Cognitive radio network has gained a lot of popularity in the era of wireless communication to overcome an issue of scarce bandwidth. Cognitive radio network (CRN) has opened with tremendous research areas toward the spectrum sensing and sharing. In this paper, we will see the different techniques used for the detection of primary user’s (PU) activity and assign the spectrum to secondary user (SU) if the spectrum is unutilized by the primary user without interfering the primary user and utilized the spectrum efficiently. This paper is an attempt to throw light on the primary idea of research work, which is going in the field of cognitive radio networks in wireless communication.