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The radio spectrum endures underutilization of the available spectrum. Assigning chunks or segments of the radio spectrum to specific users through the static model of spectrum assignment, by and large known as the command and Control Model (CCM) is the regular model of spectrum allocation. The increasing growth in the use wireless and mobile devices triggers the call for more dynamic and efficient access techniques for both licensed and unlicensed spectrum. Dynamic Spectrum Access (DSA) is a promising technology to increase spectrum efficiency by encouraging spectrum usage, this technology is a new model of spectrum access that allows other users to access the white spaces or spectrum holes, temporarily in a licensed spectrum band. Cognitive Radio (CR) technology is one of these dynamic access tools, this radio can monitor, observe and reacts to Radio Frequency (RF) events in its specified environment. This paper makes an extensive overview of the cognitive radio features, cognitive radio functional blocks, and furthermore examines cognitive radio research challenges.