Joint Secure Beamforming for Cognitive Radio Networks with Untrusted Secondary Users

In this paper, we consider a cognitive radio network (CRN) consisting of a primary transmitter-receiver pair and an untrusted secondary transmitter-receiver pair, and each pair is a multiple-input single-output (MISO) link. We consider two transmission schemes, namely underlay scheme and cooperative scheme. For the underlay scheme, the secondary user (SU) is allowed to transmit simultaneously in the presence of primary transmission. For the cooperative scheme, the secondary transmitter acts as a relay node to increase the secrecy rate of primary transmission in exchange for its own transmission. For both schemes, the SU is untrusted and considered as a potential eavesdropper. Our goal is to minimize the total power consumption while satisfying the primary user (PU)'s required secrecy rate and the SU's required information rate. By suitable optimization tools, we design the joint secure beamforming for both schemes. The simulation results show that in the considered system model, the underlay scheme outperforms the cooperative scheme, especially with high rate requirements and a large number of antennas at secondary transmitter.