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We study the performance of coded modulation schemes where the Gallager code is used as the underlying frequency-space code over MIMO fading ISI channels. Two systems are compared: one is the OFDM based system; the other is the single-carrier system, which employs the turbo-iterative decoding, and equalization at the receiver. With simulations over a number of ISI channels, we confirm that in all channel settings detection performance-perhaps already well known. In this paper, a particular attention is given to answer the question: how close is the performance of OFDM system to the performance of the optimal time-domain equalization when the LDPC code is used as the frequency-space code. It is shown that over the ensemble of MIMO fading channels, the performance degradation of OFDM is only about 2 dB, when two transmit and two receive antennas are used.