Wideband digital predistortion linearization of radio frequency power amplifiers with memory

The use of spectrum-efficient digital modulation techniques and the cost advantage of utilizing a single power amplifier in multi-carrier radio communication systems are the two major contributors to the increase in the linearity requirements during the processing of a signal. The simplest technique to improve the linearity of a power amplifier is to reduce its output power, so the device is operated in its linear region. Though this approach is valid, it severely degrades the power efficiency. Because the remaining components in a communications system usually consume little power compared to the final power amplifier, the efficiency of the latter becomes of extreme importance. Any systematic method for improvement of linearity (or nonlinear distortion reduction) is called linearization. Because baseband signal generation in modern communications systems is performed digitally, it makes sense to implement the linearization right at baseband, in the form of one more stage in the baseband generation process. The predistortion method of nonlinear distortion correction is the most suitable technique for this purpose, because the whole process is carried out before the power amplifier and immediately after the modulation takes place. This thesis generically analyses the predistortion method of nonlinear distortion reduction, and applies the concepts to implement a practical wideband digital predistorter. Methods for the cancellation of imperfections in the linearizer itself are developed, and its performance evaluated as a function of a variety of system parameters. The cancellation of memory effects in the power amplifier is also considered. Experimental results are shown for various signals, and for traveling wave tubes or solid-state power amplifiers, demonstrating correction bandwidths of up to 100 MHz, and cancellation of distortion products of up to 25 dB. These results are considered state of the art for today's standards. The bandwidth is only limited by the processing speed of the available digital processing devices; wider bandwidths are certainly possible in the near future without changes to the algorithms developed.