  Assume that a multi-user multiple-input multiple-output (MIMO) communication
system must be designed to cover a given area with maximal energy efficiency
(bit/Joule). What are the optimal values for the number of antennas, active
users, and transmit power? By using a new model that describes how these three
parameters affect the total energy efficiency of the system, this work provides
closed-form expressions for their optimal values and interactions. In sharp
contrast to common belief, the transmit power is found to increase (not
decrease) with the number of antennas. This implies that energy efficient
systems can operate at high signal-to-noise ratio (SNR) regimes in which the
use of interference-suppressing precoding schemes is essential. Numerical
results show that the maximal energy efficiency is achieved by a massive MIMO
setup wherein hundreds of antennas are deployed to serve relatively many users
using interference-suppressing regularized zero-forcing precoding.
