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Cognitive radio (CR) has been shown to be a very effective way to address this challenge by sharing the spectrum resources with the existing wireless networks. In general, CR can be classified into three categories based on the underlining spectrum access paradigms, namely interweave, underlay and overlay [25].
In interweave mode, the secondary users cannot access the licensed spectrum if the primary users are detected being active [26]. 如果检测到主用户处于活动状态，则次级用户不能访问许可的频谱。
When underlay mode is adopted, the secondary user can access the spectrum as long as it does not cause too much interference towards the primary users [27]. 只要不对主用户造成太大干扰，次级用户就可以访问频谱。
In overlay mode, the secondary users obtained the access opportunities by the aid of the primary user transmission [28]. 次级用户通过辅助主用户的传输获得访问机会。
Among the above spectrum access modes, interweave mode is limited by the operating conditions of the primary networks, while overlay mode requires excessive cooperative overhead between secondary and primary networks. In contrast, underlay mode allows secondary users to access the spectrum more freely than interweave mode, but requires less collaboration overhead than overlay mode. Accordingly, much research effort has been devoted to underlay CR networks.
《Physical layer security of cognitive ambient backscatter communications for green Internet-of-Things》
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