An Accurate Three-Input Nonlinear Model for Joint Compensation of Frequency-Dependent I/Q Imbalance and Power Amplifier Distortion

The analog front of the direct conversion transmitter suffers from the effects of various unavoidable nonlinearities, such as local oscillator (LO) leakage, power amplifier (PA) nonlinearities, and in-phase and quadrature (I/Q) branch amplitude and phase imbalance, etc. To overcome these nonlinear effects, an accurate three-input nonlinear model is proposed for joint compensation of frequency-dependent I/Q imbalance and PA nonlinear distortion in this paper. The proposed nonlinear model is an augmented version of the modulator I/Q imbalance compensation model, where the magnitudes of the input signal are included as the model input, and a three-input nonlinear compensation model with linear parameters is developed. The advantage of the proposed model is that the augmented envelope-dependent terms can produce the basis function sets for compensating the PA nonlinear distortions in the transmitter. Furthermore, the computational complexity of the proposed model is analyzed in detail for practical digital implementation. To verify the validity of the proposed model, the imperfect transmitters based on single-device gallium nitride (GaN) PA and GaN Doherty PA, which are driven by different communication signals, are used for experimental verification and analysis. The experimental results show that the proposed model is able to give improved modeling and distortion mitigation capability than the other reported I/Q imbalance compensation methods when the effects of I/Q imbalance and nonlinear characteristics of PA are considered together.