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Utilization of chaotic signals for covert communications remains a very promising practical application. Multiple studies indicate that the major shortcoming of recently proposed chaos-based communication schemes is their susceptibility to noise and distortions in communication channels. In this paper, we review a new approach to communication with chaotic signals, which demonstrates good performance in the presence of channel distortions. This communication scheme is based upon chaotic signals in the form of pulse trains where intervals between the pulses are determined by chaotic dynamics of a pulse generator. The pulse train with chaotic inter-pulse intervals is used as a carrier. Binary information is modulated onto this carrier by the pulse position modulation method, such that each pulse is either left unchanged or delayed by a certain time, depending on whether "0" or "1" is transmitted. By synchronizing the receiver to the chaotic-pulse train we can anticipate the timing of pulses corresponding to "0" and "1" and thus can decode the transmitted information. Based on the results of theoretical and experimental studies we discuss the basic design principles for the chaotic-pulse generator, its synchronization, and the performance of the chaotic-pulse communication scheme in the presence of channel noise and filtering. We also discuss the possibilities of multiuser communication using CPPM and its advantages over standard communication techniques.

We report on experimental studies of synchronization phenomena in a pair of analog electronic neurons (ENs). The ENs were designed to reproduce the observed membrane voltage oscillations of isolated biological neurons from the stomatogastric ganglion of the California spiny lobster Panulirus interruptus. The ENs are simple analog circuits which integrate four-dimensional differential equations representing fast and slow subcellular mechanisms that produce the characteristic regular/chaotic spiking–bursting behavior of these cells. In this paper we study their dynamical behavior as we couple them in the same configurations as we have done for their counterpart biological neurons. The interconnections we use for these neural oscillators are both direct electrical connections and excitatory and inhibitory chemical connections: each realized by analog circuitry and suggested by biological examples. We provide here quantitative evidence that the ENs and the biological neurons behave similarly when coupled in the same manner. They each display well defined bifurcations in their mutual synchronization and regularization. We report briefly on an experiment on coupled biological neurons and four-dimensional ENs, which provides further ground for testing the validity of our numerical and electronic models of individual neural behavior. Our experiments as a whole present interesting new examples of regularization and synchronization in coupled nonlinear oscillators., R.D. Pinto was supported by the Brazilian Agency Fundaçao de Amparo a Pesquisa do Estado de Sao Paulo - FAPESP. P.V. acknowledges support from MEC. Partial support for this work came from the U.S. Department of Energy, Office of Science, under Grants Nos. DE-FG03-90ER14138 and DE-FG03-96ER14592.

In this paper we propose a chaotic frequency modulation (CFM) technique for spread spectrum communications. Existence and stability of the synchronous mode is demonstrated analytically and numerically. Unlike synchronization of two conventional chaotic oscillators via an FM channel, the proposed method provides selectivity with respect to interference within the frequency range of chaotic frequency oscillations. This method provides a viable alternative to a recently proposed ‘interpolated’ frequency hopping for spread-spectrum communication systems. Copyright © 1999 John Wiley & Sons, Ltd.

Synchronization of chaotic systems by control using a phase feedback loop is investigated. Suggested algorithm of synchronization can be used both for discrete time and for continuous time systems. Due to similarity to the usual phase lock loop, this method allows generalization of the majority of applications of phase lock-in for chaotic systems.

Experimental evidence is provided for the synchronization of chaotic self-excited oscillators and chaotic synchronous response. With unidirectional coupling between two chaotic nonlinear circuits, synchronous chaotic oscillations can result from either forced synchronization or the response phenomenon. With mutual coupling, two originally chaotic oscillators can become time periodic, a result that we have shown to persist for values of the coupling down to noise level.

A new type of synchronized chaos conditioned by the threshold synchronization of relaxation oscillators with chaotic behaviour is studied experimentally. It is shown that for a certain parameter ratio the pulses generated by the chaotic oscillator may be synchronized by periodic and chaotic pulse sequences generated by the drive oscillator. It is found that the threshold synchronization regime enables one to detect modulation of chaotic signals.

In this letter we investigate a communication strategy for digital ultra-wide bandwidth impulse radio, where the separation between the adjacent pulses is chaotic arising from a dynamical system with irregular behavior. A pulse position method is used to modulate binary information onto the carrier. The receiver is synchronized to the chaotic pulse train, thus providing the time reference for information extraction. We characterize the performance of this scheme in terms of error probability versus E/sub b//N/sub 0/ by numerically simulating its operation in the presence of noise and filtering.

Experimental results on the synchronization of chaotic oscillations and data transmission in the system “oscillator-response subsystem” are presented.

A new spread spectrum communication system utilizing chaotic frequency modulation of sinusoidal signals is discussed. A single phase lock loop (PLL) system in the receiver is used both to synchronize the local chaotic oscillator and to recover the information signal. We study the dynamics of the synchronization process, stability of the PLL system, and evaluate the bit-error-rate performance of this chaos-based communication system.