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This paper considers the performance degradation of a high QAM orthogonal frequency division multiplexing (OFDM) system over frequency selective time-varying fading channels. The time variation of the channel during one OFDM frame destroys the orthogonality of different sub-carriers and generates power leakage among the sub-carriers, known as inter-carrier interference (ICI). The statistics of the ICI for M-QAM systems are shown to be non-Gaussian distributed. However, the ICI distribution is well approximated by the derived 2-dimensional Gram-Charlier series. Finally, the degradation in the performance of OFDM resulting from Doppler effects over time varying, frequency selective Rayleigh fading channel is obtained by averaging the approximated ICI statistics over the ensemble fading.