  The dual use of radio signals for simultaneous wireless information and power
transfer (SWIPT) has recently drawn significant attention. To meet the
practical requirement that energy receivers (ERs) operate with much higher
received power than information receivers (IRs), ERs need to be deployed closer
to the transmitter than IRs. However, due to the broadcast nature of wireless
channels, one critical issue is that the messages sent to IRs cannot be
eavesdropped by ERs, which possess better channels from the transmitter. In
this paper, we address this new secrecy communication problem in a multiuser
multiple-input single-output (MISO) SWIPT system where a multi-antenna
transmitter sends information and energy simultaneously to one IR and multiple
ERs, each with a single antenna. By optimizing transmit beamforming vectors and
their power allocation, we maximize the weighted sum-energy transferred to ERs
subject to a secrecy rate constraint for the information sent to the IR. We
solve this non-convex problem optimally by reformulating it into a two-stage
problem. First, we fix the signal-to-interference-plus-noise ratio (SINR) at
the IR and obtain the optimal beamforming solution by applying the technique of
semidefinite relaxation (SDR). Then the original problem is solved by a
one-dimension search over the optimal SINR value for the IR. Furthermore, two
suboptimal low-complexity beamforming schemes are proposed, and their
achievable (secrecy) rate-energy (R-E) regions are compared against that by the
optimal scheme.
