The next generation of wireless systems, 5G, is intended to cover plenty of new emerging applications such as ultra-high resolution multimedia systems, industry 4.0, self-driving vehicles and smart cities. All those different applications require a technology able to deal with three main type of scenarios: enhanced Mobile Broadband (eMBB) systems, Ultra-Reliable and Low Latency Communications (URLLC) and massive Machine-Type Communications (mMTC). The necessity of achieving high data rates in wireless systems have led to multi-antenna configurations and Multiple Input Multiple Output (MIMO), and massive MIMO, geometries have been essential to improve the global performance.
Otherwise, finding a solution that fits in all cases is challenging and several approaches may be considered according to the initial recommendations from the standardization entities. In this thesis, a system point of view is provided to address the problem, placing an emphasis on the physical analysis of the communications channel. In particular, different solutions are proposed and compared, from the most immediate future approach (at microwave frequencies) to mid- or long-term solutions (in millimeter-wave bands).