Complex Baseband Predistorters for Nonlinear Wideband RF Power Amplifiers

RF power amplifiers (PA) of wireless communication systems are usually driven in their nonlinear region to obtain a higher efficiency. The price paid for the increased efficiency is the distortion of the transmission signal. To fulfill a given spectral mask for the transmission signal and to avoid an unacceptable high bit-error rate on the receiver side, the amplifier must be linearized. If wideband signals as WCDMA or OFDM are used, static nonlinear predistorters are in general not sufficient to obtain the required performance. For this reason, we consider the most general nonlinear system, described by a complex baseband Volterra series, as a starting point for our investigations. Although Volterra series provides the best performance, a serious drawback is the huge number of free parameters and the high implementational complexity. Therefore we reduce the complex baseband Volterra system (43 parameter) to a complex baseband system with a lower complexity (11 parameter) and apply the developed predistorters to a physical 2.2-GHz 90-W Class AB RF power amplifier. Although we have reduced the number of parameters dramatically, the spectral performance is comparable to the full Volterra predistorter.