  Cellular networks have changed the world we are living in, and the fifth
generation (5G) of radio technology is expected to further revolutionise our
everyday lives, by enabling a high degree of automation, through its larger
capacity, massive connectivity, and ultra-reliable low-latency communications.
In addition, the third generation partnership project (3GPP) new radio (NR)
specification also provides tools to significantly decrease the energy
consumption and the green house emissions of next generations networks, thus
contributing towards information and communication technology (ICT)
sustainability targets. In this survey paper, we thoroughly review the
state-of-the-art on current energy efficiency research. We first categorise and
carefully analyse the different power consumption models and energy efficiency
metrics, which have helped to make progress on the understanding of green
networks. Then, as a main contribution, we survey in detail -- from a
theoretical and a practical viewpoint -- the main energy efficiency enabling
technologies that 3GPP NR provides, together with their main benefits and
challenges. Special attention is paid to four key enabling technologies, i.e.,
massive multiple-input multiple-output (MIMO), lean carrier design, and
advanced idle modes, together with the role of artificial intelligence
capabilities. We dive into their implementation and operational details, and
thoroughly discuss their optimal operation points and theoretical-trade-offs
from an energy consumption perspective. This will help the reader to grasp the
fundamentals of -- and the status on -- green networking. Finally, the areas of
research where more effort is needed to make future networks greener are also
discussed.
