A Chaotic Jerk System with Different Types of Equilibria and its Application in Communication System

In this paper, a new jerk system is designed. This system can display different characters of equilibrium points according to the value of its parameters. The proposed nonlinear oscillator can have both self-excited and hidden attractors. Dynamical properties of this system are investigated with the help of eigenvalues of equilibria, Lyapunov exponents' spectrum, and bifurcation diagrams. Also, an electronic circuit implementation is carried out to show the feasibility of this system. As an engineering application of this new chaotic jerk system, a chaotic communication system is realized by correlation delay shift keying. When the results of the communication system are examined, the transmitted information signal is successfully obtained in the receiving unit, and its performance efficiency is investigated in the presence of additive white Gaussian noise.


INTRODUCTION
Most of the known chaotic systems are third-order differential equations with a finite number of equilibrium points [1][2][3]. Designing chaotic systems with particular features has been an exciting topic in the last years (for example chaotic systems with multi-wing [4,5], with multi-stability [6], and fractional-order [7]). The numerical difficulties associated with the locating dynamical states whose basin of attraction does not overlap equilibrium points leads to the term "hidden attractor" [8]. Initially chaotic systems without equilibria were considered as "incomplete" or "misformulated" [9]. The first noequilibrium chaotic system (Sprott A) was reported in 1994 [10]. Since then dynamical analysis of such systems became attractive and noticeable [11,12]. Leonov & Kuznetsov studied [13,14] and developed [15,16] analytical and numerical methods to study the chaotic and hyper-chaotic hidden attractors.
The simplest of three-dimensional chaotic systems are the jerk systems [17]. Some chaotic jerk systems with two quadratic nonlinearities [18] and three quadratic nonlinearities [19] have been reported in the literature.
In [20] Piecewise-linear jerk function has been used to model simple chaotic systems. Sun and Sprott used a simple piecewise exponential nonlinear function and concluded that increasing nonlinearity will not end up with more chaos [21]. Vaidyanathan et al. announced a chaotic jerk system with two hyperbolic sinusoidal nonlinearities [22].
In chaos-based digital communication systems, each symbol to be transmitted is represented by a piece of the chaotic signal. Even if the same symbol information is repeatedly transmitted on this channel, the transmitted information will always be different because the chaotic signal is not periodic. Chaos-based communication systems have low predictability [23,24]. for the field of communication [25][26][27][28]. In this study, chaos-based communication system by CDSK method with the new jerk chaotic system is designed for engineering application.
Motivated by the above discussions, we report a unique jerk system with different types of equilibrium points (depending on the parameters). Such systems are reported less in literature. We analyze the dynamical properties such as Lyapunov exponents' spectrum, eigenvalues, and bifurcation plots. We introduce the new chaotic jerk system in Section 2. In Section 3 the feasibility of this new system is verified through circuit implementation. Section 4 belongs to communication design, and finally, Section 5 is the conclusion of the paper.
The novelty of the system is that a simple 3D jerk system is showing multiple equilibrium types with selfexcited and hidden oscillations. Also by just controlling the values of the parameters, we could change the type of the equilibrium.

JERK SYSTEM
Sprott proposed several cases of simple jerk systems [18,19] of the form 2 3 which shows chaotic oscillations for e.g. a = 3.6, b = −1 and c = 1. We present a new chaotic jerk system by modifying Eq. (1) x y y z z a z a x a y a x a xy a The character of Eq. (2) can be discussed in two conditions a6 = 0, a2 ≠ 0 and a6 ≠ 0, a2 = 0. The equilibrium points of Eq. (2) for a6 = 0, a2 ≠ 0 are   Bifurcation plots are useful in investigating the characteristics of dynamical systems (see Fig. 2). We investigate the bifurcation of system (2) for the condition a6 = 0 with a2 and a3 while the other parameters are fixed as a1 = −3.6, a3 = 0.1, a4 = −1, a5 = 1and the initial condition for the first iteration is taken as [2, 2, −0.4] and is initialized to the end values of the state trajectories and the local maxima of the states plotted with the parameters. The jerk system exits chaos with period halving for the parameter a2 and period doubling route to chaos with a3.

