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Cognitive radio (CR) is one of the most promising solutions to the scarcity of radio spectrum. In CR systems, the channel sensing technique to detect the appearance of a primary user (PU) directly affects the performances of both CR user and PU. In this paper, we propose a nonquiet channel sensing scheme for the orthogonal frequency division multiple access (OFDMA)- based CR systems. The proposed nonquiet PU detection scheme is based on the reasoning that a CR user can detect the PU even though the signal transmitted from other CR user interferes with the detection, if the interference power is sufficiently low. Since the interference power decreases as the channel condition between the transmitter and receiver CR users gets worse, the proposed scheme regards the sensing results with deeper channel fading as more reliable, given that the received signal power from PU is unknown. We design a linear combining scheme which integrates multiple sensing results to obtain spectral, temporal, and spatial diversity gain. We also suggest a system-wide PU detection mechanism based on the proposed combining scheme. The simulation results show that the proposed scheme can detect the appearance of PU effectively while achieving high utilization of the CR system.