## 1 Introduction
Section 3 describes the hardware realization on FPGA of the proposed scheme. The performance analysis of the proposed secure communication systems for voice encryption is presented in Section 4, using multiple metrics when the number of cascaded levels is increased from N = 1 to 20. Section 5 presents the results of a comparative analysis. Finally, the conclusions are discussed in Section 6. Model Description of the N-Levels Cascaded Chaotic-Based Secure Communication System for Voice Encryption  
Chaos-based encryption systems are highly effective for secure communication due to their sensitivity to initial conditions and complexity, generating hard-to-decipher, pseudo-random signals. As the demand for stronger security grows, particularly in fields like voice encryption and real-time data transmission, researchers have explored using cascaded chaotic systems. Cascading multiple chaotic systems significantly enhances encryption strength by increasing signal complexity and expanding the key-space, making the encryption more secure. This section will give a general overview of the structure of secure communication system for voice encryption based on N-level cascaded 4D unified hyperchaotic system. Additionally, it will include a detailed description of the system's structure and its mathematical representation, highlighting the underlying principles and assumptions that govern its operation. First, we introduce the mathematical representation of the 4D unified hyperchaotic system used in this paper, which is given in (1) as follows:  
\[ \begin{split}\dot{x}_{1}&=a_{1}(x_{2}-x_{1})+x_{4}\\ \dot{x}_{2}&=a_{2}x_{1}-x_{1}x_{3}+b_{4}x_{2}\\ \dot{x}_{3}&=x_{1}x_{2}-a_{3}x_{3}\\ \dot{x}_{4}&=0.2x_{4}+0.1x_{2}x_{3}\end{split}  \tag{1}\]  
where \((x_{1},x_{2},x_{3},x_{4})\in R^{4}\) are state variables. \(a_{1}\), \(a_{2}\), \(a_{3}\) and \(a_{4}\) are positive parameters. The proposed system has a chaotic behaviors when the parameters are \(a_{1}=45\), \(a_{2}=3\), \(a_{3}=10\) and \(10\leq a_{4}\leq 40\). Based on the value of parameter \(a_{4}\), the proposed oscillator shows a hyperchaotic behaviors in different regions:  
1. When \(a_{4}=13\), the output hyperchaotic attractor is similar to Lorenz attractor. 2. When \(a_{4}=27\), the output hyperchaotic attractor is similar to Lu attractor. 3. When \(a_{4}=36\), the attractor is similar to Chen attractor. Figure 1, reveals the chaotic attractors generated by Equations (1) for different values of parameter a. When \(a=13,27\), and \(37\), the resulting attractors will be similar to the Lorenz, Lu, and Chen systems, respectively. The fourth-order Runge-Kutta integrator, which has high algorithmic accuracy in numerical simulation and due to its high accuracy and efficiency in solving ordinary differential Equations (ODEs), which are critical for accurately modeling the behavior of the chaotic systems [37]. The used simulation parameters are fixed step size (0.001), \(a_{1}=45\), \(a_{2}=3\), \(a_{3}=10\), \(a_{4}=[13,27,36]\), and the initial conditions are \(x(0)=[1,1,1,1]^{T}\). Extended detailed analysis of proposed system available in [38]. Figure 2 shows the principle of cascaded structure of \(N_{th}\)chaotic systems using the unified 4D hyperchaotic system. The principle of cascaded structure is mainly to construct a new hyperchaotic system with its own performance. In this context, the additive cascaded technique used in this paper, enhances the overall complexity of the chaotic signals generated, significantly increasing the key-space and improving the security of the encryption process. To illustrate the proposed secure communication scheme using the additive cascaded technique, consider a series connection of the 4D unified hyperchaotic system given in (1), where the output of the first 4D hyperchaotic system is fed into the input of the second one producing another chaotic signal that is connected to the input of the next oscillator recursively till the \(N_{th}\) oscillator.