Cognitive radio is the best solution for the spectrum scarcity and spectrum underutilization problem in wireless communication system, because of its ability to rapidly and automatically adapt operating parameters as per requirements and conditions. In this paper, a literature review of cooperative spectrum sensing techniques for cognitive radio is presented. In Cognitive radio, spectrum sensing is the main function which prevents the interference with the Primary Users (PU) and identifies the available white spaces to enhance the spectrum utilization. We examine the use of cooperative spectrum sensing in cognitive radio systems to enhance the reliability of detecting primary users. We have presented principle, classifications and components of CSS and CSS techniques based on decision fusion rules in detail. Also advantages and disadvantages of detection techniques are investigated and summarized. Finally research issues and challenges are given. Keywords: Cognitive, radios,cooperative, Spectrum Sensing,hard and soft decision fusion
