## 1 Introduction  
The rapid expansion of wireless networks has produced a massive increment in data traffic. The fundamental requirements for future networks such as 5G include massive connectivity of users and the Internet of Things (IoT), low latency, high energy efficiency, and higher data rates. Non-orthogonal multiple access (NOMA) has attracted extensive attention in the literature because it can provide superior energy efficiency, higher data rates, and low transmission latency [1, 2]. NOMA is based on superposition coding at the transmitter and successive interference cancellation (SIC) at the receiver, where multiple users can simultaneously access the same frequency with different transmit power levels [3, 4]. To further improve the coverage of the network, cooperative communications utilize a relay or a cooperative user to forward messages to the intended users who are very far away from the base station (BS) and who do not have a direct link because of obstacles or the shadowing effect. Security represents one of the major problems in wireless networks because the signals transmitted can be intercepted in the shared wireless medium. Physical layer security (PLS) was introduced to complement traditional security techniques, and it exploits the nature of physical layer dynamics to provide a secure transmission [5]. Several studies in the literature have investigated the security in NOMA systems with the objectives of secrecy sum rate maximization [6; 7; 8] and secrecy energy efficiency (SEE) [3; 9]. [PERSON] et al. [6] considered a single-input single-output NOMA system with a passive eavesdropper, where the objective was to maximize the secrecy sum rate (SSR) under the constraint of minimum data rate at the users. The authors compared their proposed solution with the orthogonal multiple access (OMA) method, obtaining a significant improvement of the SRR by using NOMA. A downlink simultaneous wireless information and power transfer (SWIPT) system with NOMA was proposed in [7]. The BS simultaneously served several information receivers and multiple EH receivers in which the EH receivers were considered potential eavesdroppers. The authors solved the SSR maximization problem under the constraints of minimum data rate required by the information receiver users and minimum harvested energy required by the EH receivers. [PERSON] et al. [8] proposed secrecy sum rate maximization in a cooperative NOMA system composed of two users and a relay. The paper highlights the importance in cooperative communications of improving the secrecy performance of the network, since simulation results showed that higher values for the secrecy sum rate were obtained by employing a cooperative relay compared to values obtained by the NOMA network without cooperative relaying. A cooperative system model with SWIPT was proposed in [10], where an EH user co-existed with a nearby user that applies SWIPT to harvest energy and work as a relay to help and guarantee a minimum data rate at the far user. The minimum transmit power problem was considered under the constraints of minimum signal to interference plus noise ratio (SINR) of the users and minimum harvested energy at the EH user. [PERSON] et al. [11] considered an underlay cognitive multiple-input single-output NOMA system with SWIPT in which the secondary BS and the multiple secondary SWIPT users co-existed with several primary users via the underlay scheme. The minimum transmit power problem is investigated subject to the constraints of minimum SINR, minimum energy harvested to satisfy the power consumption at the users, and maximum interference power with the primary network. [PERSON] et al. [9] considered a downlink NOMA system composed of a BS and multiple legitime users under the presence of an eavesdropper. The Secrecy energy efficiency was maximized subject to the constraints of minimum data rate and maximum available power at the BS. A NOMA cognitive radio (CR) system with energy harvesting (EH) users was proposed by [PERSON] et al.