  We consider wireless-powered amplify-and-forward and decode-and-forward
relaying in cooperative communications, where an energy constrained relay node
first harvests energy through the received radio-frequency signal from the
source and then uses the harvested energy to forward the source information to
the destination node. We propose time-switching based energy harvesting (EH)
and information transmission (IT) protocols with two modes of EH at the relay.
For continuous time EH, the EH time can be any percentage of the total
transmission block time. For discrete time EH, the whole transmission block is
either used for EH or IT. The proposed protocols are attractive because they do
not require channel state information at the transmitter side and enable relay
transmission with preset fixed transmission power. We derive analytical
expressions of the achievable throughput for the proposed protocols. The
derived expressions are verified by comparison with simulations and allow the
system performance to be determined as a function of the system parameters.
Finally, we show that the proposed protocols outperform the existing fixed time
duration EH protocols in the literature, since they intelligently track the
level of the harvested energy to switch between EH and IT in an online fashion,
allowing efficient use of resources.
