## 1 Introduction  
Over the last decades, mobile communications have caught a lot of interest in our everyday life. During the 1980s, the first standard of mobile communications was launched and our lives changed dramatically in many aspects. Studies such as [1, 2] show how the use of mobile devices has changed our behavior in the social sphere. Before the advent of wireless communications, we were not able to do things that we now label as everyday activities, such as call the office while driving our car or having multiple video-calls with our friends scattered over the world. The digitization process started in the early 1990s with the roll-out of the second generation of mobile communication devices (2G). The continuous deployment of new services, with the increase in data demand [3], has led to the novel fifth generation standard (5G) for wireless communications. During the past years, research about mobile communications has been moving quickly through the different standards of communications, but there is still a certain distance between theoretical studies and practical applications. The validation of analytical models became a crucial step in the standardization process. Ideally, it is preferable to test everything through physical, real time prototypes, but this is possible only after standardization due to the high cost of development. Usually, many trials may be conducted in the field to help the research stage of the technology, although they bring a limited contribution. Field trials can be combined with numerical simulations to validate the studies during the research stage of the standardization. Therefore, in order to obtain significant numerical results for a communication system, a strong channel model is required to regard the behavior of the signals through the physical space between transmitters and receivers, attenuations through the paths, atmospheric conditions, etc. The millimeter wave (mmWave) frequency spectrum presents many attractive features able to support high data demand for 5G NR technologies such as good propagation behavior in line-of-sight (_LOS_) scenarios. Available bandwidths in that range of frequencies under 5G regulatory consideration, i.e., from 27 to 71 GHz, are much wider than all currently adopted cellular allocations [4; 5; 6]. The reduced wavelengths will pave the way for the implementation of massive multiple-input and multiple-output (MIMO) techniques such as precoding or beamforming. However, mmWave communications bring new challenges for the research community in order to develop a unified channel model, as this range of frequencies is more vulnerable to path attenuation, noise, interference, and atmospheric attenuation due to adverse weather conditions and other environmental effects. Channel models used for 4G communications make the assumption of 2-D propagating waves, which is plausible due to the fact that the angular spectrum is negligible. In mmWave communications, this assumption is no longer valid, so that a good channel model for 5G has to overtake the assumption of propagation only on the horizontal plane, and it needs to foresee a 3-D channel model [7]. Several new technologies have been proposed for 5G cellular networks, such as Massive MIMO, Non-orthogonal multiple access (NOMA) [8], and cognitive radios [9; 10] by means of spectrum sensing [11; 12; 13; 14] to support operations in unlicensed spectrum. In particular, Space Division Multiple Access (SDMA) or Multi-User MIMO techniques are one of the most promising features to improve the operation of 5G networks, essential to deal with the growing demand of data rates, caused by new services such as Ultra-High Definition (UHD) video, virtual reality (VR), and augmented reality (AR).