Energy‐Aware D2D Communications

Device‐to‐device (D2D) communications, which enable proximity users to communicate directly without passing through BS, have been considered as one of the key techniques in the future 5G networks. By taking advantage of the physical proximity, D2D communications can significantly enhance the quality of experience (QoE) of users and increase resource utilization. Meanwhile, due to the ever‐increasing power consumption of information and communication technology (ICT) industry and the slow progress in battery technology of the smart devices, energy efficiency has been considered as a key performance indicator for the 5G networks. Thus, energy‐efficient D2D communications design has attracted considerable attention from both the industrial and academic communities. There have been plenty of discussions on this issue, for instance, (i) energy‐efficient D2D transmission technologies including efficient power control, mode selection, cross‐layer optimization, and energy harvesting; (ii) novel D2D‐based energy‐saving network deployment including D2D relay cooperative communications and D2D‐aided heterogeneous networks; and (iii) D2D multiple‐input and multiple‐output (MIMO) transmission. In this article, we provide an extensive overview of these technologies and present the state of the art of each aspect. In addition, we summarize the main findings of the discussed techniques and also identify some open research problems.