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This paper looks into the allocation of unused frequency bands to secondary users. We propose a centralized model, where the secondary radio has a base station which can support the transmission of secondary users having either OFDM-based or CDMA-based communication systems, and the frequency bands assigned to each secondary user can be dynamically assigned. We set up a nonlinear binary integer programming model to allocate frequency bands to all secondary users, either in the presence or absence of primary users. We show that the problem can be converted to a quadratic integer programming problem with linear constraints to compute the optimal allocation. Finally, a suboptimal heuristic approach is proposed.