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It is well known that the realization of enormous capacity of multiple-input multiple-output (MIMO) channels depends greatly on the accuracy of channel estimation at the receiver. And the problem of designing optimal pilot or training sequences is crucial to channel estimation. In this paper, the optimal pilot sequences are designed by maximizing the mutual information (carried by pilot tones) between the channel and the received sequences during the training over multiple OFDM symbols. Therein the MIMO channel is assumed to be frequency-selective and obey block-fading law with both spatial fading correlation and multipath tap gain correlation known at both transmitter and receiver. Simulations show that the designed optimal pilot sequences outperform the orthogonal pilot sequences. It is also concluded that this performance gap between the two types of pilot sequences is dominated by different conditions of spatial fading correlation and multipath tap gain correlation.