Secrecy Communications for Wireless-Powered Cooperative NOMA Systems With Imperfect CSI: A Relay Selection and Friendly Jamming Approach

In this paper, we investigate the physical layer security of wireless-powered cooperative non-orthogonal multiple access (NOMA) systems under imperfect channel state information (CSI). In particular, the eavesdropper (Eve) intends to wiretap the transmitted signals from the source, and a friendly jammer, with energy harvesting (EH) function, emits a jamming signal to confuse the Eve. We propose a two-phase relay selection and cooperative jamming scheme. In the direct transmission phase, an active relay is selected from M candidates to serve a pair of NOMA users, and the base station (BS) transmits a superimposed signal to the selected relay and wireless-powered jammer, which harvests energy from the radio frequency signals. In the cooperative transmission phase, the selected relay serves the NOMA users, and the jammer utilizes the harvested energy to emit the artificial noise to confuse the Eve. Under the imperfect CSI, the closed-form expressions for the connection outage probability (COP) and secrecy outage probability (SOP) of a pair of NOMA users are derived. Numerical results verify the correctness of the theoretical analysis and demonstrate the superiority of the proposed scheme compared to the existing scheme.