  In this paper, we consider a wireless-powered cooperative communication
network, which consists of one hybrid access point (AP), one source and one
relay to assist information transmission. Unlike conventional cooperative
networks, the source and relay are assumed to have no embedded energy supplies
in the considered system. Hence, they need to first harvest energy from the
radio-frequency (RF) signals radiated by the AP in the downlink (DL) before
information transmission in the uplink (UL). Inspired by the recently proposed
harvest-then-transmit (HTT) and harvest-then-cooperate (HTC) protocols, we
develop a new adaptive transmission (AT) protocol. In the proposed protocol, at
the beginning of each transmission block, the AP charges the source. AP and
source then perform channel estimation to acquire the channel state information
(CSI) between them. Based on the CSI estimate, the AP adaptively chooses the
source to perform UL information transmission either directly or cooperatively
with the relay. We derive an approximate closed-form expression for the average
throughput of the proposed AT protocol over Nakagami-m fading channels. The
analysis is then verified by Monte Carlo simulations. Results show that the
proposed AT protocol considerably outperforms both the HTT and HTC protocols.
