Non Linear RF is an extensive area of research. The importance of this is due to the boom in cell phones and restrictions placed by the FCC. Due to bandwidth constraints set by the FCC, every service provider should ensure that there are no out of band components transmitted. The core cause of this has been the power amplifier which is generally a Non Linear Amplifier, such as Class E or Class F, which have excellent gain but very poor linearity. They cause intermodulation products and spectral regrowth which can result in poor transmission. Lot of techniques have been developed to remove these non linearities, such as Pre Distortion, wherein the signal is distorted before sending it to the amplifier so that the non linearities get cancelled. Thus, accurate models of the amplifiers are essential to predict their behavior for the purpose of pre distortion.
This study focuses on the modeling of the non linearities present in the amplifier, focussing on fitting the signal both in time and frequency domain. The modeling is based completely on B-Splines, to fit the responses of a 16 tone, 64 Side bands WiMAX Signal at 2.65 GHz. The experimental data is obtained from the Large Signal Network Analyzer (LSNA) which recorded the reflection coeffiecients for all the harmonics at different power levels. The data from the LSNA is first fitted in time domain and later in the frequency domain. We explore the advantages and the limitations of both kinds of fitting. The goal of the work is to fit the fundamental band using the B-Spline Model and predict the spectral regrowth. Entire fitting process has been implemented in MATLAB after suitable synchronization of phase and delay which are introduced by the test equipment.