  The data traffic in wireless networks is steadily growing. The long-term
trend follows Cooper's law, where the traffic is doubled every two-and-a-half
year, and it will likely continue for decades to come. The data transmission is
tightly connected with the energy consumption in the power amplifiers,
transceiver hardware, and baseband processing. The relation is captured by the
energy efficiency metric, measured in bit/Joule, which describes how much
energy is consumed per correctly received information bit. While the data rate
is fundamentally limited by the channel capacity, there is currently no clear
understanding of how energy-efficient a communication system can become.
Current research papers typically present values on the order of 10 Mbit/Joule,
while previous network generations seem to operate at energy efficiencies on
the order of 10 kbit/Joule. Is this roughly as energy-efficient future systems
(5G and beyond) can become, or are we still far from the physical limits? These
questions are answered in this paper. We analyze a different cases representing
potential future deployment and hardware characteristics.
