## 2 The Fifth Generation of Cellular Communications  
5G new radio (NR) is the new standard of cellular communications [20] and is being built under the performance requirements stated by the International Telecommunication Union (ITU) in 2015 [21]. Contrary to its previous generations, 5G will be much more than just cellular communications and will have different use cases to provide different services. In fact, one of the most interesting features regarding the design of 5G is its flexibility, which is brought about by so-called network slicing [18] and enables different services that go considerably beyond what was offered by its predecessors [22]. As shown in Figure 2, 5G communications involve three main use cases: enhanced mobile broadband (eMBB), massive machine-type communications (mMTC) and URLLC. These use cases will provide very different applications, such as virtual reality, autonomous vehicles, and smart cities, which are key aspects of Industry 4.0. Figure 3 represents the 5G standardization timeline of the Third Generation Partnership Project (3 GPP) [23]. The first study item of 5G appeared in 3 GPP Release 14, which was still concerned with the development of 4G-Long Term Evolution (4G-LTE). Figure 3: Third Generation Partnership Project (3 GPP) 5G standardization timeline. However, the 5G standardization process only started after 3 GPP Release 15. In order to accelerate this process, the 3 GPP has defined two distinct phases. In the first phase, the focus was the improvement of broadband wireless cellular services (i.e., the eMBB use case) so that they have two distinct modes: the non-standalone (NSA) mode, where the core is still 4G, and the standalone (SA) mode, where the core is 5G. Both modes were standardized during 2018. The second phase, also known as 3 GPP Release 16, will be finalized in 2020, and it is concerned with the other 5G use cases, (i.e., mMTC and URLLC use cases). When it comes to the eMBB use case, substantially higher peak data rates and user-experienced data rates are expected. In fact, the area throughput \(R_{A}\) (measured in bits/s/km\({}^{2}\)) of 5G will be much higher than in 4G. This substantial increase in \(R_{A}\) will be accomplished by creating a heterogeneous cellular network; in other words, there will be different solutions to increase \(R_{A}\), depending on the cell characteristics. For small cells dedicated to increasing the capacity in a small geographic area, mm-wave communications will be used [24]. Under these circumstances, the area throughput will be augmented by increasing the bandwidth of the communication channels. However, the use of carrier frequencies with tens of gigahertz poses considerable challenges, since the path loss in mm-wave communications is very high. 5G will overcome this problem with m-MIMO techniques, such as beamforming. The use of beamforming leads to large array gains at reception so that the signal-to-noise ratio (SNR) of the received signals is not too degraded. Nevertheless, it should be noted that the use of mm-wave carriers is beneficial in terms of cell density. In fact, as a large path loss yields lower interference levels, cell density can be increased by reducing the inter-base station distance, increasing the system's capacity. For large cells responsible for the coverage and mobility tiers, there is no available bandwidth to increase the capacity. Therefore, massive MIMO techniques will be employed, since they allow for huge gains in spectral efficiency [25]. These gains come from the use of appropriate precoding and decoding schemes that can take advantage of the large number of antennas and yield a very large array and multiplexing gains. This technique is also known as multi-user multiple-input multiple-output (MIMO), and it enables more users to be multiplexed in a given time-frequency resource, which leads to a higher throughput.