Beamforming design for secure SWIPT systems under a non-linear energy harvesting model

Simultaneous wireless information and power transfer (SWIPT) is an appealing solution to extend the lifetime of wireless nodes and, hence, alleviate the energy bottleneck of energy-constrained wireless communication networks. SWIPT advocates the dual use of radio frequency signals for conveying information and energy concurrently, which introduces a paradigm shift in system design. This chapter focuses on the use of multiple antennas to improve the efficiency of wireless power transfer (WPT) and secure information transmission. In particular, our objective is to maximize the secrecy rate of a SWIPT system via beamforming. To this end, we formulate a non-convex optimization problem based on a practical nonlinear energy harvesting model. The problem formulation allows for the use of an energy signal to improve WPT efficiency and to provide communication security. The globally optimal solution of the design problem is obtained via a one-dimensional search and semidefinite programming (SDP) relaxation. Numerical results demonstrate that the proposed design can achieve a significant gain in secrecy rate compared to two baseline schemes.