Blind Compensation of Frequency-Selective I/Q Imbalances in Quadrature Radio Receivers: Circularity -Based Approach

Gain and phase differences between the analog in-phase (I) and quadrature (Q) branches of a quadrature receiver are unavoidable, and seriously degrade its image rejection capabilities. Furthermore, this so-called I/Q imbalance problem is in general a frequency-dependent phenomenon, which is often ignored in many otherwise excellent work. In this paper, we take this frequency-dependency into account and study a class of I/Q imbalance compensators based on widely linear (WL) processing of the received mismatched signal, under the assumption that the ideal baseband signal is proper (or circular). In other words, the complementary autocorrelation function of the ideal baseband signal is assumed to vanish, which is a valid assumption for most practical communications signals. Under I/Q imbalance the observed baseband equivalent signal becomes improper, and I/Q imbalance compensation can be performed by making the observation proper again. We propose a simple blind (non-data aided) WL compensator structure for suppressing the mirror-frequency interference. It shows impressive performance and has many additional desirable features, such as immunity to channel noise and the fading channel.