Cognitive radio (CR) is a new paradigm that utilizes the available spectrum
band. The key characteristic of CR system is to sense the electromagnetic
environment to adapt their operation and dynamically vary its radio operating
parameters. The technique of dynamically accessing the unused spectrum band is
known as Dynamic Spectrum Access (DSA). The dynamic spectrum access technology
helps to minimize unused spectrum bands. In this paper, main functions of
Cognitive Radio (CR) i.e. spectrum sensing, spectrum management, spectrum
mobility and spectrum sharing are discussed.