Traditional wireless networks including wireless sensor networks working in an unlicensed spectrum band are suffered from uncontrolled interference, as this unlicensed spectrum band gets crowded day by day. This problem is not because of spectrum band deficiency, rather it is because of the misuse/inefficient usage of the spectrum bands i.e., fixed spectrum assignment policy. Cognitive Radio (CR) is a promising technology to solve this challenging spectrum allocation problem. The CR opportunistically access vacant spectrum bands in a licensed spectrum. When the current band/channel becomes unavailable, the device can switch to another available channel. However, for the realization of the cognitive radio networks, spectrum sensing is the ground work and also regulatory bodies need to adopt flexible and non-fixed policies & techniques. In this paper, we analyze different spectrum sensing techniques to detect the presence of the Primary User (PU). The techniques covered in this paper are fuzzy logic cooperative spectrum sensing, asynchronous cooperative spectrum sensing, cooperative spectrum sensing based on network coding, cooperative spectrum sensing with relay diversity, and distributed cooperative spectrum sensing based network coding.