The disclosure relates to an access point (AP) device for controlling spectrum usage of a hierarchical communication system, in which a spectrum reserved for an Incumbent is usable by at least one user equipment (UE) for transmission when the spectrum is not required by the Incumbent, the AP device comprising a processor configured to: enforce the at least one UE into a mode protecting use of the spectrum by the Incumbent based on a spectrum request indication from the Incumbent; and enable transmission of the at least one UE using the spectrum reserved for the Incumbent based on a spectrum availability indication from the Incumbent.