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Dynamic spectrum access has become a promising approach to fully utilize the scarce spectrum resources. In this paper, two new dynamic spectrum access schemes for secondary users are proposed, which are based on continuous-time Markov chains (CTMC). Both the interactions between primary and secondary users and the effects of imperfect spectrum sensing(i.e. miss-detection and false alarm) are considered in the schemes. Due to the fact that miss-detection would lead to collision between primary and secondary users, we derive the optimal access probabilities for secondary users, so that the QoS of primary users in terms of collision probability is guaranteed. Simulation results show that the proposed dynamic spectrum access approach under proportional fairness criterion achieves much higher fairness over max-throughput criterion. Moreover, the scheme with buffering mechanism can raise the channel occupancy remarkably.