Editorial: Special Issue “Massive MIMO and mm-Wave Communications”

Massive multiple-input multiple-output (mMIMO) communication systems and the use of millimeter-wave (mm-Wave) bands represent key technologies that are expected to meet the growing demand of data traffic and the explosion of the number of devices that need to communicate over 5G/6G wireless networks [1,2]. In mMIMO, the number of antennas is much larger than in traditional MIMO systems with the purpose of obtaining a high spectral efficiency. The use of mm-Wave bands, thanks to the short wavelength, may help to have compact arrays that can accommodate such a large number of antennas and allow overcoming the scarcity of the available spectrum, though the signal propagation is affected by severe path loss, sensitivity to blockage, directivity, and narrow beamwidth [3]. In addition, at mm-Wave bands, the size of the cells turns out to be much smaller than that required by traditional sub-6 GHz wireless networks such as 4G networks, and small-cell heterogeneous network (HetNet) architectures are required [4]. This Special Issue presents contributions to various aspects of the design and implementation of mMIMO systems that use mm-Wave bands. Topics such as precoding techniques, channel estimation, antenna design, network coverage, and health and safety issues are addressed, and the most advanced solutions are proposed.