Enhanced mobile broadband (eMBB) is one of the key use⁃cases for the development of the new standard 5G New Radio for the next generation of mobile wireless networks. Large⁃scale antenna arrays, a.k.a. massive multiple⁃input multiple⁃output (MIMO), the usage of carrier frequencies in the range 10-100 GHz, the so⁃called millimeter wave (mm⁃Wave) band, and the network densification with the introduction of small⁃sized cells are the three technologies that will permit implementing eMBB services and realizing the Gbit/s mobile wireless experience. This paper is focused on the massive MIMO technology. Initially conceived for conventional cellular frequencies in the sub⁃6 GHz range (μ⁃Wave), the massive MIMO concept has been then progressively extended to the case in which mm⁃Wave frequencies are used. However, due to different propagation mechanisms in urban scenarios, the resulting MIMO channel models at μ⁃Wave and mm⁃Wave are radically different. Six key basic differences are pinpointed in this paper, along with the implications that they have on the architecture and algorithms of the communication transceivers and on the attainable performance in terms of reliability and multiplexing capabilities.
millimeter wave; microwave; channel modeling; massive MIMO; doubly massive MIMO