Blind Compensation of I/Q Filter Imbalance in the LTE Downlink

In direct conversion receivers differences of the analog components in the I- and the Q-branch lead to I/Q mismatch. Digital signal processing can be used to reduce the specification for the RF circuitry. In this paper we focus on the compensation of frequency dependent I/Q mismatch due to different cut-off frequencies of the lowpass filters. This mismatch, which is an issue due to the wide bandwidth used e. g. in LTE systems, is compensated with a second order statistic-based approach. The blind adaptive algorithm recovers a property of complex signals, called properness which is disturbed by the I/Q mismatch. The algorithm achieves a very good performance even with a mismatched upconversion stage. It is able to compensate cut-off differences higher than 10% and is independent of special pilot structures and Rayleigh fading channels.