Frequency Dependent Modulator Imbalance in Predistortion Linearization Systems: Modeling and Compensation

Impairments, i.e., gain/phase imbalance and dc offset, in the analog quadrature modulator, can adversely impact the performance of a digital predistortion linearization system. In the literature, gain/phase imbalance is usually looked upon as a frequency-independent phenomenon. However, our experiments suggest that the gain/phase imbalance exhibits frequency-dependent behavior when the input signal bandwidth becomes wide. In this paper, we first propose channel models to describe the effects of dc offset and frequency-dependent gain/phase imbalance on transmitted I/Q data streams. We then develop algorithms to estimate the parameters of these channel models and to construct I/Q compensators. Performance of the algorithms and the I/Q compensators is shown together with digital predistortion.