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Space-time coded wireless transmission techniques with multiple transmit and receive antennas can provide considerable increases in both the link quality as well as link capacity when compared to ordinary single-antenna techniques. However, multiantenna transmission basically calls for multiple parallel radio implementations, and the resulting link performance is found to be very sensitive to the possible nonidealities of the individual analog radio front-ends. One important practical example is the so-called I/Q imbalance problem related to the amplitude and phase matching of the I/Q branches of the transmitters and receivers. In this paper, we analyze the I/Q imbalance effects in space-time coded transmit diversity system context, in terms of the resulting signal-to-interference ratio as a function of the imbalance properties, assuming the individual transmitter and receiver analog front-ends are based on the so-called direct-conversion radio architecture. The obtained results indicate that the I/Q imbalance effect is fundamentally more challenging in the multiantenna context compared to traditional single-antenna systems. In addition, two digital compensation methods are proposed for combating the resulting signal distortion on the receiver side. The first approach is based on algebraic properties of the derived signal models combined with proper pilot data, while the second one is blind, stemming from the blind signal separation principles. The resulting link-level performance of the proposed algorithms is evaluated using extensive computer simulations. Based on the obtained results, the I/Q imbalance effects can be efficiently compensated using the proposed techniques, the resulting link performance being practically identical to that of the ideal perfectly matched reference case. Furthermore, the proposed methods are also shown to correct for channel estimation errors, in addition to I/Q impairments, and are also reasonably robust against residual carrier offs- - ets.