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In Alamouti coded OFDM systems, the simple decoding at the receiver relies on the assumption that the subchannels are flat fading, decoupled, and quasistatic so that each subchannel does not change over an Alamouti codeword period (two consecutive OFDM symbol periods). When the channel is fast fading, however, these assumptions are not met, resulting in severe performance degradation. In this paper, a sequential decision feedback sequence estimation (SDFSE) scheme with an adaptive threshold (AT) is used to mitigate the performance degradation at a moderate complexity. A new method to set a threshold value is proposed. Simulation results show that the SDFSE with an AT shows significant performance improvement at the cost of a moderate complexity increase.