 The exponential increase in mobile data traffic during the decades of the 1990s and 2000s has led to the massive deployment of wireless systems.As a consequence, the limited available RF spectrum is subject to an aggressive spatial reuse and co-channel interference has become a major capacity limiting factor.Therefore, there have been many independent warnings
of a looming "RF spectrum crisis"
as the mobile data demands continue to increase while the network spectral efficiency saturates despite newly introduced standards and great technological advancements in the field.