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With emerging technologies and with the ever increasing number of wireless devices, the radio spectrum is becoming scarcer everyday. On the other hand, measurements show that wide ranges of the spectrum are rarely used most of the time while other bands are heavily used. However, those unused portions of the spectrum are licensed and thus cannot be utilized by users other than the license owners. Hence, there is a need for a novel technology that can benefit from these opportunities. Cognitive radio arises to be a tempting solution to spectral crowding problem by introducing the opportunistic usage of frequency bands that are not heavily occupied by licensed users [3].

Cognitive radio, which is a recent concept introduced by Mitola [1], has been attracting significant interest and has the potential of shaping the future of wireless communication systems. The basic idea of the concept can be stated as employing smart wireless devices that are furnished with awareness, sensing, learning, and adaptation capabilities in order to utilize the available radio resources as efficiently as possible.

Technology is futile without its application. One of the major driving forces of a new technology is the combination of man’s endless thirst for knowledge, unequal intuitive mind, and limitless ambition to better his life. The other important factor consists of user demand, user necessity, and increasing user desire to have it all in one single device.

When we look at the evolution of wireless communication systems, as in most systems, two major developments are most prominent: 1) addition of new features and 2) improvement of already existing capabilities. The first development arises from the fact that communication through a wireless medium makes life easier. Every development brings several new features and these new features contribute positively to this fact. These developments can be seen clearly in the evolution of cell phones. In the past, cell phones were used only for transmitting voice and short text-based messages. Currently, there are cell phones in which an operating system runs and several multimedia applications are available.