I. Introduction
Spectral efficiency (SE) remains the criterion of choice for assessing system performances and designing reliable and scalable communication systems. However, network operators require the next generation of communication systems to be low-energy consuming for both ensuring the economic and environmental sustainability of their activity. In this respect, energy efficiency (EE) is becoming increasingly important as a system performance criterion, and has recently attracted a surge of interest in the research community [1]–[3]. Although, EE has been extensively discussed in the past for power-limited as well as battery-driven systems [4]–[6], it remains a fairly new concept for power-unlimited communication systems, such as cellular networks [7], [8].