Secrecy design in full-duplex multi-user MIMO wireless networks with untrusted non-linear energy harvesting

In this paper, we consider a secure full-duplex (FD) multi-user multiple-input multiple-output (MU-MIMO) network where an FD base station simultaneously communicates with users and transfers energy for untrusted energy harvesting (EH) devices. Considering that the EH users are untrusted and try to wiretap the signals, the base station adds artificial noise (AN) signals into its transmitted signals to enhance the system security. We focus on two single-objective optimization problems including (i) the sum secrecy rate maximization problem, (ii) the harvested energy maximization problem, and a multi-objective optimization problem of the secure information and EH trade-off. The goal is to jointly optimize the transmit covariance matrices of the precoding and AN jamming under the constraints on transmit power budgets, harvested energy requirements, and minimum secrecy rates. Three considered optimization problems are highly non-convex due to the non-convexity of the sum secrecy rate. To handle this challenge, we first recast the sum secrecy rate as the inner convex approximation one thanks to the difference of two convex functions (DC) approach. Then, the sequential convex programming (SCP) approach is utilized to iteratively find the optimal solution. The numerical simulations are provided to investigate the impacts of the system settings and residual self-interference on the achievable system performances in terms of the secrecy rate and harvested energy. Furthermore, the numerical results also reveal the interesting trade-offs between the secrecy rate and harvested energy.