  We consider the downlink of a cell-free massive multiple-input
multiple-output (MIMO) network where numerous distributed access points (APs)
serve a smaller number of users under time division duplex operation. An
important issue in deploying cell-free networks is high power consumption,
which is proportional to the number of APs. This issue has raised the question
as to their suitability for green communications in terms of the total energy
efficiency (bits/Joule). To tackle this, we develop a novel low-complexity
power control technique with zero-forcing precoding design to maximize the
energy efficiency of cell-free massive MIMO taking into account the backhaul
power consumption and the imperfect channel state information.
