The original motivation for cognitive radios came as a natural evolution of software-defined radios (SDR): automating multimode and multiband operation. However, as mentioned in Chapter 1, the notion of cognitive radios emerged around the same time that many measurement campaigns pointed out the high underutilization of many desirable radio spectrum bands. Regulatory agencies such as the US Federal Communications Commissions (FCC) were looking to encourage new approaches to address the perceived scarcity of radio spectrum due to this wasteful usage. The question was, is it possible to improve the spectrum utilization in already assigned frequency bands without harming the existing legacy systems? Due to their inherent ability to comprehend and respond to the spectrum state, the cognitive radio concept seems to provide a solution. Thus, immediately after its initial introduction, cognitive radio concept was viewed as an ideal vehicle to improve spectrum utilization by capitalizing on the unused spectrum opportunities.

In its 2002 Spectrum Policy Task Force report referred to in Chapter 1, the FCC noted several important trends on spectrum usage below 1 GHz [14]. For instance, it observed that in many places the VHF and UHF frequencies that are historically designated for broadcast TV are not always used. Frequencies that are used by a TV station in a particular geographical area tend to have continuous use. But ...

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