  Green radios jointly consider the spectrum efficiency and the energy
efficiency in wireless networks in order to provide sustainable development. In
the past two decades, various green radio solutions for different types of
terminals and radio access networks have been developed, some of which have
been used in the designs of wireless products. In this paper, we provide a
historical view on some fundamental works and practical issues. In addition to
providing a comprehensive overview on theoretical achievements, we also discuss
several important power saving solutions with notable engineering impacts, such
as Doherty power amplifier and separated baseband unit - remote radio unit
architecture, which might be overlooked in previous publications. Moreover,
with the huge growth of wireless traffic in the near future, green radio design
for future wireless networks shall involve an end-to-end energy efficiency
investigation of mobile terminals, radio access and core networks, and
different applications. By introducing green radio schemes for advanced
terminals and future wireless networks, this article will not only be
beneficial for readers with only preliminary background in communications and
signal processing but also have reference and historical values for researchers
and practical engineers in the area.
