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Wireless Communications and Mobile Computing
Volume 2018, Article ID 7906957, 8 pages
https://doi.org/10.1155/2018/7906957
Research Article

Power-Splitting Scheme for Nonlinear Energy Harvesting AF Relaying with Direct Link

1School of Management, Xi’an Polytechnic University, Xi’an, China
2Army Academy of Border and Coastal Defence, Department of Information and Arms, Xi’an, China

Correspondence should be addressed to Liqin Shi; moc.621@niqiltcennoc

Received 3 May 2018; Revised 6 June 2018; Accepted 10 June 2018; Published 2 July 2018

Academic Editor: Fuhui Zhou

Copyright © 2018 Xiaobo Bai et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Simultaneous wireless information and power transfer (SWIPT) is a promising technique to prolong the lifetime of energy-constrained relay systems. Most previous works optimize power-splitting (PS) scheme based on a linear or a simple two-piecewise linear energy harvesting (EH) model, while the employed EH model may not characterize the properties of practical EH harvesters well. This leads to a mismatch between the existing PS scheme and the practical EH harvester available for relay systems. Motivated by this, this paper is devoted to the design of PS scheme in a nonlinear EH amplify-and-forward energy-constrained relay system in the presence of a direct link between the source and the destination. In particular, we formulate an optimization problem to maximize the system capacity according to the instantaneous channel state information, subject to a nonlinear EH model based on the logistic function. The objective function of the formulated problem is proven to be unimodal and there is no closed-form expression for the optimal PS ratio due to the complexity of logistic function. In order to reduce overhead cost of optimizing PS ratio, a simpler nonlinear EH model based on the inverse proportional function is employed to replace the nonlinear EH model based on the logistic function and we further derive the closed-form expression for the optimal PS ratio. Simulation results reveal that a higher system capacity can be achieved when the PS scheme is optimized based on nonlinear EH models instead of the linear EH model, and that there is only a marginal difference between the capacity under the two optimal PS schemes optimized for two different nonlinear EH models.