I/Q imbalance compensation techniques for wideband receiver architectures

Abstract

As the demand for high data rates and robust communication systems increases, the mitigation of receiver impairments becomes essential. An important challenge is the frequency-selective In-Phase (I)/Quadrature (Q) imbalance, which causes interference between the desired signal and its image frequency in baseband. This limits the theoretically infinite image rejection of an ideal quadrature receiver and leads to significant signal degradation. This thesis investigates the application of frequency-selective I/Q imbalance compensation to wideband single-carrier signals. Therefore, various state-of-the-art methods are being considered, and the Real-Valued Compensator (RVC) is used for further investigations, as it is a robust approach that can be applied to different receiver architectures. While existing coefficient estimation methods are mainly studied for multicarrier signals with comparatively narrow bandwidths, their application in wideband single-carrier signals remains less understood. This thesis studies the blind closed-form adaptation method based on second-order statistics together with the blind adaptive gradient-descent method for wideband single-carrier signals. The closed-form method delivers promising results, whereas the gradient-descent approach suffers from unreliable gradient estimation for this signal. To overcome this limitation, a numerical gradient calculation using central finite differences is proposed, which enables the gradient-descent method to also achieve valuable results for the considered signal. For these two methods of estimating the coefficients of the RVC, this thesis examines two different simulated data sets with exactly known receiver IQ imbalances and channel effects. In addition, two measurements with unknown impairments are considered in order to assess what improvements can be expected in practice. These considerations are based primarily on the Image Rejection Ratio (IRR) and the Error Vector Magnitude (EVM). For wideband single-carrier signals, an improvement in the IRR of up to 18 dB is observed. Furthermore, it is concluded that the number of filter taps of the RVC mainly depends on the strength of the frequency-selective I/Q imbalance. In all considerations, the closed-form coefficient estimation method provides comparable results to the gradient-descent method, with the latter mainly improving the IRR slightly more.