Secure Primary Transmission Assisted by a Secondary Full-Duplex NOMA Relay

In this paper, secure primary transmission is proposed by using a multi-antenna secondary full-duplex non-orthogonal multiple access (NOMA) relay in cognitive radio (CR) networks. First, the primary signal is transmitted from the primary transmitter to the relay. Artificial noise is generated by using part of the antennas at the relay to disrupt eavesdropping, without affecting the primary transmission. Then, superimposed signals are transmitted from the relay to the primary receiver and secondary receivers via NOMA. The primary security is guaranteed by the modified decoding order and beamforming optimization, which is converted to convex and solved by an iterative algorithm. Simulation results are presented to show the effectiveness of the proposed scheme in guaranteeing the primary security in CR networks.