  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. Then DSA models are discussed
along with different methods of DSA such as Command and Control, Exclusive-Use,
Shared Use of Primary Licensed User and Commons method. Game-theoretic approach
using Bertrand game model, Markovian Queuing Model for spectrum allocation in
centralized architecture and Fuzzy logic based method are also discussed and
result are shown.
