In this paper, a distributed power and spectrum allocation scheme is proposed to maximize the system energy efficiency (EE) for unlicensed device-to-device (D2D-U) networks. A non-convex optimization problem is formulated while considering the co-channel interference on licensed bands as the global constraint. To deal with the non-convex situation, the object function is converted into an equivalent convex one. Then, a distributed algorithm is developed to solve the optimization problem in which each D2D-U pair can find the optimal allocation strategy independently. The signaling overheads are analyzed and numerical results are presented to show that the proposed scheme is capable of achieving the optimal EE while confining the cochannel interference and guaranteeing the fair coexistence.

Unlike the conventional device-to-device (D2D) networks, the unlicensed D2D (D2D-U) pairs can not only reuse the licensed channels with the base station (BS) but also share the unlicensed channels with the WiFi stations. One challenge arises from the fact that the co-channel interference on licensed channels and the collision probability on unlicensed channels may cause extra power consumption at the terminals. Accordingly, we first propose a channel access method for the D2D-U pairs on unlicensed channels. Then, a decentralized joint spectrum and power allocation scheme is designed to minimize the power consumption at D2D-U pairs. Different from the existing distributed schemes, the proposed scheme can guarantee the global minimization of power consumption across the D2D-U pairs. Simulation results validate the theoretical analysis and verify the performance from the proposed scheme.