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Cognitive Radio (CR) in a new emerging technology which can increase the spectrum efficiency by opportunistically spectrum access and can solve the problem of spectrum scarcity, thus meeting the increasing demand of the RF spectrum. It can be realized in two different ways, either opportunistic use of RF band when it is not used by licensed user or dynamic spectrum access coexisting with the licensed user, which are known as, spectrum overly and spectrum underlay approaches, respectively. In this paper, a comprehensive study of signal detection techniques for spectrum sensing in CR systems is presented. Specifically, the state-of-the-art research results, challenges and future perspectives of spectrum sensing in CR systems are outlined, and also a comparison of different methods is given. With this article, readers can have a comprehensive insight of signal processing methods of spectrum sensing for cognitive radio networks and the ongoing research and development in this area.