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Cognitive radio makes it possible for an unlicensed user to access a spectrum unoccupied by licensed users. Efficient subcarricr and power allocation algorithm in CR networks is a challenging problem, especially for uplink. In this paper, an efficient multiuser water-filling algorithm is proposed under the interference temperature constraints. L'sing this algorithm, subcarriers are allocated to the users with the best channel quality initially and then adjusted to improve the system performance. As is shown in the simulation results, the proposed algorithm has a much lower computational complexity and the performance is close to optimal scheme. Finally, it is proved that there is an upper bound of the throughput loss by using this algorithm.