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In common models of cognitive radio networks (CRNs), priority in spectrum access of primary users (PUs) must be guaranteed, but the quality of service (QoS) for secondary users (SUs) is mostly ignored. Subject to meeting PUs' blocking probability objective, this paper proposes a channel reservation scheme to enhance QoS for SUs. The proposed scheme employs a centralized control manager (CCM) to coordinate the dynamic spectrum access among PUs and SUs by dividing the spectrum into two parts: the normal access channels (NACs) that may be taken back anytime by PUs, and the reserved secondary channels (RSCs) that are locked by occupied SUs until their call sessions are complete. A Markov chain model is developed to analyze the proposed spectrum access scheme and compare its performance to that of a CRN without reserved channels. Furthermore, the number of reserved channels is optimized based on SUs' utility functions. Numerical results show that by optimally selecting the number of reserved channels to adapt to various traffic loads, the proposed scheme can significantly improve SUs' QoS while meeting PUs' QoS objectives.