Cognitive radio is an exciting and new way of thinking and researching about wireless communications. Indeed, it is already being considered as one of the key candidate technologies for the fourth-generation wireless systems. There are several drivers for the development of cognitive radio. Perhaps by far the most pressing of them is improved utilization of the electromagnetic radio spectrum: a highly valuable natural resource. Careful studies of the current usage of the radio spectrum by several agencies have already revealed that a large fraction of the radio spectrum is inadequately utilized. This basic finding has led to numerous research initiatives. For instance, five European projects addressing a multitude of cognitive radio topics were recently approved with a total budget of more than 50 M/spl Euro/, including ARAGORN (www.ict-aragorn.eu), EUWB (www.euwb.eu), PHYDYAS (www.ist-phydyas.org), E3 (www.ict-e3.eu), SENDORA (www.sendora.eu). Moreover there are emerging companies who apply cognitive radio principles for sensing the environment (e.g. self-organizing femtocells) in order to efficiently exploit the bandwidth resources and also to minimize interference (e.g., www.ubiquisys.com, www.adaptrum.com). Several cognitive radio test trials were created by the United States Federal Communications Commission (FCC) to investigate the impact of cognitive radios in "white spaces" (i.e., the under-utilized TV frequencies). Recently, even a "white space coalition" consisting of eight large companies was founded in order to promote the usefulness of the not yet publicly available analog television broadcast frequency bands (http://en.wikipedia.org/wiki/White_Spaces_Coalition). © 2008 IEEE.