and irregular operator-deployment of low-cost and low-power base stations (BSs), with higher density where the user load is higher. Bringing the BSs and user equipments (UEs) closer to each other can increase the area throughput, while significantly reducing the radiated signal power. However, this may come at the price of a substantial increase of the circuit power consumption (per km$^2$) due to the larger amount of hardware and infrastructure [@Bjornson2013e]. In contrast, the massive MIMO technology aims at evolving the conventional BSs by using arrays with a hundred or more small dipole antennas. This allows for coherent multi-user MIMO transmission where tens of users can be multiplexed in both the uplink (UL) and the downlink (DL) of each cell. It is worth observing that, contrary to what the name “massive” suggests, massive MIMO arrays are rather compact; 160 dual-polarized antennas at 3.7 GHz fit into the form factor of a flat-screen television [@Vieira2014a]. In massive MIMO systems, the area throughput is improved by the multiplexing gain, while the array gain from coherent processing allows for major reductions in the emitted power. Similar to small-cell networks, however, the potential throughput gains from massive MIMO come from deploying more hardware (i.e., multiple 