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munications Magazine ? April 2008 72 0163-6804/08/$ ? 2008 IEEE The views presented here are those of the authors and do not necessarily reflect the views of the IEEE SCC41 or its pro- jects. I NTRODUCTION The idea of cognitive radio (CR) as an alterna- tive munication paradigm, capable of managing and executing itself in real-time without human intervention, was first proposed by Joseph Mitola III and Gerald Q. Maguire, Jr. [1]. In the utopian world of put - ing, each device would be equipped only with CR, thus empowering it municate on any available spectrum. The question, however, is where to look for the unused spectrum with the myriads of wireless devices employed around the globe and with a status quo in spectrum regula- tions. As demand for radio resources increases, industry and academia are searching for efficient methods to share the spectrum — the wireless domain real-estate . Essentially, there are two models of spectrum allocation and usage: ? Owned — where spectrum is sold or allo- cated to service providers or agencies who have the sole ownership of the spectrum. ? Common — where anyone can use the spec- trum with an agreed upon etiquette . The first model is dominating the current wire - less world, but it is inefficient. This observation was strengthened by a study of the New Ameri- can Foundation with the cooperation of the Shared pany to find the exten