Extended Hammerstein Behavioral Model Using Artificial Neural Networks

In this paper, a novel extended Hammerstein model is presented to accurately mimic the dynamic nonlinearity of wideband RF power amplifiers (RFPAs). Starting with a conventional Hammerstein model scheme, which fails to predict the behavior of the RFPA with short-term memory effects, two areas of improvements were sought and found to allow for substantial improvement. First, a polar feed-forward neural network (FFNN) was carefully chosen to construct the memoryless part of the model. The error signal between the output and the input signal of the memoryless sub-model was then filtered and then post-injected at the model output. This extra branch, when compared to the conventional Hammerstein scheme, allowed for an extra mechanism to account for the memory effects due to dispersive biasing network that was present otherwise. The excellent estimation capability of the polar FFNN together with the additional filtered error signal post-injection led to remarkable accuracy when modeling two different RFPAs both driven with four-carrier wideband code division multiple access signals. Despite its simple topology and identification procedure, the extended Hammerstein model demonstrated is capable in accurately predicting the dynamic AM/AM and AM/PM characteristics and the output signal spectrum of the RFPA under test.