Hybrid pseudo-random noise and chaotic signal implementation for covert communication

1. A covert communication system comprising:

a transmitter for sending a covert signal, the transmitter comprising:

a modulator to modulate a data signal entering the transmitter to create a transmitter modulated signal,

a first transmitter multiplying device coupled to the modulator to multiply the transmitter modulated signal with a chaotic signal to create a chaotic modulated signal,

a transmitter chaos generator coupled to the first multiplying device to generate the chaotic signal, wherein the chaotic modulated signal reenters the transmitter chaos generator;

a transmitter pseudo-random noise sequence generator to generate a transmitter pseudo-random noise sequence, and

a second transmitter multiplying device coupled to the first multiplying device and to the transmitter pseudo-random noise sequence generator to multiply the chaotic modulated signal with the transmitter pseudo-random noise sequence to create the covert signal; and

a receiver for receiving the covert signal, wherein the receiver comprises:

a receiver pseudo-random noise sequence generator to generate a receiver pseudo-random noise sequence identical to the transmitter pseudo-random noise sequence,

a first receiver multiplying device coupled to the receiver pseudo-random noise sequence generator to multiply the covert signal with the pseudo-random noise sequence to create a de-spread signal,

a receiver chaos generator coupled to the first receiver multiplying device to generate a receiver chaotic signal, wherein the receiver chaos generator is of similar configuration as the transmitter chaos generator and wherein the de-spread signal enters the receiver chaos generator,

a complex conjugate device coupled to the receiver chaos generator to complex conjugate the receiver chaotic signal to generate a conjugated signal,

a second receiver multiplying device coupled to the first receiver multiplying device and the complex conjugate device to multiply the conjugated signal to the de-spread signal to create a receiver modulated signal, and

a demodulator coupled to the second receiver multiplying device to demodulate the receiver modulated signal to generate a received data signal approximating the data signal entering the transmitter.

2. The system of claim 1 , wherein the receiver further comprises a pseudo-random noise sequence synchronization device comprising:

a delay device;

a supplement complex conjugate device; and

a complex multiply algorithm; and

wherein the pseudo-random noise sequence synchronization device determines a delay necessary to synchronize the receiver pseudo-random noise sequence with the transmitter pseudo-random noise sequence.

3. The system of claim 1 , wherein the transmitter chaos generator further comprises:

a non-linear device to receive the chaotic signal and generate a filtered signal;

a buffer coupled to the non-linear device to sample the filtered signal to generate sampled signals;

a selection device coupled to the buffer for selecting from sampled signals to generate a selected sampled signal;

an integrator coupled to the selection device to solve differential equations with variable coefficients for state variables as a function of the selected sampled signal to generate an integrated signal;

a variable gain control coupled to the integrator to scale the integrated signal; and

a numerically controlled oscillator coupled to the variable gain control to generate a constant amplitude and varying phase within the chaotic signal.

4. The system of claim 3 wherein the non-linear device comprises:

a first filter;

an instantaneous nonlinear function coupled to the first filter; and

a second filter coupled to the instantaneous nonlinear function.

5. The system of claim 4 wherein the first filter is selected from a finite impulse response filter or an infinite impulse response filter.

6. The system of claim 4 wherein the second filter is selected from a finite impulse response filter or an infinite impulse response filter.

7. The system of claim 4 wherein the instantaneous nonlinear function is a square law device.

8. The system of claim 4 wherein the instantaneous nonlinear function is a lookup table device.

9. The system of claim 1 , wherein the transmitter chaos generator is digitally programmable.

10. The system of claim 1 , wherein the complex conjugate device is digital.

11. The system of claim 1 , wherein the modulator is selected from a minimum shift keying modulator, a staggered quadrature phase shift keying modulator, or a modulator with different amplitude and phase shift keying.

