Physical Layer Security Meets Privacy Requirements for Downlink NOMA

Recently, cutting-edge techniques, such as Non-Orthogonal Multiple Access (NOMA), have been highlighted to enable wireless networks to handle massive access scenarios and further improve the spectral efficiency. Nevertheless, these advantages come at the price of privacy exposure since a received signal may contain information belonging to several users. Usual security mechanisms, such as upper-layer encryption and sophisticated authentications, are not well suited to low-capacity Internet of Things (IoT) devices. These technical challenges have spotlighted Physical Layer Security (PLS) as a key enabling technology since it takes advantage of the wireless communication characteristics to secure communications without adding complex encryption mechanisms at higher layers. In this paper, we propose a PLS approach based on a network coding technique in order to ensure secure NOMA-based downlink transmissions taking into account the Quality of Service (QoS) requirements of the users. By doing so, decoding successfully some of the transmitted packets by an eavesdropper does not reveal useful information about the data. In fact, we develop a sequence-based algorithm with the aim of ensuring the confidentiality by changing the users’ positions in the Successive Interference Cancellation (SIC) decoding process. Henceforth, using its corresponding sequence, the legitimate user becomes the only one able to decode the information sent by the Base Station (BS). We show that the eavesdropper decoding complexity increases exponentially with the sequence length making the task intractable for relatively long ones.