  We consider the wireless powered communications where users harvest energy
from non-dedicated sources. The user follows a harvest-then-transmit protocol:
in first phase of a slot time the source node harvests energy from a nearby
conventional Access Point, then transmit information to its destination node or
relay node in the second phase. We obtain the optimal\textit{ harvesting ratio}
to maximize the expected throughput for direct transmission (DT )and decode
forward (DF) relay under outage constraint, respectively. Our results reveal
that the optimal harvest ratio for DT is dominated by the outage constraint
while for DF relay, by the data causality .
