Low probability of intercept communication system
First Claim
1. A communication system for conducting low probability of intercept communications between a transmitter site and a receiver site comprising:
- at said transmitter site,first means for generating, during a prescribed time slot, a plurality N of carrier frequencies having respective amplitudes, and phase angle values that are randomly distributed with respect to one another;
second means, coupled to said first means, for performing an inverse fast Fourier transformation of said plurality of carrier frequencies so as to obtain a time domain pulse waveform representative thereof; and
third means, coupled to said second means, for modulating said time domain pulse waveform with information signals and transmitting said modulated time domain pulse waveform; and
at said receiver site,fourth means for receiving the modulated time domain pulse waveform that has been transmitted by said transmitter site;
fifth means, coupled to said fourth means, for performing a fast Fourier transformation of the received time domain pulse waveform, so as to obtain therefrom a distribution of the frequency components thereof; and
sixth means, coupled to said fifth means, for processing the frequency components obtained by said fifth means, so as to recover said information signals.
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Accused Products
Abstract
A low probability of intercept communication system (CCSK)--modulates information signals onto an inverse fast Fourier transformation of a large number of simultaneous frequencies that have been determined to be reasonably `quiet` within a given system bandwidth, so as to produce a time domain pulse waveform. The amplitude of each transmitted frequency is weighted. Within the receiver equipment of each participant in the system, the incoming pulse waveform produced by the inverse fast Fourier transformation mechanism at the source is coupled to a fast Fourier transform operator, so as to separate the time domain signal into a plurality of frequency components that contain the modulated data. These components are then convolved with a replica of the plurality of quiet channels to derive a time domain output waveform from which the data modulation can be identified and recovered. Even if a jamming threat is injected into one or more of the `quiet` channels that has been selected as a participating carrier, by virtue of the signal analysis and recovery process employed by each unit for incoming signals, jamming spikes are effectively excised.
136 Citations
30 Claims
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1. A communication system for conducting low probability of intercept communications between a transmitter site and a receiver site comprising:
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at said transmitter site, first means for generating, during a prescribed time slot, a plurality N of carrier frequencies having respective amplitudes, and phase angle values that are randomly distributed with respect to one another; second means, coupled to said first means, for performing an inverse fast Fourier transformation of said plurality of carrier frequencies so as to obtain a time domain pulse waveform representative thereof; and third means, coupled to said second means, for modulating said time domain pulse waveform with information signals and transmitting said modulated time domain pulse waveform; and at said receiver site, fourth means for receiving the modulated time domain pulse waveform that has been transmitted by said transmitter site; fifth means, coupled to said fourth means, for performing a fast Fourier transformation of the received time domain pulse waveform, so as to obtain therefrom a distribution of the frequency components thereof; and sixth means, coupled to said fifth means, for processing the frequency components obtained by said fifth means, so as to recover said information signals. - View Dependent Claims (2, 3, 4, 5)
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6. A method of conducting covert communications in the presence of one or more jamming/intercept threats comprising the steps of:
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at a transmission site, (a) modulating information signals onto an inverse fast Fourier transformation of a plurality of frequencies that have been selected within a given system bandwidth, the amplitude of each transmitted channel being weighted in accordance with the inverse power spectrum density of said bandwidth, and the phases of which are irregularly distributed, thereby producing a time domain pulse waveform; at a reception site, (b) coupling a received time domain pulse waveform to a fast Fourier transform operator, so as to separate the time domain pulse waveform into a plurality of frequency components that contain modulated information signals; (c) convolving the frequency components of step (b) with a replica of the plurality of frequencies so as to derive a time domain output waveform; and (d) recovering said information signals from said time domain output waveform. - View Dependent Claims (7, 8, 9, 10, 11, 12)
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13. A method of conducting low probability of intercept communications between a transmitter site and a receiver site comprising the steps of:
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at said transmitter site, (a) generating, during a prescribed time slot, a plurality N of carrier frequencies having respective amplitudes, and phase angle values that are randomly distributed with respect to one another; (b) performing an inverse fast Fourier transformation of said plurality of carrier frequencies so as to obtain a time domain pulse waveform representative thereof; and (c) modulating said time domain pulse waveform with information signals and transmitting said modulated time domain pulse waveform; and at said receiver site, (d) receiving the modulated time domain pulse waveform that has been transmitted by said transmitter site; (e) performing a fast Fourier transformation of the received time domain pulse waveform, so as to obtain therefrom a distribution of the frequency components thereof; and (f) processing the frequency components obtained by step (e), so as to recover said information signals. - View Dependent Claims (14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24)
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25. A communication system for conducting covert communications between a transmission site and a reception site in the presence of one or more jamming/intercept threats comprising, in combination:
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at said transmission site, a time domain pulse waveform transmitter which modulates information signals onto an inverse fast Fourier transformation of a plurality of frequencies that have been selected within a given system bandwidth, the amplitude of each transmitted channel being weighted in accordance with the inverse power spectrum density of said bandwidth, and the phases of which are irregularly distributed, thereby producing a time domain pulse waveform; and at said reception site, a time domain pulse waveform receiver to which a received time domain pulse waveform is coupled, said receiver including a fast Fourier transform operator which separates the time domain pulse waveform into a plurality of frequency components that contain modulated information signals, a frequency domain convolver which convolves said frequency components with a replica of the plurality of frequencies so as to derive a time domain output waveform, and decoder which recovers said information signals from said time domain output waveform. - View Dependent Claims (26, 27)
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28. For use with a communication system in which information signals are transmitted in burst format and at a prescribed burst repetition rate, a method of aligning a burst recovery receiver with transmitted bursts comprising the steps of:
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at a transmitting site, (a) transmitting an acquisition preamble a first portion of which contains a first sequence of the same preselected information symbol, followed by plural repetitions of a second sequence of different information symbols; at a receiver site, (b) monitoring said acquisition preamble transmitted in step (a) to locate and align said burst recovery receiver with the occurrence of one of the same preselected information symbols in said first sequence; and (c) monitoring said second sequence of different information symbols and deriving therefrom an indication of which of a plurality of successive timeslots, within said burst repetition interval, said burst recovery receiver is aligned.
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29. For use with a communication system in which information signal bursts are processed to produce a time domain correlation characteristic, a method of processing said time domain correlation characteristic so as to recover an intended information signal burst in the presence of a multipath signal burst comprising the steps of:
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(a) sending two symbols such that the second is a time reversal of the first. (b) rotating the time domain correlation characteristic of the second symbol in step (a) about the center of the time domain interval, thereby causing a complementary translation of a desired attribute of said time domain correlation characteristic back to its original time domain location, while causing a displacement of a multipath signal correlation; and (c) combining the original time domain correlation characteristic with the rotated characteristic, and thereby emphasizing the desired information signal attribute, so that the intended signal can be readily identified. - View Dependent Claims (30)
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Specification