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This paper studies a relay-assisted wireless-powered communication network (WPCN), where a hybrid relay node (HRN) broadcasts radio-frequency energy to energy-harvesting users and forwards these users' data to a base station in amplify-and-forward (AF) or decode-and-forward (DF) fashions. The joint time duration and power allocation optimization problems for both AF and DF relay-assisted WPCNs are investigated to maximize the energy efficiency of HRN, which are challenging to solve due to the nonconvexity and strong coupling of variables. For the DF relay-assisted WPCN, we propose an optimal time duration and power allocation algorithm by decomposing the original problem into three subproblems, i.e., time duration allocation subproblem for users' information transfer, time duration allocation subproblem for HRN's energy and information transfer, and HRN's power allocation subproblem. For the AF rely-assisted WPCN, we propose an iterative algorithm to optimize the transmit power and energy transfer time of HRN, by proving the quasi-concavity of the objective function where the optimal time duration allocation has been derived. The simulation results verify that the proposed algorithms can render significant energy efficiency gains, e.g., the performance gains of the proposed algorithms are about 300% and 150% compared with the existing schemes for the AF relay-assisted WPCN and DF relay-assisted WPCN, respectively, when the noise power is relatively large.

Energy Efficiency Maximization for Relay-Assisted WPCN: Joint Time Duration and Power Allocation