I. Introduction
Simultaneous wireless information and power transfer (SWIPT) and energy efficiency (EE) transmission are considered as two prominent approaches to the ever-increasing demand for energy in the context of green communication. More specifically, in SWIPT-based communications, receivers are able to salvage electromagnetic energy from radio-frequency (RF) signals. On the other hand, energy-efficient communication aims to achieve a certain rate-energy balance by maximizing EE, defined as the number of delivered bits per unit energy. Traditionally, SWIPT and EE are often independently investigated. Recently, there is growing interest in combining these two concepts together to maximize EE in the SWIPT system, e.g., in [1]–[3].