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In this paper, receiver designs for orthogonal frequency-division multiplexing (OFDM) systems that exploit the Alamouti transmit diversity technique are addressed. In Alamouti space-time coded OFDM systems, the simple Alamouti decoding at the receiver relies on the assumption that the channels do not change over an Alamouti codeword period (two consecutive OFDM symbol periods). Unfortunately, when the channel is fast fading, the assumption is not met, resulting in severe performance degradation. In this paper, a sequential decision feedback sequence estimation (SDFSE) scheme with an adaptive threshold (AT), a traditionally single-carrier equalization technique, is used to mitigate the performance degradation. A new method to set the threshold value is proposed. For small signal constellations like BPSK and QPSK, the SDFSE with an AT requires much lower complexity than a previous minimum mean square error approach at the cost of a small performance degradation. Furthermore, we show that the performance difference becomes smaller when channel estimation error is included. An adaptive effort sequence estimation (AESE) scheme is also proposed to furthur reduce the average complexity of the SDFSE scheme with an AT. The AESE scheme is based on the observation that a high Doppler frequency does not necessarily mean significant instantaneous channel variation. Simulations show the efficacy of the proposed SDFSE with an AT and the AESE. [ABSTRACT FROM PUBLISHER]