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OFDM has been widely regarded as an effective modulation technique for mitigating the effects of ISI in a frequency selective fading channel and for providing reliable high-data transmission over wireless links. Adaptive antenna arrays at the transmitter can achieve further increases in system capacity and bandwidth efficiency, as well as, in QoS improvement in conventional OFDM systems. But the performance of such of systems will severely degrade as the quality of the channel information available at the transmitter decreases. In this paper, we propose a space-time block coded adaptive transmit antenna arrays for broadband OFDM communication systems by utilizing partial channel side information. By employing space-time coding and adaptive power assignment between the sub-channels obtained by applying eigenvalue decomposition on the conditional channel correlation matrix of each sub-carrier, the proposed system combines the benefits of both space-time block coding and adaptive antenna arrays while being robust to channel imperfection. Simulation results show that the proposed system yields significant gains over conventional adaptive antenna arrays based systems without space-time coding under imperfect channel information available at the transmitter. Simulation results also show that the proposed power allocation in the frequency domain can further improve the performance of the space-time block coded adaptive transmit antenna arrays based OFDM systems.