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.

本文介绍了第五代（5G）射频技术，以及第三代伙伴关系项目（3GPP）新射频规范，提供了用于显著降低下一代网络能耗和温室气体排放的工具。重点介绍了能够实现能效的关键技术，例如大规模多输入多输出（MIMO），轻载波设计和先进的空闲模式，探讨了其在能源消耗方面的最佳操作点和理论折衷。最后，讨论了未来网络需要更多努力实现绿色网络的研究领域。