On the connection of I/Q imbalance and channel equalization in direct-conversion transceivers

Due to its simple structure and reduced requirements for radio frequency filtering, the direct-conversion principle has recently gained a lot of interest in radio transceiver design. This paper considers the so-called inphase/quadrature (I/Q) imbalance problem in such structures under the assumption of frequency-selective transmission channel requiring equalization on the receiver side. It will be shown that I/Q imbalance and channel equalization are strongly interdependent, and thus, I/Q mismatch should be taken into account in the equalizer design, and vice versa, particularly with high-order symbol alphabets. Most importantly, the amount of signal distortion caused by transmitter and receiver I/Q imbalances to the equalized received signal will be analytically analyzed through signal-to-interference ratio (SIR) analysis. This gives a quantitative measure to analyze and judge the role of I/Q imbalances in practical communication system design, without having to resort to lengthy data simulations. Furthermore, the analysis shows that imbalance levels that are tolerable in additive white Gaussian noise channels can turn into a serious problem in multipath channels requiring equalization. This gives further motivation and invaluable insight in the overall receiver and system design and in developing digital signal processing-based techniques to mitigate the I/Q imbalance effects on the receiver side. In general, the analytical outcomes and results are verified using extensive computer simulations.