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Recent studies and measurements have shown that, with the traditional spectrum access approach, the radio spectrum assigned to primary (licensed) users is vastly underutilized. While many spectrum methods have been proposed to use spectrum effectively, the opportunistic spectrum access has become the most viable approach to achieve near-optimal spectrum utilization by allowing secondary (unlicensed) users to sense and access available spectrum opportunistically. Opportunistic spectrum access approach is enabled by cognitive radios which are able to sense the unused spectrum and adapt their operating characteristics to the real-time environment. However, a naive spectrum access for secondary users can make spectrum utilization inefficient and increase interference to adjacent users. In this paper, we propose a novel approach using Fuzzy Logic System (FLS) to control the spectrum access. Three descriptors are used: spectrum utilization efficiency of the secondary user, its degree of mobility, and its distance to the primary user. The linguistic knowledge of spectrum access based on these three descriptors is obtained from a group of network experts. 27 fuzzy rules are set up based on this linguistic knowledge. The output of the FLS provide the possibility of accessing spectrum band for secondary users and the user with the greatest possibility will be assigned the available spectrum band.