Physical layer security for NOMA systems

Non-orthogonal multiple access (NOMA) has been envisioned as a promising candidate for the next generation of wireless communication systems. By multiplexing users over the same time-frequency resource block, NOMA can increase the number of served users and enhance the system spectral efficiency (SE), compared with conventional orthogonal multiple access (OMA). Nonetheless, from the security perspective, sharing the same time-frequency resource among users imposes secrecy challenges. In this regard, physical layer security (PLS) has been introduced as an additional protecting layer to traditional encryption methods for securing communication confidentiality, via exploiting the randomness nature of wireless transmission media. The application of PLS to NOMA networks has drawn great attention recently. Existing works on this cover various scenarios, including single-input single-output (SISO), multiple-input multiple-output (MIMO), and massive MIMO systems. Moreover, PLS has also been considered when NOMA is combined with other advanced transmission technologies, such as simultaneous wireless information and power transfer (SWIPT), relay, full-duplex (FD), and millimeter wave (mmWave). This chapter aims to provide a comprehensive survey on the research progress of PLS-assisted NOMA systems. Toward this, we first introduce the fundamentals of NOMA and PLS, respectively. Then, we classify the existing PLS-assisted NOMA frameworks into three categories based on the number of antennas at the base station (BS), namely, SISO-, MIMO-, and massive MIMO-based systems. A detailed presentation of the state-of-the-art on PLS-assisted NOMA systems is further provided.