  Cognitive radios process their sensed information collectively in order to
opportunistically identify and access under-utilized spectrum segments
(spectrum holes). Due to the transient and rapidly-varying nature of the
spectrum occupancy, the cognitive radios (secondary users) must be agile in
identifying the spectrum holes in order to enhance their spectral efficiency.
We propose a novel {\em adaptive} procedure to reinforce the agility of the
secondary users for identifying {\em multiple} spectrum holes simultaneously
over a wide spectrum band. This is accomplished by successively {\em exploring}
the set of potential spectrum holes and {\em progressively} allocating the
sensing resources to the most promising areas of the spectrum. Such exploration
and resource allocation results in conservative spending of the sensing
resources and translates into very agile spectrum monitoring. The proposed
successive and adaptive sensing procedure is in contrast to the more
conventional approaches that distribute the sampling resources equally over the
entire spectrum. Besides improved agility, the adaptive procedure requires
less-stringent constraints on the power of the primary users to guarantee that
they remain distinguishable from the environment noise and renders more
reliable spectrum hole detection.
