Dense Networks for Maximal Energy Efficiency: Small Cells Meet Massive MIMO'
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Introduction {#sec:intro}
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The biggest challenges for next generation wireless communication systems (5G) are to support the ever-growing demands for higher data rates and to ensure a consistent quality of service (QoS) throughout the entire network [@Andrews2014a]. To meet these demands, the network area throughput (in bit/s/km$^2$) needs to increase by a [factor of 1000]{} over the next 10–15 years [@Qualcomm]. At the same time, the power consumption of the information and communication technology (ICT) industry and the corresponding energy-related pollution are becoming major societal and economical concerns [@Fehske2011a]. Credited sources foresee that, to meet such a $1000 \times$ higher data traffic without increasing the ICT footprint, new technologies that improve the overall energy efficiency (EE) by $1000 \times$ must be developed [@Greentouch2013]. Hence, higher network area throughput on the one hand and less power consumption on the other are seemingly contradictory 5G requirements [@Bjornson2014c]. There is a broad consensus that these goals can only be jointly achieved by a substantial network densification.

Two promising technologies towards network densification are small-cell networks [@Hoydis2011c; @Sanguinetti2015bJSAC] and massive MIMO systems [@Marzetta2010a; @Larsson2014a; @Yang2013b]. The first technology relies on an ultra-dense 