Ch. K. Volos

Synchronization Phenomena in Coupled Colpitts Circuits

In this work, the case of coupling (bidirectional and unidirectional) between two identical nonlinear chaotic circuits via a linear resistor, is studied. The produced dynamical systems have different structure, in regard to other similar works, due to the choice of coupling nodes. As a circuit, a modification of the most well-known nonlinear circuit that can operate in a wide range of radiofrequencies, the Colpitts oscillator, is chosen. The simulation and the experimental results show a variety of dynamical phenomena, such as periodic, quasi-periodic and chaotic behaviors, as well as anti-phase and complete synchronization phenomena, depending on the value of the coupling coefficient.

The memristor as an electric synapse - synchronization phenomena

Today many scientists see nonlinear science as the most important frontier for the fundamental understanding of Nature. Especially, the recent implementation of the memristor has led to the interpretation of phenomena not only in electronic devices but also in biological systems. Many research teams work on projects, which use memristor to simulate the behavior of biological synapses. Based on this research approach, we have studied using computer simulations, the dynamic behavior of two coupled, via a memristor, identical nonlinear circuits, which play the role of “neurons”. The proposed memristor is a flux-controlled memristor where the relation between charge and magnetic flux is a smooth continuous cubic function. Very interesting synchronization phenomena such as inverse π-lag synchronization and complete chaotic synchronization were observed.

Chaotic Dynamics from a Nonlinear Circuit based on Memristor with Cubic Nonlinearity

In this work, we present the study via computer simulations of the dynamic behavior of a chaotic circuit based on a memristor. The proposed memristor is a flux controlled memristor described by the function W(φ(t)), which is called memductance, and defined as, W(φ) = dq(φ)/dφ, where q(φ) is a cubic function. We discovered very important phenomena concerning the Chaos theory, such us, the great sensitivity of the circuit on initial conditions, the route to chaos through the mechanism of period doubling, and the phenomenon of coexisting attractors.

Image encryption process based on a chaotic True Random Bit Generator

In recent years, information security requires randomly selected strong keys. Most of the proposed methods provide keys based on chaotic cryptographic algorithms. In this paper, an image encryption scheme based on a chaotic True Random Bit Generator (TRBG) is proposed. The chaotic generator is a nonlinear electronic circuit which produces double scroll chaotic attractors. The values of the circuit parameters and the initial conditions are the keys of the cryptographic scheme. Since the dynamic behavior of the circuit is so unpredictable, a true random bits sequence is produced via a well known method. We adopt the FIPS 140-1 statistical suite to test the distribution of the bits sequence. This sequence is used to encrypt and decrypt images. Also, the security analysis of the encrypted image demonstrates the high security of the proposed scheme.

Kinematic Control of a Robot by Using a Non-autonomous Chaotic System

In this chapter, a new navigation strategy by designing a path planning generator, which ensures chaotic motion to an autonomous robot, has been investigated. This new technique is implemented by using the well-known from the nonlinear theory Poincare map, which is produced by a non-autonomous chaotic system. For this reason one of the most studied non-autonomous systems, the Duffing—van der Pol system has been chosen, presenting suitable results. The path planning generator produces an unpredictable trajectory by converting the chaotic strange attractor, produced by system’s Poincare map, into the robot’s “random” trajectory in a chosen workspace. As a consequence the autonomous robot covers the whole workspace with unpredictable way. The simulation results verify that the proposed generator achieves the aforementioned goal. So, with this work, new perspectives on use of non-autonomous chaotic systems in robotic applications are opened.

Experimental Synchronization of Two Unidirectionally Coupled Double Scroll Circuits

In this work, we have studied the case of chaotic synchronization between two identical double scroll chaotic circuits, unidirectionally coupled via a linear resistor and we found out, that the system was synchronized for different values of the coupling resistor. The nonlinear element of the double scroll circuit was a saturated circuit, based on operational amplifiers. Finally, we compared the theoretical results from the simulation, with the experimental results and we saw that the system had the expected dynamical behavior.

