  We consider a three-node decode-and-forward (DF) half-duplex relaying system,
where the source first harvests RF energy from the relay, and then uses this
energy to transmit information to the destination via the relay. We assume that
the information transfer and wireless power transfer phases alternate over time
in the same frequency band, and their {\it time fraction} (TF) may change or be
fixed from one transmission epoch (fading state) to the next. For this system,
we maximize the achievable average data rate. Thereby, we propose two schemes:
(1) jointly optimal power and TF allocation, and (2) optimal power allocation
with fixed TF. Due to the small amounts of harvested power at the source, the
two schemes achieve similar information rates, but yield significant
performance gains compared to a benchmark system with fixed power and fixed TF
allocation.
