In this paper, preamble design for estimation of frequency selective channels and In-phase/Quadrature-phase (I/Q) imbalance in Multiple Input Multiple Output Orthogonal Frequency Division Multiplexing (MIMO-OFDM) systems is proposed. First the authors utilize convex optimization to optimize power of all active subcarriers, then they employ Cross Entropy (CE) optimization techniques to select optimal preamble sequence that minimizes the channel estimate Mean Squared Error (MSE) while suppressing the effect of I/Q mismatch. To mitigate inter-antenna interferences, disjoint preamble sequences are utilized for each transmit antenna. An algorithm to guarantee that the preamble sequences are disjoint for each transmitter is proposed. Numerical simulations are provided to verify the advantages and effectiveness of the proposed preamble sequences over the conventional sequences.