Optimal Mobile Association in Device-to-Device-Enabled Heterogeneous Networks

With device-to-device (D2D) communications, a user terminal(UT) can naturally be used as a relay node (RN) and thus inherently support multi-hop transmission. Thus, cell-edge or deeply faded users can obtain a more uniform connectivity experience. In this paper, we investigate mobile association for the UT with the capability of D2D communications in heterogeneous networks (HetNets). We will develop a framework on joint mobile association and transmission mode switching between the direct and the D2D relay modes to improve the system spectrum efficiency (SE) and energy efficiency (EE). We first formulate the optimization problems, and then obtain closed-form solutions. Simulation results show that with the proposed schemes, both SE and EE of the network can be significantly improved compared to the traditional solutions without D2D communications. We also discuss the trade-off between the minimum rate requirement and EE of a network.