A Cognitive Radio is an extension of modern Software Defined Radio. This extension creates new capabilities for users.

The main potential advantages introduced by cognitive radio are improving spectrum utilization and increasing communication quality.

A cognitive radio is an intelligent radio that can be programmed and configured dynamically. Its transceiver is designed to use the best wireless channels in its vicinity. Such radio automatically detects available channels in wireless spectrum, then accordingly changes its transmission and reception parameters to allow more concurrent wireless communications in given spectrum band at one location.

Cognitive radio network is complex multi-user wireless communication system to provide efficient use of radio spectrum.

The idea of cognitive radio extends by using the concepts of hardware radio and software defined radio from a simple , single function device to a radio that senses its operating environment.

The concept of cognitive radio is that unlicensed users (cognitive radio users) can access the spectrum owned by licensed users (primary users) while they cannot interfere with primary users when exploiting spectrum.

Thus to realize the technique of cognitive radio, a cognitive radio user must have the ability to measure, to sense and to learn channel characteristics and availabilities.

In addition primary users can claim the spectrum anytime when they have data to send, thus cognitive radio users should be able to identify the presence of primary users in time and vacate the occupied bands immediately to prevent the interference to primary users.

Cognitive Radio Cycle

In cognitive radio cycle, a cognitive radio monitors spectrum bands and captures their information and then detects spectrum spaces.

The characteristics of the spectrum spaces that are detected spectrum sensing are estimated. Then appropriate spectrum band is chosen according to its characteristics and user requirements.

Once operating spectrum band is determined, the communication can be performed over this spectrum band.

1. Spectrum Sensing: It refers to detect the unused spectrum and sharing it without harmful interference with other users. It is an important requirement of the Cognitive Radio network to sense spectrum holes, detecting primary users is the most efficient way to detect spectrum holes.
2. Spectrum Management: It is the task of capturing the best available spectrum to meet user communication requirements.
3. Spectrum Mobility: It is defined as the process where the cognitive user exchanges its frequency of operation
4. Spectrum Sharing: This refers to providing a fair spectrum scheduling method among the users. Sharing is the major challenge in the open spectrum usage

Spectrum Sensing

Spectrum sensing is defined as the task of finding of spectrum holes by sensing the radio spectrum in the local neighbourhood of the cognitive radio receiver in unsupervised manner.

The spectrum holes stands for those sub bands of the radio spectrum that are underutilised at particular instant of time and specific geographic location.

Spectrum Sensing is the capability to determine and sense whether license user is present or absent.

Objective of cognitive radio is that unlicensed user needs to detect the presence of licensed user or shift to another frequency band or stay in the same band by changing its modulation scheme to avoid interference.

Spectrum Sensing involves the detection of the presence of a transmitted signal, by a given Receiver.

The ability of a cognitive Radio to dynamically access the spectrum holes that dynamically appear is predicated upon its ability to detect these white spaces in the first place.

Spectrum sensing technique can be categorized into two types. They are: Direct and Indirect Techniques.

Direct Technique is also called as frequency domain out in which estimation is carried out directly from signal approach. Where as in Indirect Technique (also called as time domain approach), in this technique estimation is performed using auto correlation of the signal.