This paper investigates the energy-efficient resource allocation problem for the energy harvesting-powered device-to-device (D2D) communications underlaying cellular networks under the harvest-then-transmit protocol. Our objective is to maximize the energy efficiency (EE) of D2D communications via joint time allocation and power control while satisfying the quality of service (QoS) requirement of cellular user (CU) and energy causality constraint. Exploiting the non-linear fractional programming, an iterative resource allocation scheme based on the Dinkelbach method is proposed to solve the formulated nonconvex optimization problem. For the optimization problem in each iteration, we decompose it into two subproblems: the power control subproblem and the time allocation subproblem. It is shown that the former subproblem fits in D.C. (difference of two concave functions) programming while the latter subproblem corresponds to a linear programming problem which can be efficiently solved. Simulation results validate the EE performance of our proposed scheme.
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