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MIMO OFDM is a key technique for achieving high data rate in future wireless communication systems. The desired low-cost, low-power and fully integrated implementation of MIMO OFDM systems leads to the application of direct (up/down) conversion architecture, whose performance can be seriously limited by I/Q-imbalance. Thus, the compensation of I/Q-imbalance is a crucial issue in the implementation of MIMO OFDM systems, which requires reliable parameter estimation. In this paper, an efficient scheme is proposed for the joint estimation of the MIMO channel and frequency-selective I/Q-imbalance both at the transmitter and the receiver. First, a multi-functional preamble is presented, which not only fulfills the optimal design rules for joint estimation but can also be used for both parameter estimation and frame detection/time synchronization, allowing low overhead. Furthermore, low Crest-Factor (CF) of the preamble is achieved. Based on this preamble, a low complexity estimator is developed, which can achieve performance close to the Cramer-Rao Lower Bound (CRLB). Numerical simulation results and complexity analysis have verified the advantages of the proposed estimation scheme over the existing schemes.