  [...] This paper explores how spectrum policy and spectrum technologies can
evolve to enable sharing among different stakeholders in the above 100 GHz
spectrum, without introducing harmful interference or disrupting either
security applications or fundamental science exploration. This portion of the
spectrum presents new opportunities to design spectrum sharing schemes, based
on novel antenna designs, directional ultra-high-rate communications, and
active/passive user coordination. The paper provides a tutorial on current
regulations above 100 GHz, and highlights how sharing is central to allowing
each stakeholder to make the most out of this spectrum. It then defines -
through detailed simulations based on standard International Telecommunications
Union (ITU) channel and antenna models - scenarios in which active users may
introduce harmful interference to passive sensing. Based on this evaluation, it
reviews a number of promising techniques that can enable active/passive sharing
above 100 GHz. The critical review and tutorial on policy and technologies of
this paper have the potential to kickstart future research and regulations that
promote safe coexistence between active and passive users above 100 GHz,
further benefiting the development of digital technologies and scientific
exploration.
