setting allows for very high spectral and energy efficiencies using simple linear signal processing in the form of conjugate beamforming or [@Marzetta10; @Marzetta16].

In , a large number of deployed within a given coverage area cooperate to jointly transmit/receive to/from a (relatively) reduced number of thanks to the availability of high-performance low-latency fronthaul links connecting the to a central coordinating node. Coordination among can effectively control (or even eliminate) intercellular interference in an approach that was first referred to as network [@Karakayali06; @Gesbert10], later led to the concept of transmission [@Irmer11] and, more recently, to that of [@Checko15]. In a , the , which are treated as a distributed system, are connected to a cloud-computing based in charge, among many others, of the baseband processing tasks of all . Conceptually similar to the architecture, but explicitly relying on assumptions specific of the massive regime, distributed massive -based have been recently termed as *cell-free massive* networks [@Ngo15; @Ngo17]. In these networks, a massive number of connected to a are distributed across the coverage area and, as in the cellular collocated massive schemes, exploit the channel hardening and favorable propagation properties to coherently serve a large number of over the same time/frequency 