We consider wireless-powered amplify-and-forward and decode-and-forwardrelaying in cooperative communications, where an energy constrained relay nodefirst harvests energy through the received radio-frequency signal from thesource and then uses the harvested energy to forward the source information tothe 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 totaltransmission block time. For discrete time EH, the whole transmission block iseither used for EH or IT. The proposed protocols are attractive because they donot require channel state information at the transmitter side and enable relaytransmission with preset fixed transmission power. We derive analyticalexpressions of the achievable throughput for the proposed protocols. Thederived expressions are verified by comparison with simulations and allow thesystem performance to be determined as a function of the system parameters.Finally, we show that the proposed protocols outperform the existing fixed timeduration EH protocols in the literature, since they intelligently track thelevel of the harvested energy to switch between EH and IT in an online fashion,allowing efficient use of resources.
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