A basic chaotic modulated voice secure communication system

    Abstract: This paper proposes a voice secure communication scheme using Chua's circuit to realize hybrid pure modulation, analyzes the synchronization performance of the system, designs a hardware experimental circuit on this basis, conducts hardware experimental research on transmitting voice signals, and provides postal experimental results.

    Keywords: Chua's circuit, chaotic synchronization, chaotic modulation, chaotic communication

In recent years, various methods for applying chaos synchronization theory to the field of secure communications have been proposed internationally, including chaos masking [1], chaotic parameter modulation [2], chaotic phase shift keying (CSK) [3] and Chaotic digital code division multiple access (CD) 2MA) [4] and so on. In order to further improve the fidelity and security performance of chaotic communication systems, people are exploring new transmission schemes [5-7].

This paper proposes a voice secure communication scheme using Chua's circuit to achieve two-level chaotic modulation. At the transmitting end, Chua's circuit is used to perform two-level chaotic modulation on the transmitted signal, and at the receiving end, the original signal is demodulated by inverse transformation. The synchronization of the transceiver system is realized based on the one-way coupling method, and the convergence characteristics of the synchronization are analyzed. The designed circuit was used to conduct hardware experimental research on transmitting speech signals. At the receiving end, the loudspeaker can restore clear and realistic speech signals.

1. Establishment of voice chaos secure communication hardware experimental system

The hardware experimental system circuit of the voice secure communication scheme using Chua's circuit to realize two-level chaotic modulation is shown in Figure 1. The component parameters of the transmitter and receiver Chua's circuit in the figure are the same: R=1.75kΩ , L=19.2mH, C1=9.8nF, C2=100.5nF, r=10Ω . NR is Chua's diode, and its volt-ampere characteristic function is f(V)=GbV+1/2(Ga-Gb)(|V+E|-|VE|), where Ga=-0.76ms, Gb=-0.41 nS, E=1V, dual operational amplifiers (TL082) and 6 linear resistors can be used to form NR[8], as shown in Figure 2, and its volt-ampere characteristics are shown in Figure 3. The working process of this system is as follows: at the origin, the chaotic signal vc2 is first used to perform first-level modulation on the information s(t) and then becomes sm(t), and then the communication linear transformation T becomes s'm(t). After VCCS Then the current is(t) acts on the originating system (vc1, vc2, iL) to achieve the second level modulation. The setting of linear transformation T can not only increase the security of the system, but also limit the size of s'm(t) and prevent over-modulation. After two levels of chaotic modulation, the useful information s(t) is modulated in the chaotic signal vc1, forming a chaotic spread spectrum signal similar to white noise. The output signal vc1 of the transmitting system is transmitted to the receiving end through the channel. When the parameters of the transmitting and receiving systems match, the receiving end performs the corresponding inverse transformation, and then the signal s(t) can be demodulated.

2 How the system works

2.1 Synchronization between sending and receiving systems

From Figure 1, the status agenda of the originating Chua's circuit can be obtained as:

C 1 dV c1 /dt=(V c2 -V c1 )/Rf(V c1 )+i s     (1)

C 2 dV c2 /dt=(V c1 -V c2 )/R+i L     (2)

Ldi L /dt=-V c2 -ri L     (3)

The status agenda of the terminating Chua circuit is:

2.2 Signal modulation and demodulation

After the parameters of the sending and receiving ends match and synchronization is achieved, the signal can be demodulated. The originating chaotic modulation signal is

is=GTg(Vc2,s(t)) (10)

Perform the corresponding inverse operation at the receiving end to get

    s'(t)=(g -1)(V'c2,T -1(G-1)i's) (11)

s'(t)=s(t) (12)

It can be seen that the receiving system can demodulate the original signal s(t).

In the formula, g(*,*) can be addition, subtraction, multiplication and division operations or more complex mixed operations, and g -1(*,*) is the inverse operation of g(*,*). In the hardware experiment, g(*,*) performs the addition operation, g -1(*,*) performs the subtraction operation, T is the attenuation coefficient, T -1 is the amplification coefficient, G and G -1 are VCCS and CCVS respectively. The G parameters and R parameters, where T=0.5, T -1=2, G=0.05mS, G -1=20k Ω , VCCS and CCVS can be realized by the operational amplifier circuit. Due to space limitations, we will not go into details here.

3 System fidelity and security performance

According to equation (12), when the circuit parameters are accurately matched, the receiving end can demodulate the signal S(t) without distortion. Theoretically, the system has high fidelity. During the hardware experiment, the components of the transceiver circuit were strictly paired, all resistors were precision adjustable resistors, and the receiving end inductor L was composed of a simulated inductor, as shown in Figure 4. Adjusting resistor R5 can achieve strict matching of the inductance at both transmitting and receiving ends.

On the other hand, only when the receiving circuit parameters accurately match the transmitting circuit parameters, the useful information s(t) can be demodulated. R, C1, C2, L, etc. play the role of key parameters. Therefore, the system It is also safe. Since this scheme uses two-level chaotic modulation, which increases the difficulty of signal deciphering, its security performance is improved compared with the general single-level chaotic system.

4 Hardware experiment results

Figure 5 shows the double scroll chaotic attractor after the originating circuit is modulated by s(t). Figure 6 shows the chaotic modulation waveform of the originating signal. Figure 7 shows the demodulation waveform of the sine wave signal. Figure 8 shows the demodulation waveform of the speech signal. , Figure 9 is the demodulation waveform of the music signal. After amplifying s'(t) and inputting it into the speaker, a very clear and realistic speech signal can be restored. In the three pictures in Figures 7 to 9, the upper waveform is the input signal s (t), the lower waveform is the demodulated signal s'(t). All pictures in this article are taken from the digital storage oscilloscope TDS3012.

This paper proposes a chaotic communication scheme that uses Chua's circuit to realize voice signal transmission. On this basis, a hardware experiment for secure communication of voice transmission is carried out. Through research on this solution, the following conclusions are drawn: (1) It has better amplitude-frequency response characteristics, improved fidelity, and can meet the requirements of transmitting speech signals; (2) Two-level chaotic modulation is used to improve improve the security performance of the system. At the same time, the circuit parameters R, C1, C2 and L play the role of key parameters. Without knowing the originating circuit parameters in advance, it is difficult to decipher the useful signal. Therefore, this article is safe; (3) The circuit implementation is not complicated and practical.