  Compared with the energy efficiency of conventional mobile communication
systems, the energy efficiency of fifth generation (5G) communication systems
has been improved more than 30 times. However, the energy consumption of 5G
communication systems is 3 times of the energy consumption of fourth generation
(4G) communication systems when the wireless traffic is increased more than 100
times in the last decade. It is anticipated that the traffic of future sixth
generation (6G) communication systems will keep an exponential growth in the
next decade. It is a key issue how much space is left for improving of energy
efficiency in mobile communication systems. To answer the question, an
entropy-based energy dissipation model based on nonequilibrium thermodynamics
is first proposed for mobile communication systems. Moreover, the theoretical
minimal energy dissipation limits are derived for typical modulations in mobile
communication systems. Simulation results show that the practical energy
dissipation of information processing and information transmission is three and
seven orders of magnitude away from the theoretical minimal energy dissipation
limits in mobile communication systems, respectively. These results provide
some guidelines for energy efficiency optimization in future mobile
communication systems.
