With the rapid deployment of new wireless devices and applications, the last decade has witnessed a growing demand for wireless radio spectrum. However, it is observed that most of the licensed spectrum is underutilized due to fixed spectrum allocation policy. The inefficient usage of the limited spectrum resources drives to find innovative way of communication technology so that the wireless spectrum can be utilized in a more intelligent and flexible way. To address the issue of spectrum efficiency the concept of Cognitive Radio (CR) is proposed. This has been getting more attention in recent years and significant developments have been in past few year. To achieve spectrum efficiency the main focus is given on spectrum management and that feature is “dynamic”. Hence Often, the broad term “CR” is used as a synonym for dynamic spectrum access.

The usage and regulation of radio spectrum resources are coordinated by national regulatory bodies like the Federal Communications Commission (FCC). The FCC assigns spectrum to licensed holders on a long term basis in a particular geographical area. These license holders are also known as primary users. However, from Figure 1 it can be observed that a large portion of the licensed spectrum remains underutilized. To utilize this spectrum efficiently, it is possible to allocate this unused spectrum temporarily to those users who are in need and do not have spectrum license. Such users are known as Secondary users. The technology where limited resources are used in a more efficient, flexible and comprehensive way without interfering with the primary users is called CR. A fully CR is also known as Mitola Radio in recognition of the engineer who first proposed it. It is defined as a radio that can change its transmitter parameters such as modulation format, multiple access method, coding as well as centre frequency, bandwidth, transmission times and so on according to the surrounding environment in which it operates. While a fully CR is interesting from a scientific point of view, it currently seems too complicated for practical purposes.

A “spectrum-sensing cognitive radio,” only adapts the transmission frequency, bandwidth, and time according to the environment. Such cognitive radio is also often called Dynamic Spectrum Access (DSA) networks. It differs from conventional radio devices in the sense that it has cognitive capability, reconfigurability and opportunistic behavior.

• Cognitive capability is defined as ability to sense and gather information from the surrounding environment, such as information about transmission frequency, bandwidth, power, modulation, etc. With this capability, secondary users can identify the best available spectrum.

• Reconfigurability refers the ability to adapt the parameters according to the sensed information rapidly.

• Opportunistic behavior is due to the fact that secondary users sense which portion of the spectrum is available, select the best available channel, coordinate spectrum access with other users, and vacate the channel whenever a primary user reclaims the spectrum.(Primary users have priority in using the spectrum).

In order to achieve these functionalities, it is necessary to perform various tasks such as real-time monitoring of the licensed spectrum to find the availability of channel, maintenance of interference limit when secondary users are allowed to transmit data simultaneously with a primary user, interference. If secondary users are transmitting alone then it is required the detection of reappearance of primary user (using energy detection, feature detection, matched filtering and coherent detection methods). Due to coexistence of primary users and secondary users in CR based networks, traditional spectrum allocation schemes and spectrum access protocols can not be used. New spectrum management approaches are developed to solve new challenges of CR, specifically, in spectrum sensing and dynamic spectrum sharing. In this chapter the CR function, architecture of a CR network and its applications are first introduced. In the succeeding section existing works in spectrum sensing are reviewed, and classification of dynamic spectrum allocation is presented in brief.