12. The system of claim 1 , wherein the transmitter further comprises an encoder coupled to the modulator for encoding the data signal before the data signal is modulated and wherein the receiver further comprises a decoder coupled to the demodulator to decode the demodulated waveform to generate a decoded waveform.

13. The system of claim 12 , wherein the encoder is selected from a Reed-Solomon encoder, a convolutional encoder, a LDPC/BCH encoder, or a Turbo Code encoder.

14. The system of claim 12 , wherein the decoder is selected from a Reed-Solomon decoder, convolutional decoder, a hard decision Viterbi algorithm decoder, a soft-decision Viterbi algorithm decoder, a LDPC/BCH decoder, or a Turbo Code decoder.

15. The system of claim 12 , wherein the transmitter further comprises an encryption device coupled to the encoder for encrypting the data signal before encoding and wherein the receiver further comprises a decryption device coupled to the decoder to decrypt the decoded waveform.

16. The system of claim 3 , wherein the transmitter further comprises an analog-to-digital converter coupled to the modulator to convert the data signal from analog to digital and a digital-to-analog converter coupled to the numerically controlled oscillator to convert the chaotic signal from digital to analog.

17. The system of claim 1 , wherein the transmitter further comprises a carrier frequency generator device coupled to the second transmitter multiplying device to transmit the covert signal on a carrier frequency and wherein the receiver further comprises a carrier frequency recovery device to remove the carrier frequency from the covert signal.

18. The system of claim 1 , wherein the selection device is selected from a multiplexer or a summation device.

19. A method for covertly communicating between a transmitter and a receiver comprising:

receiving a data signal at the transmitter;

modulating the data signal at a modulator in the transmitter to create a transmitter modulated signal;

generating a chaotic signal from a transmitter chaos generator in the transmitter;

multiplying the modulated signal with the chaotic signal in the transmitter to create a transmitter chaotic carrier signal;

generating a transmitter pseudo-random noise sequence from a transmitter pseudo-random noise sequence generator in the transmitter;

multiplying the transmitter chaotic carrier signal to the transmitter pseudo-random noise sequence in the transmitter to create a covert signal;

transmitting the covert signal over a channel from the transmitter;

receiving the covert signal transmitted from the transmitter over the channel at the receiver;

generating a receiver pseudo-random noise sequence from a receiver pseudo-random noise sequence generator identical to the transmitter pseudo-random noise sequence;

multiplying the receiver pseudo-random noise sequence to the covert signal in the receiver to create a de-spread signal;

generating a receiver chaotic carrier signal at a receiver chaos generator, wherein the receiver chaos generator is of similar configuration as the transmitter chaos generator;

complex conjugating the receiver chaotic carrier signal from the receiver chaos generator at a complex conjugate device to generate a conjugated signal in the receiver;

multiplying the conjugated signal to the de-spread signal to create a receiver modulated waveform; and

demodulating the receiver modulated waveform at a demodulator in the receiver to generate a received data signal approximating the data signal received at the transmitter.

20. The method of claim 19 , wherein generating a chaotic signal from the transmitter chaos generator in the transmitter further comprises:

receiving the chaotic signal at a non-linear device in the transmitter chaos generator;

filtering the chaotic signal at the non-linear device to create a filtered signal;

sampling the filtered signal at a digital buffer in the transmitter chaos generator;

selecting from the sampled signals at a selection device to generate a selected sampled signal;

integrating the selected sample signal at an integrator for solving differential equations with variable coefficients for state variables as a function of the selected sampled signal to generate an integrated signal;

scaling the integrated signal at a variable gain control; and

generating a constant amplitude and varying phase in the chaotic signal at a numerically controlled oscillator.

21. The method of claim 20 , wherein integrating the selected sample signal further comprises solving for Lorenz differential equations.

22. The method of claim 20 , wherein integrating the selected sample signal further comprises integrating with trapezoidal differential equations.

23. The method of claim 20 , wherein integrating the selected sample signal further comprises integrating under Simpson's rule for differential equations.