CIRCUIT DESIGN FOR THE JERK SYSTEM
In this section, we introduce a circuit design which provides a physical way to realize the theoretical jerk Eq. (2).
The circuit in Fig. 3 has been designed by using methods in [30][31][32][33], which are based on five operational amplifiers. As can be seen in Fig. 3, X, Y and Z are the voltages at the operational amplifiers U1, U2 and U3. From Fig. 3, the circuit equation is given by:  The electronic circuit has been implemented with PSpice. Fig. 4 presents PSpice chaotic attractors of the circuit for the case a2 = −1, a6 = 0.  Figure 4 The 2D phase portraits of the electronic circuit obtained by using PSpice

COMMUNICATION DESIGN WITH THE NEW CHAOTIC JERK SYSTEM USING CDSK MODULATION
In this part, the new chaotic jerk system tested communication designs with the modulation method. Then, the BER performances of these communication systems were compared.
The CDSK modulation method can be considered as a type of DCSK method. The CDSK modulation method was proposed by Sushchic et al. in 2000. In the CDSK transmitter unit (TU), the chaotic signal c(t) and delayed of this chaotic signal are multiplied by the information signal m(t), then the chaotic signal is summed with the multiplication output signal and transmitted to the receiver unit. Block diagram of the CDSK transmitter unit is given in Fig. 5.  [28] The modulated signal sent in the CDSK modulator is, In Eq. (4), τ denotes the specified delay. In the CDSK receiver unit (RU), the noisy incoming n(t) signal r(t) and its delayed version are multiplied and integrated into the correlator. The signal from the correlator is passed through the threshold detector to obtain the transmitted IS ~ m(t).
The signal at the correlator output o(iTb) at the CDSK receiver unit,

r t t s t n t s t n t t s t s t t s t n t t n t .s t t n t .n t t
In the Eq. (5), n(t) denotes the noise. Eq. (5) becomes equal to Eq. (6) since n(t) = 0 in noiseless environment.
The principle scheme of the CDSK RU is given in Fig.  6. Technical Gazette 27, 3(2020), 681-686 Figure 6 CDSK receiver unit block diagram [28] ( ) The TU of the CDSK modulated communication system designed in the Matlab-Simulink ® program using the new jerk CS is given in Fig. 7. In the TU, the x state variable of the system is used for the CS c(t). The CS and the delayed of this chaotic signal by half the bit period (τ = Tb/2) are multiplied, and this value is summed with the IS. The modulated signal thus obtained is sent to the RU. The RU designed in the Matlab-Simulink ® program using the new jerk chaotic system is given in Fig. 8. The modulation signal with noise r(t) which is received by the RU is delayed by half the time of the bit period (τ = Tb/2). Then these two signals are multiplied and integrated into the correlator unit. Thus the energy value of the signal is calculated. The correlator output signal is sent to the threshold detector and the IS m(t) sent from the TU according to the threshold level is estimated, and the IS is obtained ~ m(t). The system was tested under noise. Additive White Gaussian Noise (AWGN) was selected as a noise signal. The test was performed between 0 dB and −15 dB Eb/N0 (Energy per bit to noise power spectral density ratio) values. In Fig. 9, the BER values graph is given. Figure 9 The BER performances of the CDSK modulated communication system using the new chaotic jerk system The signal sent from the transmitter unit has been successfully received from the receiver unit up to a noise value of −8 dB Eb/N0. In Fig. 10, the system simulated under the noise of −8 dB Eb/N0 is given the IS, and the retrieved IS, respectively.

CONCLUSION
A three-dimensional chaotic jerk system which shows different types of equilibrium points was reported in this research. Dynamical properties of this new system were analyzed by the help of Lyapunov exponents' spectrum and bifurcation plots. Analog circuit implementation was presented to show that the jerk system can be effectively realized in hardware. A correlation delay shift keying communication system was designed using the jerk system, and the performance efficiency of the communication model was analyzed in the presence of additive white Gaussian noise.