  This paper investigates a wireless energy harvesting cooperative network
(WEHCN) consisting of a source, a decode-and-forward (DF) relay and a
destination. We consider the relay as an energy harvesting (EH) node equipped
with EH circuit and a rechargeable battery. Moreover, the direct link between
source and destination is assumed to exist. The relay can thus harvest and
accumulate energy from radio-frequency signals ejected by the source and assist
its information transmission opportunistically. We develop an incremental
accumulate-then-forward (IATF) relaying protocol for the considered WEHCN. In
the IATF protocol, the source sends its information to destination via the
direct link and requests the relay to cooperate only when it is necessary such
that the relay has more chances to accumulate the harvested energy. By modeling
the charging/discharging behaviors of the relay battery as a finite-state
Markov chain, we derive a closed-form expression for the outage probability of
the proposed IATF. Numerical results validate our theoretical analysis and show
that the IATF scheme can significantly outperform the direct transmission
scheme without cooperation.
