IM3 injection technique for improving the linearity of an RF power amplifier

Linearity and efficiency are two challenging parameters in an RF power amplifier. This is because there is a trade-off between linearity of a PA and its efficiency. This can be a bottleneck in many of modern wireless systems especially those are to be designed for transmission of non-constant envelope signals. This paper presents an efficient predistortion method based on digital baseband injection of appropriate signals to the input to lower the Adjacent Channel Power Ratio (ACPR) parameter. A mathematical method is given to describe the nonlinear relationship between input and output of the PA The optimum values for the injection signals at the intermodulation frequencies are obtained. In comparison with previous works, the proposed method is relatively low cost and high efficient. Simulation results show that the proposed method is capable of remarkably reducing spectral regrowth in adjacent channels for different non-constant envelope signals. For a QPSK signal, the spectral regrowth and ACPR of the PA are decreased by 20dB and 26dB, respectively.