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MIMO OFDM is a very promising technique for high-speed wireless transmission. By applying the Direct Conversion Architecture (DCA), low-cost, low-power and fully integrated implementation of such systems is enabled. However, the performance of DCA may be seriously limited by I/Q-imbalance, which shows frequency selectivity in broadband systems. In this paper, we consider an indoor scenario and present effective preamble based joint compensation of the MIMO wireless channels (block fading) and the TX- and RX-frequency-selective I/Q-imbalance. Two low overhead preambles are proposed, which are constructed in time- and frequency domain, respectively. Both of them have their own advantages. Compared to the literature, our paper considers more practical issues and shows how to choose the adequate preamble design according to system parameters to achieve the best performance with relatively low computational complexity. Effectiveness of the proposed schemes is verified by numerical simulation.