Cognitive Radio (CR) is aimed at increasing the e ffi ciency of spectrum utilization by allowing unlicensed users to access, in an op- portunistic and non-interfering manner, some licensed bands temporarily and / or spatially unoccupied by the licensed users. The analysis of CR sys- tems usually requires the spectral activity of the licensed system to be rep- resented and characterized in a simple and tractable, yet accurate manner, which is accomplished by means of spectrum models. In order to guarantee the realism and accuracy of such models, the use of empirical spectrum oc- cupancy data is essential. In this context, this paper explains the complete process of spectrum modeling, from the realization of field measurements to the obtainment of the final validated model, and highlights the main rel- evant aspects to be taken into account when developing spectrum usage models for their application in the context of the CR technology

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