  Next-generation communication networks will likely involve the
energy-efficient transfer of information and energy over the same wireless
channel, for which the physical layer will become more vulnerable to cyber
attacks by potential multi-antenna eavesdroppers. To address this issue, this
paper considers transmit time-switching (TS) mode, in which energy and
information signals are transmitted separately in time by the BS. This protocol
is not only easy to implement but also delivers the opportunity of
multi-purpose beamforming, in which energy beamformers during wireless power
transfer are useful in jamming the eavesdropper. In the presence of imperfect
channel estimation and multi-antenna eavesdroppers, the energy and information
beamformers and the transmit TS ratio are jointly optimized to maximize the
worst-case user secrecy rate subject to UEs' harvested energy thresholds and a
BS transmit power budget. New robust path-following algorithms, which involve
one simple convex quadratic program at each iteration are proposed for
computational solutions of this difficult optimization problem and also the
problem of secure energy efficiency maximization. The latter is further complex
due to additional optimization variables appearing in the denominator of the
secrecy rate function. Numerical results confirm that the performance of the
proposed computational solutions is robust against the channel uncertainties.
