# Physical-Layer Security in Power-Domain NOMA Based on Different Chaotic Maps
## 1 Introduction  
Nonorthogonal multiple access (NOMA) is a multiple-access technology for next-generation (5G) mobile networks. It is a nonorthogonal multiplexing method that allows users to be multiplexed in the power domain, which had previously been neglected by wireless mobile systems. Many users' signals are combined at the transmitter and then segregated via successive interference cancellation (SIC) at the receiver [1]. In recent years, the multiple access (MA) system has been considered a significant high-tech advancement that has defined each generation of wireless communication networks. From the 1G mobile communication system to the 4G system, orthogonal MA (OMA) has been extensively used in current wireless systems. OMA systems include the widely used frequency-division MA for 1G, time-division MA for 2G, code-division MA (CDMA) for 3G, and orthogonal-frequency-division MA for 4G. Notably, only a limited number of users are multiplexed orthogonally within the frequency, time, or code domain in these OMA methods [2]. Several promising technologies have been proposed in recent years to address this critical issue. One such technology is nonorthogonal multiple access (NOMA): a way of serving numerous users from a single wireless resource. NOMA can be accomplished in different domains, including power, code, and others. For participation in the total resource, code-domain NOMA uses user-specific spreading sequences, whereas power-domain NOMAuses channel-gain changes among users for multiplexing through power allocation [3]. Owing to the open propagation environment, wireless networks are prone to interception from unauthorized receivers, and the enormous rise in NOMA users and related data traffic could result in more information security breaches, thereby making standard wireless security techniques depend on upper-layer cryptography. Because use of upper-layer security in NOMA systems with power-limited devices is ineffective due to cost and complexity restrictions, a reliable and economical physical-layer security (PLS) strategy that depends on channel-coding methods has been developed [4; 5; 6]. This strategy improves the performance gap between the intended user and the eavesdropper, using basic channel features such as noise, fading, diversity, and interference [4]. In addition, when combined with encryption-based systems, it can increase the entire system's security [7; 8]. Several types of encryption algorithm exist, each designed for a specific purpose. When existing algorithms become insecure, new ones are developed. The Data Encryption Standard, the Advanced Encryption Standard, Blowfish encryption, and Rivest Cipher 4 are some of the most well-known cryptographic algorithms. Some recent encryption methods have been shown to be untrustworthy for ciphering [9]. Owing to its sensitivity to control parameters and initial conditions, chaos-based encryption is inherent in chaotic methods of meeting security demands. Several studies have described various chaos-based secure communication (CBSC) methods that offer cost effective and robust PLS for multiuser wireless applications. Chaos-based, secure power-domain NOMA was introduced in [6] for the uplink large-scale and Rician channels, with effective dynamic power control for transmitted multilevel chaos-shift-keying signals. For multiuser detection, an advanced receiver scheme that depends on SIC and chaos demodulation is suggested. The authors in [7] introduced a downlink (DL) chaotic NOMA (C-NOMA) method that used the C-MIMO idea to achieve high-capacity allocation and PLS. In [8], an uplink C-NOMA transmission technique, in which C-MIMO was used with NOMA to improve system throughput while providing PLS to other cell users and an eavesdropper, was proposed. NOMA successfully replicated the impacts of channel coding and PLS in C-MIMO. The authors in [10] offered a DL-C-NOMA transmission technique that achieves better capacity while providing PLS that is resistant to eavesdroppers and other users without initial keys.