Cognitive radio offers a tempting solution to increase the spectrum usage efficiency by proposing opportunistic usage of frequency bands among licensed users and secondary users. This paper focuses on mitigating the interference that may occur when the secondary user is unaware of the primary user's active period ; known as hidden terminal problem. Path attenuation and shadowing effect are also considered. In this paper, in order to enhance the sensing capability of the cognitive radio system, we propose a cognitive radio system with the combination of active and passive awareness types, by the collaboration of carrier sense and cellular system. Here, we present the simulation results of the system with microwave relays as a primary user and ad hoc cognitive radio terminals as a secondary user. Three different cases are evaluated ; the conventional system, the system with carrier sense, and the system with carrier sense assisted by cellular system.
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