  This paper maximizes the achievable throughput of a relay-assisted wirelessly
powered communications system, where an energy constrained source, helped by an
energy constrained relay and both powered by a dedicated power beacon (PB),
communicates with a destination. Considering the time splitting approach, the
source and relay first harvest energy from the PB, which is equipped with
multiple antennas, and then transmits information to the destination. Simple
closed-form expressions are derived for the optimal PB energy beamforming
vector and time split for energy harvesting and information transmission.
Numerical results and simulations demonstrate the superior performance compared
with some intuitive benchmark beamforming scheme. Also, it is found that
placing the relay at the middle of the source-destination path is no longer
optimal.
