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In an OFDM-MIMO system, for each sub-carrier the singular values decomposition (SVD) decouples the MIMO matrix channel into parallel sub-channels and the water-filling algorithm allocates the power optimally for the sub-channels. Since there are multiple OFDM sub-carriers, the SVD produces a large amount of subchannels and it is computation-intensive to find the exact water-filling solution. Fortunately, the singular values in an OFDM-MIMO system exhibit asymptotic features, which can be utilized to simplify the power allocation process. Our scheme achieves the simplification by using a continuous curve to fit the distribution of singular values and computing only the useful singular values. Simulation results verify the accuracy of our scheme and show that the capacity loss due to the approximation is less than 3%.