Resumen
Cognitive radio holds great promise as a research area nowadays. By increasing the spectral efficiency, it is applicable to various areas where spectrum is a rare resource. Nevertheless, it still presents key challenges such as the interference and the false alarm possibilities. The community of research is committed to find the suitable solution in these cases. In some situations, it could be that the primary transmitter and the secondary receiver are very distant from each other and thus, direct communication is not possible. Relay nodes are used for establishing communication in such cases. Though selecting the best relay among others is a very difficult task. This paper offers a summary of cognitive radio, the significance of relaying network and the algorithm of relay selection.