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Due to THE growing demand for high data rates, wireless communication systems require more and more terrestrial frequency resources. Usable frequencies, however, are typically fully allocated to all kinds of wireless technology. One possible solution to meet the increasing data traffic demands is to build smaller radio cells to reuse spectrum more efficiently. However, the associated costs for new infrastructure and increased energy consumption make this option economically unviable. Therefore, efforts are being made to make use of locally vacant spectrum that is primarily licensed to other wireless systems?on a license-exempt or secondary license basis. Locally underutilized spectrum can be found in many frequency ranges (e.g., in military, aeronautical, and TV spectrum or in frequency bands allocated to historic communication systems) and is usually referred to as spectrum holes or white spaces . Before these frequency resources are allowed to be exploited by secondary users, regulators must ensure that primary users will not be negatively affected by additional interference.