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In this paper we study the resource allocation problem for an OFDM-based multi-cell cognitive radio (CR) system, in which the secondary user (SU) of the CR system shares radio spectrum conditionally with the primary user (PU) of a licensed system. The SUs can use the whole licensed bandwidth of PUs, as long as the interference power introduced to the PUs is within the tolerable margin. Since the SUs of different CR cells can transmit signals using the same subchannels, the inter-carrier interference among the SUs should also be considered. We propose an efficient algorithm to allocate bits of OFDM subchannels in a single CR cell by jointly considering the transmitting power and interference constraints. An iterative algorithm is adopted to update the interference level of each cell until an equilibrium point is reached, which means the interference introduced to all PUs is below the predefined threshold while obtaining the maximum capacity. Simulation results show the proposed algorithm performs better than other methods.