I. Introduction
The next generation of wireless communication networks is expected to provide intelligence, limitless connectivity, and full synchronization of the physical and digital worlds, paving the way for the development of novel applications both indoors and outdoors. Specifically, the key performance indicator (KPI) expectations of academia and industry are converging toward an ever-increasing deployment of machine-type nodes in 2-D/3-D service areas (10 Gb/s/m3), and the need for even higher reliability ( frame error rate), lower latency (0.1 ms), higher data rates (1 Tb/s), and higher energy efficiency (1 pJ/bit) [1], [2], [3].