Low-complexity subspace method for I/Q imbalance estimation in low-IF receivers with unknown fading

In low-intermediate-frequency (low-IF) receivers, I/Q imbalance (IQI) causes interference on the desired signal from the blocker signal transmitted over the image frequencies. Conventional approaches for pilot-aided IQI estimation in zero-IF receivers are not applicable to low-IF receivers, where the image interference is unknown at the receiver. We propose a low-complexity subspace method for the estimation of IQI parameters in low-IF receivers in the presence of unknown fading, where we utilize knowledge of the pilots to null out the signal part. This reduces the variance of the sample mean estimate and leads to faster convergence. The proposed nulling method offers significantly better image rejection at low input signal-to-interference power ratio (SIR) than existing methods. Performance analysis of the output SIR as well as computer simulations are also provided.