  This paper investigates a wireless-powered cooperative communication network
consisting of a source, a destination and a multi-antenna decode-and-forward
relay. We consider the relay as a wireless-powered node that has no external
power supply; but it is equipped with an energy harvesting (EH) unit and a
rechargeable battery such that it can harvest and accumulate energy from
radio-frequency signals broadcast by the source. By fully incorporating the EH
feature of the relay, we develop an opportunistic relaying protocol, termed
accumulate-then-forward (ATF), for the considered WPCCN. We then adopt the
discrete Markov chain to model the dynamic charging and discharging behaviors
of the relay battery. Based on this, we derive a closed-form expression for the
exact outage probability of the proposed ATF protocol. Numerical results show
that the ATF scheme can outperform the direct transmission one, especially when
the amount of energy consumed by relay for information forwarding is optimized.
