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The space-time coded orthogonal frequency division multiplexing (STC-OFDM) system is a promising scheme for broadband communications due to the benefits from the diversity gain within multiple input multiple output (MIMO) of space-time coding (STC) and the OFDM modulation technique to mitigate inter-symbol interferences (ISI). However, STC insults in an impairment of the system ability to suppress interferences as the signals transmitted from two transmit antennas are superposed and interfered at the receive antennas. In this paper, we developed an adaptive beamforming based on minimum mean squared error (MMSE) algorithm with broad null synthesis to reduce co-channel interferences (CCI) for the STC-OFDM system. We compare the proposed beamformer with the null steering beamformer which requires known directions of arrival (DOAs). The space-time decoder corresponding to both beamformers are given by the maximum likelihood rules. The performance improvement is achieved as shown from the simulation results.