Energy harvesting is considered as a promising solution to efficiently prolong the lifetime of energy-constrained wireless networks. In particular, wireless energy harvesting, which scavenges energy from ambient radio signals, has recently received an increasing attention. In this paper, we consider a self-powered wireless system with one transmitter and one receiver, in which the transmitter has no fixed power supplies and extract energy only via wireless energy harvesting from ambient radio signals. It is assumed that the transmitter follows a save-then-transmit protocol, which specifies that a fraction (referred to as save-ratio) of time is devoted exclusively to energy harvesting while the remaining fraction is used for data transmission. We focus on the optimal save-ratio selection and achievable throughput maximization in two cases with regard to energy harvesting rate: the deterministic case, in which energy harvesting rate is known in advance, and the stochastic case, in which energy harvesting rate is unknown and only its statistical properties (e.g. probability distribution) are available. The optimal save-ratio for the two cases is theoretically derived as a function of energy harvesting rate (or its statistical properties). The experimental results characterize how the optimal save-ratio and the maximal achievable throughput vary with energy harvesting rate and validate the optimality of save-ratio selection.
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