PAs present impairments which have to be taken into account during the system design because they can degrade the modulation accuracy (EVM) and also produce unwanted out-of-band emissions, due to spectral regrowth, causing interference to neighboring channels (Liang et al., 1999; Zhou and Kenney, 2002).
The distortions of the PA are generally modeled using AM/AM (amplitude to amplitude) and AM/PM (amplitude to phase) curves which can be measured or represented as mathematical functions. If we consider a complex baseband signal x(t) = a(t)e jφ(t), the output of the PA can be written
where AM(a(t)) is the AM/AM function describing the PA output amplitude as a function of the input signal amplitude, and PM(a(t)) is the AM/PM function describing the PA output phase as a function of the input signal amplitude. The AM/AM causes amplitude distortion whereas AM/PM introduces phase shift.
The aim of the PA modeling is to develop mathematical functions for including AM/AM and AM/PM characteristics in the system simulations. In this section we will briefly describe the main models used in the literature.
Note that the models presented below do not depend on frequency and are called memory-less because their outputs do not depend on the past, only on the instantaneous input. However, with wideband communication systems memory effects ...