I. Introduction
Recent years have seen increasing demand for high-speed wireless communications. Moreover, reducing energy consumption in wireless networks is of significant interest among academic and industrial researchers. This is due to the fact that there are rising energy costs and carbon footprint of operating wireless networks with an increasing number of customers [1]. Consequently, energy efficiency (EE), which is conventionally defined as the number of transmitted information bits per unit energy (bits/Joule), needs to be considered as one of the key design metrics for future networks [2], [3]. The improvement of EE in wireless systems could be tackled at the component level (e.g., improve power amplifier (PA) efficiency), link level (e.g., discontinuous transmission and sleep modes), or network level (e.g., the layout of networks and their management) [3], [4].