Cognitive radio has been proposed in order to promote spectrum utilization by exploiting the spectrum holes. Observing the spectrum resources can be matched to traffic demand of secondary users with the help of relay node. In this article a new research direction for cognitive radio networks by utilizing cooperative relay to assist transmission and improve communication efficiency. Since wireless networks are characterized to have limited resources which are accessed by a large number of mobile

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... towers with distinct capabilities. In such challenged environment the dynamic resource control is used to eliminate the limitations of wireless networks. Such augmented wireless resource usage can be incorporated based on cooperative relaying schemes, which have the capability to support the network lifetime and desired system performance. However, the introduction of cooperative relay raises several problems such as the relay selection issue and resource allocation. Due to the significant number of different cooperative relaying techniques, the aim of this survey is to provide a systematic and thorough analysis and classification of major relay selection procedures, and to identify research directions as well as the evaluation methods for an efficient analysis of different approaches.