Generalized Transmitter Compensation of Frequency Dependent I/Q Imbalance

This paper considers the formulation of a digital pre-distorter to eliminate distortions that are generated by a frequency-dependent imbalance between the analog in-phase (I) and quadrature (Q) channels of a direct conversion transmitter. Estimation of the imbalance occurs in the frequency domain, while compensation occurs in the time domain. We demonstrate the efficacy of the approach by estimating and eliminating transmitter distortions whose digital-to-analog converters (DACs) and filters have differing impulse responses, as well as a phase and amplitude imbalance between the local oscillators (LOs). The estimator developed includes a separation of the linear and nonlinear transmitter response, facilitating the application of nonlinear pre-distortion without architectural restriction. Furthermore, the estimator not only compensates for I/Q imbalances, but fully characterizes the frequency response and identifies the source of the imbalance. An interesting outcome of the analysis is that it may not be possible with baseband processing to compensate for an imbalance that occurs after up-conversion but prior to I/Q summing.