Locally-Optimum Detection signal processing techniques and achieves the capabilities of both techniques. The present invention also relates to a system that combines Adaptive Filtering, Adaptive Antenna Arraying and Adaptive Locally-Optimum Detection signal processing techniques and achieves the capabilities of all three techniques. The system is primarily designed for incorporation within systems using Direct Sequence Spread Spectrum signals, such as Global Positioning System (GPS) signals. In a preferred embodiment the inventive system is incorporated within a conventional GPS receiver, which tends to be vulnerable to jamming by interference signals.
2. Description of the Related Art
Adaptive Filtering, Adaptive Antenna Arraying and Adaptive Locally-Optimum Detection are three complementary signal processing techniques used by an Adaptive Filter (AF) system, an Adaptive Antenna Array (AAA) system and an Adaptive Locally-Optimum Detector (ALOD) system, respectively, for detecting weak signals (such as radio and radar) in the presence of strong man-made interference or jamming signals.
Each one of these techniques has its own limitations. An AF, such as Frequency Excision (FX) or Adaptive Wiener Filtering (AWF), can be effective against a large number of narrowband jammers, but its performance declines with interference bandwidth. An AF is ineffective against full-band jammers. An AAA, such as a Controlled Radiation Pattern Antenna (CRPA), is effective against all types of jamming, but requires an array of multiple antenna elements. An ALOD, such as Nonlinear Adaptive Processor (NONAP) technology developed by The Johns Hopkins University/Applied Physics Lab (JHU/APL), achieves gain against all power-efficient (and therefore non-Gaussian) types of jamming and requires relatively little processing power, but produces little gain when confronted with nearly Gaussian interference, such as more than two or three simultaneous independent jammers of comparable power.
The AAA system suppresses interference signals based on their direction of arrival or polarization. A conventional prior-art AAA system accomplishes this by weighting the signal from each antenna element and summing them together to form an output. For applications where the signal of interest is weaker than the antenna noise so that any detectable signal above the antenna noise is interference and should be suppressed, the output may be used as an error feedback signal to drive the AAA weights towards the values that minimize the output and therefore suppress the interference contribution to the output. Such a prior-art AAA system configuration is shown by FIG.
1
.
For appropriate applications, a conventional prior-art AAA system can incorporate “beam-forming” capability, i.e. the ability to protect a known direction of arrival from which the signal of interest is expected so that the AAA will not form a null in that direction. For example, a conventional Griffiths-Jim beamforming pre-processor (as described in Griffiths, L. J., and Jim, C. W., “An Alternative Approach to Linearly Constrained Adaptive Beamforming,”
IEEE Trans. Antennas Propag
., vol. AP-30, pp. 27-34, (January 1982)) can simply be inserted just in front of the AAA system as shown by FIG.
2
. The Griffiths Jim pre-processor first weights the signal from each of the M elements, X, so that if summed together without further weighting they would steer the array in the direction of the desired signal. The Griffiths-Jim reference output component is taken to be that unweighted sum of the steered input components, and the M-1 Griffiths-Jim auxiliary components are simply the differences between independent pairs of the steered elements of X.
Since a standard AAA system can only suppress M-1 spatially distinct interferers, where M is the number of antenna elements, prior art includes augmenting an AAA system with an AF system. Adding the AF component increases the number of simultaneous sources of interference that may be countered if some of these sources are narrow-band.
An AF system can improve the performance of an AAA system in another way: If, as is usual, the AAA weights are simple multipliers that shift signal phase but don't alter the time delay, then the ability of the AAA system to cancel interference declines as interference bandwidth increases, or if signal echoes, such as may be caused by multi-path propagation, are present. Under such conditions, an AF system can improve antijam (AJ) gain by altering the effective time delay of signals from a given element, or even by suppressing echoes.
A conventional prior-art means of combining AF and AAA techniques is to filter the M signals from the antenna elements with M separate filters prior to applying these signals to the AAA system. As with the AAA system, the output of the AAA system is used as an error feedback signal, but in the AF-AAA system this error signal is used to update not only the AAA weights, but also a set of filter weights (one for each filter “tap” or degree of freedom) for each filter. Such a prior-art AF-AAA system, sometimes referred to as a Space-Time Adaptive Processor (STAP), is shown by FIG.
3
.
An amplitude-domain ALOD, such as described in (1) Higbie, J. H., “Adaptive Nonlinear Suppression of Interference,” in Proc. MILCOM 88, IEEE, New York, p. 23.3.1 (1988); (2) Higbie, J. H., “Adaptive Locally-Optimum Detection Signal Processor and Processing Methods,” U.S. Pat. No. 5,018,088, issued May 21, 1991; and (3) Radcliffe, S. T., and Higbie, J. H., “Evaluating Transform Estimators for Locally Optimum Signal Processors,” Unclassified paper in Classified Proc. MILCOM 92, IEEE, New York, p. 162 (1993), enables suppression of non-Gaussian interference sources (i.e. ones whose amplitudes do not follow a Rayleigh probability distribution). This class includes all man-made interference sources, such as jamming. An ALOD is especially attractive in an AJ system because it can suppress wide-band jamming, unlike an AF, and doesn't require multiple antenna elements, as does an AAA. Another advantage of an ALOD implemented as described in References 1-3 is that its adaptation is rapid, so that it can combat even rapidly time-varying jamming, which is difficult for conventional AAA or AF signal processing designs that are based on using error feedback to update weights.
From the perspective of the jammer, it is possible to design jamming waveforms against which an ALOD system does not provide usable antijam (AJ) gain, however transmitting such waveforms nearly always requires a reduction in the power that can be devoted to jamming the victim receiver. The jamming power must be reduced either by spreading its energy outside the frequency band occupied by the victim signal, or by modulating the amplitude of the jamming waveform. (Amplitude modulation reduces the average jamming power relative to the peak jamming power. Since the jammer's peak power is typically fixed, amplitude modulation causes an absolute reduction in the average jamming power, which reduces jamming impact.)
It is advantageous to augment an ALOD system with an AF system, in order to increase the number of simultaneous independent jammers that may be countered. An effective prior-art AF-ALOD system configuration is to feed the (single-element) input signal to the AF system and to feed the output from the AF system to the ALOD system.
Cooch Francis A.
Le Duy K
The Johns Hopkins University
Urban Edward F.
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