Terrestrial wireless systems have largely restricted their operation
to the relatively slim range of microwave frequencies
that extends from several hundred MHz to a few GHz and corresponds
to wavelengths in the range of a few centimeters up to
about a meter. By now though, this spectral band—often called
“beachfront spectrum”—has become nearly fully occupied, in
particular at peak times and in peak markets. Regardless of the
efficacy of densification and offloading, much more bandwidth
is needed [59], [60].