Sources of linearity degradation in LINC transmitters for hybrid and outphasing combiners

The LINC (linear amplification using non linear components) is a potential candidate as a transmitter system for providing both high linearity and high efficiency. In this paper, we review all the origins of linearity degradation in LINC transmitters for both matched and outphasing combiners and group them into three main sources. We first show that the linearity degradation in the LINC transmitter with an outphasing combiner is inherent to the combiner structure. We further show the impact of phase and amplitude imbalance in the two LINC branches on the transmitter linearity. Finally the filtering effect on the linearity in the LINC transmitter is studied. Extensive simulations using the advanced design system (ADS) software with CDMA signals (7 dB of peak-to-average) have been carried out and results are presented to validate the approach used in each case studied.