  The use of millimeter waves for wireless communications is one of the main
technological innovations of 5G systems with respect to previous generations of
cellular systems. Their consideration, however, has been up to now mainly
restricted to the case in which analog or, at most, hybrid analog-digital
beamforming structures were used, thus posing a limitation on the multiplexing
capabilities and peak data rates that could be theoretically achieved at these
frequencies. Recent progress in the field of electronics, however, has made the
energy consumption of digital beamforming structures at least on par with that
of analog beamforming, thus redeeming them from the ghetto they had been placed
in over the last year. Digital beamforming, coupled with the use of large
antenna arrays at both sides of the communication link, promises thus to be one
of the secret weapons of future 6G networks, capable of unleashing
unprecedented values of spectral and energy efficiency for ultra-broadband
connectivity.