Experimental Observation of Antimonotonicity in a Nonlinear R-L-Diode Circuit

In this work the antimonotonicity phenomenon is experimentally investigated in the case of a driven nonlinear R-L-Diode circuit, with or without a DC bias voltage. The nominated electronic circuit is one of the simplest and extensively studied nonlinear circuits that can be made in breadboard. Due to this fact, this is not an introductory paper but focuses specifically to the study of how the frequency of the sinusoidal voltage source acts to the dynamical behavior of the circuit, using appropriate nonlinear tools such as the bifurcation diagram, phase portrait, return map and correlation dimension. Furthermore, except of the antimonotonicity, a number of other interesting phenomena related to chaos has

Block error rate estimation for wireless optical communication links over strong turbulence channels with pointing errors

Free-space optical communication systems are gaining popularity as a high-capacity, cost-effective and license-free wireless technology, addressing the bandwidth demands of existing and future wireless networks. The deployment scenarios of free pace optical links usually concern secure, fast and reliable connections. Thus, in this work, the performance of optical wireless communication systems is studied in terms of the average block error rate which constitutes a very significant metric for their reliability. The optical signal transmission is assumed to be hampered by the joint effects of strong atmospheric turbulence, modeled by the Negative Exponential and the pointing errors effect which affects the alignment between the transmitter and the receiver. Accurate closed-form expression for the evaluation of the average block error rate is obtained, including the aforementioned effects, while finally the corresponding numerical results, for realistic optical wireless links, are presented.

DF relayed FSO communication systems with time dispersion over Gamma Gamma turbulence and misalignment

Free Space Optical (FSO) communications have recently attracted a renewed commercial and research interest. However, the performance of such terrestrial wireless optical links is strongly degraded by the pointing errors effect between the trans-receivers terminals and by the atmospheric turbulence effect whose impact becomes even more detrimental by the propagation distance. In order to combat this combined distance-dependent turbulence-induced and misalignment-induced fading we often resort to place relays along the line-of-sight propagation path. Thus, intermediate shorter hops are created which can lead to significant performance improvements. Additionally, by adjusting as information bit carriers, negative chirped Gaussian pulses with narrow initial pulse width, the also distant-dependent time dispersion effect can further ameliorate the FSO performance for some initial, i.e. hop distance. In view of the above, a typical terrestrial FSO system is considered that may employ serial Decode-and-Forward (DF) relaying configurations over a wide turbulence range, modeled through the very accurate Gamma-Gamma distribution model, along with the presence of time dispersion effect and different amounts of pointing mismatch. Under these assumptions, the performance of the FSO system is evaluated by means of its probability of fade metric. Closed-form expressions for the probability of fade of such DF relay-assisted FSO systems are derived, which along with the numerical results, demonstrate the beneficial impact on the FSO performance due to DF relays employment and time dispersion effect, under specific links characteristics.

On the influence of time dispersion at the availability of optical wireless links with diversity and spatial jitter over Malaga-modeled turbulence

In this work we investigate the combined detrimental impact of atmospheric turbulence and spatial jitter, i.e. pointing errors, effects on the performance of a terrestrial free space optical (FSO) system, by taking into consideration several time diversity schemes and the time dispersion of the longitudinal optical chirped pulses which are employed. The scintillation effect is modeled through the recently launched Malaga distribution, which accurately emulates irradiance fluctuations due to weak to strong turbulence conditions and unifies most of the well-known statistical models for such fluctuations alike. Additionally, we estimate the availability of those FSO systems by deriving closed-form mathematical expressions for the estimation of the probability of fade of the system and we demonstrate that its value is significantly reduced when time diversity schemes with negative chirped pulses are implemented. Finally, proper numerical results are provided so as to present the influence of each effect at the systems availability.