resources. Typically using simple linear signal processing schemes, they are claimed to provide uniformly good to the whole set of served irrespective of their particular location in the coverage area.

Since the microwave radio spectrum (from 300 MHz to 6 GHz) is highly congested, the use of massive antenna systems and network densification alone may not be sufficient to meet the demands in next generation wireless communications networks. Thus, another promising physical layer solution that is expected to play a pivotal role in and beyond communication systems is to increase the available spectrum by exploring new less-congested frequency bands. In particular, there has been a growing interest in exploiting the so-called bands [@Rappaport13; @Boccardi14; @Akdeniz14; @Rappaport17]. The available spectrum at these frequencies is orders of magnitude higher than that available at the microwave bands and, moreover, the very small wavelengths of , combined with the technological advances in low-power CMOS miniaturization, allow for the integration of a large number of antenna elements into small form factors. Large antenna arrays can then be used to effectively implement massive schemes (see, for instance, [@Gao18; @Busari18] and references therein) that, with appropriate beamforming, can more than compensate for the orders-of-magnitude increase in free-space 