Joint Beamforming and Artificial Noise Optimization for Secure Transmissions in MISO-NOMA Cognitive Radio System with SWIPT  
[PERSON]  
1 School of Electrical and Computer Engineering, University of Ulsan, Ulsan 680-749, Korea; [EMAIL_ADDRESS] (C.E.G. ); [PERSON]L_ADDRESS]om (M.R.C.) [PERSON]  
1 School of Electrical and Computer Engineering, University of Ulsan, Ulsan 680-749, Korea; [EMAIL_ADDRESS] (C.E.G. ); [PERSON]SS] (M.R.C.) 1 School of Electrical and Computer Engineering, University of Ulsan, Ulsan 680-749, Korea; carli.garcia27@hotmail. tries to minimize the risk of any interference with the PUs at the expense of spectrum use, allowing SUs to use the licensed spectrum in the absence of the PUs [7; 8]. Meanwhile, NOMA has surged as another powerful technology to enhance spectrum efficiency while providing high transmission rates and high user densities for future mobile communications [9; 10]. NOMA can be categorized into code-domain and power-domain NOMA. Complex encoding or decoding techniques are required in code-domain NOMA, while the key idea of power-domain NOMA is to assign different power levels to users according to their channel strength, such that higher transmission power is assigned to users with weaker channels. Moreover, on the receiver side, users with stronger channels apply successive interference cancelation (SIC) to first decode the signals of the weaker users, and then, they decode their own signals. In this paper, we focus on power-domain NOMA, which can be further combined with other communications technologies (multiple-input single-output (MISO), multiple-input multiple-output (MIMO), cooperative communications) to reach a better yield [11; 12; 13; 14; 15; 16]. Furthermore, energy-efficient techniques have been investigated for future wireless networks in support of environmental concerns and battery-limited devices, such as sensors and wearables [17]. In this sense, achieving high energy efficiency and recycling energy are the main goals of energy-efficient techniques, where simultaneous wireless information and power transfer (SWIPT) brings technological innovations and plays a key role in energy and information transmission [18; 19; 20]. In contrast to the conventional energy-harvesting (EH) technologies that rely on unpredictable natural sources, or where the communications nodes are only able to recharge from electromagnetic radiation, such as wireless power transfer (WPT), SWIPT enables users to perform information decoding (ID) and radio-frequency (RF) EH simultaneously [21; 22]. Hence, the study of SWIPT in NOMA with CR networks is of meaningful importance in supporting massive numbers of mobile devices with limited battery power [23; 24; 25]. However, a NOMA-CR network assisted by SWIPT technology operates in a wireless transmission environment, which is vulnerable to malicious receivers that can intercept confidential information transmitted from the sources to either the legitimate or the unlicensed users. Therefore, security is a critically challenging issue that must be handled for the suitable deployment of wireless communications networks. In this sense, physical-layer security (PLS) has emerged as a complementary technology to address the eavesdropping security issue by exploiting the physical characteristics of wireless channels, e.g., propagation delay, multipath fading, and so on. Although traditional cryptographic encryption and decryption techniques have been pointed out to secure communication systems using complex algorithms and encryption keys. They require key management, maintenance and distribution procedures that use network resources and may become an issue in large-scale decentralized and heterogeneous wireless network. Moreover, the encryption keys are prone to being broken by high computational capabilities of an eavesdropper [26; 27]. These drawbacks are the motivations for the study and research on PLS to complement cryptographic techniques.