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In this paper, we analyze the multiuser diversity gain in a cognitive radio (CR) system where the secondary transmitters opportunistically utilize the spectrum licensed to primary users only when it is not occupied by the primary users. To protect the primary users from the interference caused by missed detection of primary transmissions at the secondary receivers, minimum average throughput of the primary network is guaranteed by transmit power control at the secondary transmitters. The traffic dynamics of a primary network are also considered in our analysis. We derive the average system throughput of the secondary network and analyze its asymptotic behaviors to characterize the multiuser diversity gain in the CR system. It is shown that the throughput gain of the CR system depends on the levels of the peak power constraints on the secondary transmitters and the reliability of spectrum sensing. The asymptotic …