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Efficient pilot designs are vital as they improve system performance and resource utilization, especially in multi-input multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. Most of the existing pilot designs do not consider in-phase and quadrature (I/Q) imbalance whose detrimental effects become more severe since the technology and applications are pushing towards smaller semiconductor node, smaller chip, cheaper equipment, and larger modulation order. This paper studies the impact of I/Q imbalance on the pilot designs, and presents several new pilot designs for equivalent channel estimation of MIMO OFDM systems with frequency-dependent I/Q imbalances. Advantages of the proposed designs are illustrated through simulation and analytical results in terms of estimation accuracy, error rate, overhead efficiency, and generality with the transmission formats (preamble-based or pilot-data-multiplexed systems, with or without null guard tones).