How would a cellular network designed for maximal energy efficiency look\udlike? To answer this fundamental question, we model future cellular networks\udusing stochastic geometry and obtain a new lower bound on the average uplink\udspectral efficiency. This enables us to formulate a tractable energy efficiency\ud(EE) maximization problem and solve it analytically with respect to the density\udof base stations (BSs), the transmit power levels, the number of BS antennas\udand users per cell, and the pilot reuse factor. The closed-form expressions\udobtained from this general EE maximization framework provide valuable insights\udon the interplay between the optimization variables, hardware characteristics,\udand propagation environment. Small cells are proved to give high EE, but the EE\udimprovement saturates quickly with the BS density. Interestingly, the maximal\udEE is obtained by also equipping the BSs with multiple antennas and operate in\uda "massive MIMO" fashion, where the array gain from coherent detection\udmitigates interference and the multiplexing of many users reduces the energy\udcost per user.
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