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Frequency-dependent I/Q imbalance is one of the major impairments in the direct-conversion receivers (DCR) for high-speed wideband wireless systems. We propose two new blind methods for compensating frequency-dependent I/Q imbalance. The first one is a time-domain approach. Specifically we develop a blind identifiability condition based on which a cost function and a gradient descent search algorithm are proposed for blind I/Q imbalance compensation. The second blind method is a frequency-domain approach for OFDM systems. Here we provide blind estimators for the frequency-selective I/Q imbalance parameters, which once obtained, the I/Q imbalance can then be compensated by a simple single-tap matrix filter inversion. We provide extensive simulation results to demonstrate the performance of the proposed algorithms.