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In this paper, a Space-Time-Frequency block code (STFBC) based on Quasi-Orthogonal designs is proposed over a frequency selective Rayleigh fading channel. The proposed code achieves rate-one and full diversity offered by the MIMO-OFDM channel. For quasi-static channel over adjacent OFDM symbol durations, as numerical results demonstrate, the proposed code introduces a significant performance gain over the existing STFBC codes in terms of bit error rate performance. By coding across the three dimensions with four frequency diversity branches, the proposed code has a four-wise maximum likelihood decoding. The system performance will be investigated under different delay spread of the channel. In addition, a performance comparison for different FFT size will be presented.