  Nowadays, the demand for wireless mobile services is copious, and will
continue increasing in the near future. Mobile cellular operators are therefore
looking at the unlicensed spectrum as an economical supplement to augment the
capacity of their soon-to-be overloaded networks. The same unlicensed bands are
luring internet service providers, venue owners, and authorities into
autonomously setting up and managing their high-performance private networks.
In light of this exciting future, ensuring coexistence between multiple
unlicensed technologies becomes a pivotal issue. So far this issue has been
merely addressed via inefficient sharing schemes based on intermittent
transmission. In this article, we present the fundamentals and the main
challenges behind massive MIMO unlicensed, a brand-new approach for technology
coexistence in the unlicensed bands, which is envisioned to boost spectrum
reuse for a plethora of use cases.
