Finite-Time Synchronization of Chaotic Systems with Different Dimension and Secure Communication

Finite-time synchronization of chaotic systems with different dimension and secure communication is investigated. It is rigorously proven that global finite-time synchronization can be achieved between three-dimension Lorenz chaotic system and fourdimensionLorenz hyperchaotic systemwhichhave certain parameters or uncertain parameters.The electronic circuits of finite-time synchronization usingMultisim 12 are designed to verify our conclusion. And the application to the secure communications is also analyzed and discussed.


Introduction
Synchronization of chaotic systems recently has become one of the most interesting topics because of the seminal work of Pecora and Carroll [1].These phenomena have arisen in various fields of science, like engineering technology [2], ecological system [3], biological neurons [4], and others [5][6][7][8] various types of synchronization.They have been deeply investigated in the previous literature.A large number of results have been reported in the past two decades [9].
From a practical point of view, it will be more reasonable to realize synchronization in a given time [10][11][12][13].This means optimality in convergence time [14].This problem is important in all fields where synchronization finds or will find practical interest [15][16][17].If we consider, for example, the application of synchronization in secure communications, the range of time during which the chaotic oscillators are not synchronized corresponds to the range of time during which the encoded message can unfortunately not be recovered or sent.More than a grave and irreversible loss of information, this is a catastrophe in digital telecommunications, since the first bits of standardized bit strings always contain signalization data, that is, the "identity card" of the message.Hence, it clearly appears that the synchronization time has to be minimized, so that the chaotic oscillators synchronize as fast as possible.In this context, it is fair to say that there is a need to study finite-time chaos synchronization and optimization problems of nonlinear systems with uncertainties.
This paper mainly addresses the problem of finitetime master-slave synchronization of the three-dimensional (3D) Lorenz system and four-dimensional (4D) hyperchaotic Lorenz system.A family of feedbacks has been proposed to achieve the finite-time synchronization of the two systems with different dimension.It is rigorously proven that global finite-time synchronization can be achieved between three-dimension Lorenz chaotic system and four-dimension Lorenz hyperchaotic system which have certain parameters or uncertain parameters.Finally, the experimental setup for the coupling process has also been designed via Multisim 12 simulations.The numerical simulation and circuit experiment show that the proposed control technique is feasible and reliable.

Finite-Time Synchronization of Different Dimension Systems
Lorenz system is considered a paradigm, since it captures many of the feathers of chaotic dynamics.The Lorenz system is described by the following nonlinear equations: where , , and  are the constants.With  = 10,  = 8/3, and  = 28, the Lorenz system shows chaotic behavior.By introducing a nonlinear feedback controller to the second equation of the system (3), the follow hyperchaotic system is obtained [18]: When  = 2,  = 10,  = 8/3, and  = 28, the four Lyapunov exponents are  1 = 0.2230,  2 = 0.1389,  3 = 0, and  4 = −14.0380.The Lyapunov dimension is D L = 3.0258, and numerical simulations have verified that system (4) indeed has a hyperchaotic attractor.

Finite-Time Synchronization of Systems with the Certain
Parameters.This subsection deals with finite-time synchronization of Lorenz system and hyperchaotic Lorenz system.Considering any initial conditions, system (3) is regarded as the master and system (4) as the slaver.

The master system
(5) could be described by where  = ( ), and  1 ,  2 , and  3 are the state constants and , , and  are state parameters of system.
Hyperchaotic Lorenz system (4) is regarded as the slaver system, and (, ) is controllers, which is determined for achieving synchronization between master system (3) and slaver system (4).Described by where  = ( 1 ,  2 ,  3 ,  4 )  is the state vector of the responder system, () = (0, − 1  3 ,  1  2 , −0.3 2  3 )  is a continuous smooth function and  = ( 1 ,  2 ,  3 ,  4 )  is a control function.To make the same structures between ( 6) and (7), a new artificial variable  4 is introduced into system (6) and set ẋ 4 () = 0. Subtract (6) from (7) and define the state errors as Then the error system between the Lorenz system and the controlled hyperchaotic Lorenz system could be written as The above four equations merge into a unified form: where ) . ( Our aim is to design a suitable controller, in which we can achieve the finite-time synchronization of Lorenz system (3) and hyperchaotic Lorenz system (4).This problem can be converted to design a controller to attain finite-time stability of error system (10).
That is to say, slave system (4) will synchronize with master system (3) in finite time.

Finite-Time Synchronization of Systems with Uncertain
Parameters.We consider the more realistic and practical case, where some of the parameters of the master system and slave system are unknown.It is valuable because practical chaos systems are often disturbed by different factors.It is that the parameters of the systems have been disturbed.Consider the master system and the slave system where Δ  ( = 1, 2, 3, 4) are the increments of the parameter, which is caused by the interruption, and  1 (),  2 (),  3 (), and  4 () are the control functions.Analogously, as in Section 3.1, a new artificial variable  4 is given and ẋ 4 () = 0, by defining the synchronization error in the following way: Mathematical Problems in Engineering and then the error system can be written as In order to solve the finite-time master-slave synchronization problem between system equations ( 15) and ( 16), we select the Lyapunov function: Differentiation of this function with respect to time yields Combining ( 18) and (20), we have We defined the active control functions  1 (),  2 (),  3 (), and  4 () as follows: One obtains From Lemma 1, it follows that ( 18) is finite-time stabilized.Thus, uncertain slave system ( 16) can synchronize uncertain master system (15) in finite time.

