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Cognitive radio technique has been recently proposed as a novel approach for improving the utilization of the precious limited radio resources, by letting secondary users access primary users' spectrum if not used. There are two major challenges for cognitive radio: spectrum sensing and hidden terminal problem. In this paper, a novel network structure is proposed to separate the spectrum sensing task from the secondary users. The service providers for the secondary users need to place sensing devices within the primary users' networks. These sensing devices sense the primary users' activity. The sensing devices also decide whether to admit a secondary user's transmission. A low-temperature handshake technique is proposed for communications between the secondary users and the sensing devices. A new cognitive cycle is proposed, and the detailed protocol is constructed. The proposed protocol is analyzed using the theory of Lame curve. The problems of optimally locating a sensing device and minimizing the number of sensing devices under the constraint of the maximal collision probability are studied and solved. A performance upper bound is also derived for a special case. From simulation results, the effectiveness of the proposed scheme is demonstrated.