Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Electrical signal parameter measurement system
Reexamination Certificate
1998-05-01
2001-04-10
Wachsman, Hal (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Electrical signal parameter measurement system
C702S190000, C380S263000, C703S002000
Reexamination Certificate
active
06216093
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to communication systems and, more specifically, to devices for estimating parameters to facilitate the demodulation of the signals including chaotic waveforms.
2. Description of the Prior Art
Recent advances in the understanding of nonlinear circuits have shown that chaotic oscillators can synchronize or become entrained. This result appears to contradict the definition of chaos: complex, unpredictable dynamics of a deterministic system characterized most commonly as extreme sensitivity to initial conditions. However, if certain mathematical requirements are met, then a chaotic circuit (called the driving system) can be designed to drive a similar system (the receiving circuit or subsystem) and obtain a correlated response.
Several existing systems employ chaotic synchronization to achieve secure communications by using the chaotic signal to mask the secure information. In one approach, a chaotic signal is added to the communications channel, thereby hiding the information signal. At the receiver, a synchronous subsystem is used to identify the chaotic part of the signal, and the chaotic part is then removed to reveal the information signal. Presumably, the security in this approach is due to the difficulty in identifying and removing the chaotic signal from the noise.
A second approach uses a shift-key approach, in which an information signal is encoded using a different chaotic attractor for each symbol to be transmitted. The resulting waveform is a sequence of chaotic bursts, each generated by an attractor corresponding to a symbol of the discrete information signal. Upon reception, separate receiver subsystems, one for each possible symbol, are used to identify the bursts by detecting which of the multiple receiver subsystems has synchronized. This approach is limited to digital modulation and requires a more complex receiver than the present approach.
In one application, communication is achieved by detecting parameter mismatch between the transmitter and the receiver by looking at the power contained in the difference between the received signal and a replica generated using cascaded synchronous subsystems. These schemes recognize that this error signal is proportional to the mismatch, which is present due to parameter modulation at the transmitter, thereby providing a crude method for demodulation. This approach allows for analog modulation and demodulation; however, it will suffer when noise is present in the system.
An inverse system approach for decoding the modulation of a chaotic waveform effectively demodulates the signal by algebraically inverting part of the circuit. Other forms of communications using chaotic waveforms have been postulated in the literature, including techniques called phase space location modulation and chaotic digital encoding.
No device or method exists which estimates a modulation parameter in real time to emodulate a signal, such as a chaotic signal.
SUMMARY OF THE INVENTION
The disadvantages of the prior art are overcome by the present invention which is an apparatus for communicating using waveforms generated using linear and nonlinear oscillators. In general, these waveforms may be periodic, quasi periodic, or chaotic. In the first embodiment disclosed, chaotic waveformns will be emphasized since they are the most general of these descriptors and the other waveform types can be considered simpler, degenerate forms of chaos. This embodiment of the present invention comprises two parts (1) method of modulating signal onto an oscillator in a transmitter, and (2) a filter for demodulating the signal at the receiver. This approach, can be applied to a wide range of nonlinear systems and waveforms.
One existing approach for communication with chaotic waveforms employs a chaotic nonlinear oscillator as the transmitter and a synchronous subsystem for the receiver. The present approach can fit within this construct, although the invention is not limited to chaotic systems, nor does it require a synchronous subsystem for implementation. In the transmitter, an analog information signal is encoded on the carrier through parameter modulation of an oscillator. The proper choice of a transmitted state and modulation parameter ensures perfect tuning of a synchronous subsystem in the receiver, independent of the modulation. In alternative embodiments, the invention can be implemented without a synchronous subsystem in the receiver by transmitting all the oscillator states which define the attractor. The receiver also contains a filter designed specifically to extract the signal from the modulated waveform. With this filter, the transmitted information is continuously decoded at the receiver.
The present invention employs two aspects: (1) parameter modulation that allows the receiver subsystem to always remain in tune, and (2) a nonlinear filter for continuous demodulation of the analog signal carried by the chaotic waveform. The invention allows for the modulation and demodulation of both analog and digital signals. Furthermore, the resulting system can be easily and inexpensively implemented using simple electrical circuitry, thereby making the technique feasible for practical communications applications.
Many prior art approaches to chaotic communications assume that the chaotic dynamics are much faster than those of the modulation. This assumption parallels the usual approach in conventional communications where the carrier signal is at a much higher frequency than the signal. Presumably, these approaches view the chaotic waveform as a carrier waveform that, for example, operates with radio frequency (RF) dynamics. In the invention, on the other hand, the time scales of the nonlinear dynamics and modulation do not have to be separated. In fact, by allowing the frequency spectra of the modulation and the carrier to overlap, this invention allows for encryption of the message signal with little or no increase in the signal bandwidth. By building such a system that operates in the range from 300 to 3000 Hz, efficient audio scrambler-descrambler systems can be built that are suitable for use with voice communications over existing phone lines.
These and other aspects of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the following drawings. As would be obvious to one skilled in the art, many variations and modifications of the invention may be effected without departing from the spirit and scope of the novel concepts of the disclosure.
REFERENCES:
patent: 5379346 (1995-01-01), Pecora et al.
patent: 5655022 (1997-08-01), Carroll
patent: 5680462 (1997-10-01), Miller et al.
Corron Ned J.
Hahs Daniel W.
Dynetics Inc.
Needle & Rosenberg P.C.
Wachsman Hal
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