Case of the Certain Parameters.
Because the values of the state variables are out of the scope of the amplifier, the linear compressor of the system variable is necessary; to be specific, variables of system are multiplied by a factor  = 0.1 [17].The transform has no influence on the dynamics of the system, so the circuit equation of the master system can be written as Compared with system equation (3), we can obtain, 1/    As shown in Figure 7, we have reduced the number of circuits and further reduced the design complexity of system circuit.The operational amplifiers LM741 and associated circuitry perform the basic implementation of addition, subtraction, and integration.The nonlinear terms of system are implemented with the analog multipliers AD633.
For the slave system, we take the same design as the master system.The circuit equation of system (4) can be written as follows: Slave circuit is implemented via operational amplifiers. 16 constitutes a subtractor circuit, with   minus   and voltage comparator LM339DG implement sign operational.Average absolute value detector circuit consists of  16 ,  16 , and others.Root circuit consists of  17 and  5 .When the parameters  = 0.5 and  = 0.5, we can get the value of the component:   and Figure 8 shows the circuit diagram of the slave system.
The circuit of the control function  consists of subtractor circuit, sign function circuit, average absolute value detector circuit, and root circuit.By adjusting the value of the   ( = 18, 19, 27, 31), the master system and the slaver system can realize the synchronization quickly and monotonically through controller circuits.The circuit of the control function is shown in Figure 9.The result of the simulation is shown in Figure 10.It can see that the ratio of   :   ( = 1, 2, 3) is in close proximity to 1 : 1, which shows that the master system and the slaver system are synchronized.

Case of the Uncertain Parameters.
In this subsection, combining with the numerical simulations of the Section 4.2, master system (15) is implemented in Figure 11. Figure 11 shows the circuit diagram of the master system and the values    of the circuit components; the frequency of the sinusoidal signal is 2000 Hz.
Figure 12 shows the circuit diagram of the state variables for slave system (16).The value of the components in Figures 11 and 12 are scaled by a factor 0.1.
The circuit schematic is composed of four submodule control circuit.Figure 13 shows the circuit diagram of control function  2 , and the design methods of the other circuit is identical to it.Figure 14 illustrates the synchronization is achieved between master system (15) and slave system (16).

Experiment on Secure Communication
Most secure communication schemes using chaotic dynamics are based on identical synchronization.These systems employing chaos are partly due to their high unpredictability and simplicity of implementation.Chaotic Masking (CM), Chaos Shift Keying (CSK), Chaos On-Off Keying (COOK), and Differential Chaos Shift Keying (DCSK) are the most common chaos modulation techniques.However, the technology of the finite-time synchronization and unpredictability of uncertain parameters can enhance the security of communication.In the paper, based on the method of finitetime synchronization, simulation test of the digital secure communication adopting Chaotic Masking (CM) technology is performed.The information signal   () is chosen, which is a square signal with frequency of 4 khz and 500 mVp.The signal is modulated with the variable of the master system.When  1 () is chosen as a marking signal, the transmitting system can be written as Figure 11: Circuit of the master system with uncertain parameters.
The receiving system is written as At the transmitter, the information signal   () is added into the master system and chaos signal mask the information signal.One obtained the encryption signal: At the receive end, for maintaining synchronization between  1 () and  1 (), the information signal   () is decoded by a subtractor: Modulation-demodulation of the chaotic communication system is shown in Figure 15.
The result of circuit simulation proves that the encryption signal   () has chaotic characteristics and contains information.The synchronous demodulation signal   () is consistent with the modulating signal   () after the finite time   ; Figure 16 shows the waves of the signals and state variables of the system.Chaos communication system showed good reducibility.

Conclusion
In this paper, the finite-time chaos synchronization between the 3D Lorenz system and the 4D hyperchaotic system is investigated.We proposed an accurate finite-time synchronization control strategy and implemented finite-time synchronization between the 3D Lorenz system and the 4D hyperchaotic system based on the finite-time stability theory.An electronic circuit is designed to realize the controller

Figure 2
shows the synchronization of the Lorenz systems with the certain parameters.

Figure 3 :Figure 4 :
Figure 3: State trajectories of the uncertain master  1 and slave system  1 .

Figure 5 :Figure 6 :
Figure 5: State trajectories of the uncertain master  3 and slave system  3 .

Figure 7 :
Figure 7: The circuit of the master system.

Figure 8 :
Figure 8: The circuit of slave system.

3 Figure 10 :
Figure 10: The ratio of the state variables of different systems.

Figure 12 :
Figure 12: Circuit of the slave system with uncertain parameters.

Figure 13 :
Figure 13: Circuit diagram of the control function  2 .

y 1 -x 1 y 2 -x 2 Figure 14 :
Figure 14: The ratio of the state variables of different systems.