On nonlinear memory effects in all digital RF-transmitters

Recent developments in the field of wireless communications emphasis the urgent need for flexible radio frequency (RF) front ends. Not only a multitude of frequency bands needs to be supported, also modulation formats come in a greater variety and tend to develop at a faster pace. While receiver topologies have been adapted by employing software defined radio (SDR) concepts, translating these developments to transmitters was hindered. Mostly due to the lack of power amplifiers that are energy efficient, linear and broadband at the same time. All digital radio frequency transmitters (DRFTx) are a promising, however, not a very mature option to overcome this issue. While offering the desired flexibility and potential energy efficient operation, these setups exhibit relatively long memory effects which also result in nonlinear inter symbol interference (nISI). In this paper it is demonstrated that the observed nonlinear memory effects, dominantly caused by the need for a reflective reconstruction filter, can be modeled reliably by a lookup table (LUT) model. This analysis is based on measured data of a DRFTx lab setup. However, they are backed up by simulations gained from readily available nonlinear transistor models within an RF computer aided design (CAD) tool.