  This work aims to design a cellular network for maximal energy efficiency
(EE). In particular, we consider the uplink with multi-antenna base stations
and assume that zero- forcing (ZF) combining is used for data detection with
imperfect channel state information. Using stochastic geometry and a new lower
bound on the average per-user spectral efficiency of the network, we optimize
the pilot reuse factor, number of antennas and users per base station.
Closed-form expressions are computed from which valuable insights into the
interplay between the optimization variables, hardware characteristics, and
propagation environment are obtained. Numerical results are used to validate
the analysis and make comparisons with a network using maximum ratio (MR)
combining. The results show that a Massive MIMO setup arises as the EE-optimal
network configuration. In addition, ZF provides higher EE than MR while
allowing a smaller pilot reuse factor and a more dense network deployment.
