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In this paper, we propose a novel orthogonal space- time-frequency block codes using Alamouti scheme for OFDM systems (STFBC-OFDM) employing two transmit antennas. Space-time block coded OFDM (STBC-OFDM) systems and space-frequency block coded OFDM (SFBC-OFDM) systems face problems in time-selective fading (i.e., time-varying channel) and frequency-selective fading environments, respectively. In the proposed STFBC-OFDM, the Alamouti code is applied over two successive OFDM symbols and neighboring subcarriers simultaneously with scaling factor to reduce the performance degradation from channel selectivities. Although the repetition of coding decreases the code rate, a SNR gain obtained by scaling can nearly compensate for the loss of code rate. The simulation results show that the proposed STFBC-OFDM outperforms STBC-OFDM in time-selective fading channels and SFBC-OFDM in frequency-selective fading channels.