## 1 Introduction  
The deployment of wireless cellular networks back in the early 1980s made feasible communications via portable devices, thus decoupling call establishment from existing location. In the next decades, technological achievements such as data exchange, which was introduced in second generation (2G) wireless cellular networks, or multimedia communications, which was a key concept of third generation (3G) networks, enabled the delivery of even higher data rates to mobile users (MUs) and a more efficient spectrum utilization compared to second generation systems [1]. In March 2009, the International Telecommunications Union-Radio communications sector (ITU-R) specified a list of requirements for fourth generation (4G) systems, named the International Mobile Telecommunications Advanced (IMT-Advanced) specification, setting peak speed requirements for 4G services at 100 Mbs for high mobility communications (such as from trains and cars) and 1 Gbs for low mobility communications (such as pedestrians and stationary users) [2]. The era of 3G and 4G networks coincided with scientific progress in other related fields, such as micro and power electronics, as well as hardware minimization and related improvements. This in turn made feasible the development of advanced transceiver architectures able to support among others large bandwidth operations and multiple Radio Frequency (RF) chains. Therefore, a quite popular transmission technique that has been studied thoroughly over the previous two decades is the use of antenna arrays at both ends of a wireless orientation, also known as multiple input multiple output (MIMO). Research on MIMO systems was mainly boosted after the pioneering work of [PERSON] [3]. As shown in [4; 5; 6], MIMO systems can provide, among other benefits, diversity and spatial multiplexing gain. In the first case, the same symbol information is sent and received over multiple antennas; hence, the mean Bit Error Rate (BER) is reduced, due to the presence of multiple diversity branches. In the spatial multiplexing mode, individual data streams are sent from different transmit antennas. Therefore, overall network throughput can be improved, at the cost, however, of increased hardware complexity, as the diversity order of the orientation is reduced. Although MIMO systems were incorporated in 3G and 4G standards [7; 8], the increasing demand for even higher data rates as well as traffic congestion (i.e., total requested throughput per area) led the scientific community to seek additional bandwidth efficient solutions [9]. Observing the evolution of generations of mobile communication systems, one easily realizes that there is an endless quest for an equilibrium between serving the exponentially increasing user needs (global wireless traffic volume in 2013 increased 30 times compared to that in 2008 [10]), and developing innovative technologies to enhance operational capabilities and network capacity given the scarce spectrum (wireless communications capacity in 2008 has increased by one million times compared to 1957 [11]). In this context, various solutions have been proposed for the deployment of 5G networks, such as mmWave transmission [12; 13; 14], massive MIMO systems [15; 16; 17], non-orthogonal multiple access (NOMA) schemes [18; 19; 20; 21] as well as flexible network deployment along with nomadic nodes [22; 23] (e.g., drones, uav, etc.). In the first case, mmWave spectrum covers the range from 30 GHz to 300 GHz (with equivalent wavelengths from 10 to 1 mm). This spectrum area is of particular interest for various reasons, as there is one order of magnitude of more spectrum available in this band than in lower bands. In addition, larger bandwidth channels can be now achieved (i.e., of 2 GHz, 4 GHz, 10 GHz, or even 100 GHz). Massive MIMO is an extension of multiuser MIMO, in which the base station (BS) transmitter simultaneously communicates with multiple mobile station (MS) receivers using the same time-frequency resources, improving the spectrum efficiency.