  In this letter, we analyze the problem of detecting spectrum holes in
cognitive radio systems. We consider that a group of unlicensed users can sense
the radio signal energy, perform some simple processing and transmit the result
to a central entity, where the decision about the presence or not of licensed
users is made. We show that the proposed cooperative schemes present good
performances even without any knowledge about the measurements statistics in
the unlicensed users and with only partial knowledge of them in the central
entity.
