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Opportunistic access has been considered by regulators for a number of different spectrum bands. In this paper, we discuss and qualitatively evaluate techniques used in the discovery of spectrum opportunities, also called white spaces, in the radar, TV, and cellular bands. These techniques include spectrum sensing, cooperative spectrum sensing, geolocation databases, and the use of beacons. We make the case that each of the three bands considered calls for a different set of spectrum access techniques. While TV bands are well matched to the adoption of geolocation databases, a database-assisted spectrum sensing mechanism may represent the most efficient solution to exploit the spectrum holes in radar bands. We drew this conclusion based on a multitude of factors, such as the radar antennas' constant motion, and the absence of a hidden node problems in these bands. The unpredictability of cellular …