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The maximum likelihood estimate of the impulse response of a frequency-selective channel in the presence of phase noise and I/Q imbalance is derived. The complexity of the joint estimator is reduced using approximate cost functions for both phase noise and I/Q imbalance. The proposed estimator is first applied to OFDM transmission in a single-input single-output system and then generalized to multi-input multi-output OFDM systems. The bit error rate performance of popular space-time codes for two and four transmit antennas is evaluated under zero-forcing, minimum mean squared error and maximum likelihood detection rules. An expression for the residual inter-carrier interference variance after phase noise and I/Q imbalance compensation is derived and compared to the uncompensated case. Significant improvement in signal-to-interference-noise is obtained with the proposed algorithm.