  This paper considers a multiuser multiple-input single-output (MISO) downlink
system with simultaneous wireless information and power transfer. In
particular, we focus on secure communication in the presence of passive
eavesdroppers and potential eavesdroppers (idle legitimate receivers). We study
the design of a resource allocation algorithm minimizing the total transmit
power for the case when the legitimate receivers are able to harvest energy
from radio frequency signals. Our design advocates the dual use of both
artificial noise and energy signals in providing secure communication and
facilitating efficient wireless energy transfer. The algorithm design is
formulated as a non-convex optimization problem. The problem formulation takes
into account artificial noise and energy signal generation for protecting the
transmitted information against both considered types of eavesdroppers when
imperfect channel state information (CSI) of the potential eavesdroppers and no
CSI of the passive eavesdroppers are available at the transmitter. In light of
the intractability of the problem, we reformulate the considered problem by
replacing a non-convex probabilistic constraint with a convex deterministic
constraint. Then, a semi-definite programming (SDP) relaxation approach is
adopted to obtain the optimal solution for the reformulated problem.
Furthermore, we propose a suboptimal resource allocation scheme with low
computational complexity for providing communication secrecy and facilitating
efficient energy transfer. Simulation results demonstrate a close-to-optimal
performance achieved by the proposed schemes and significant transmit power
savings by optimization of the artificial noise and energy signal generation.
