Cognitive Radio has been recognized as a promising way to improve spectrum efficiency of wireless communications by exploiting under-utilized licensed spectrum in temporal, frequency, and spatial domains. As a remedy to the limited capability of a single node or network, cooperative communication technique has a potential to further improve the performance of cognitive networks, especially for spectrum sensing based cognitive networks. Given that spectrum efficiency of a cognitive network is determined jointly by spectrum sensing and access capabilities, it makes sense to employ cooperative techniques to enhance their performance. This article offers a tutorial on various cooperative techniques in cognitive networks, with its emphasis on spectrum sensing and access based cooperation, interference constraint based adaptive cooperative feedback, rateless network coding based cooperative transmission, and limited cooperation based interference coordination. Finally, numerical results are given to showcase advantages of the proposed cooperative schemes for cognitive networks.
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