Secure beamforming with nonorthogonal multiple access transmission in cooperative CR networks for Internet of Things

In this paper, secure beamforming design in cooperative cognitive radio networks for Internet of Things (IoT) is investigated, where we maximize the secrecy sum rate for the IoT devices (IoDs) to protect their communication and employ the nonorthogonal multiple access scheme at the cognitive access point (AP) to serve the cognitive users. The formulated secure beamforming optimization problem is a non-convex problem and its global optimal solution is very hard to find. To overcome this difficulty, we propose an iterative algorithm based on semidefinite programming and monotonic optimization method to obtain the global optimal solution, which has high computational complexity and is suitable for the case that the cognitive AP has a powerful computing ability. For some application scenarios in which the cognitive AP has a lower computing ability, we propose suboptimal solutions based on zero-forcing scheme, which have much lower computational complexity. Furthermore, we derive the limit value of the secrecy sum rate for the IoDs when the transmit power of the cognitive AP goes to infinity, which can serve as an upper bound for our proposed solutions. Numerical results are also provided to verify the effectiveness of our proposed solutions.