Distortion Mitigation in RF Power Amplifiers Through FPGA-Based Amplitude and Phase Predistortion

Strong requirements are currently imposed to limit the negative effects of distortion or bad synchronization between emitter and receiver in digital communication channels. Distortion is mainly caused by the output stage of the emitter´s power amplifier. It can be mitigated by applying to the input signals a correction, causing them a distortion contrary to that occurring in the amplifier (i.e., a predistortion). This paper presents a new method which combines amplitude and phase predistortion for digital video broadcasting-terrestrial (DVB-T) gap fillers (intermediate reemitters) in single frequency networks. These systems impose severe limitations to the time available for processing, which prevents the application of existing predistortion techniques. The method is based on a new high-frequency model of the output stage of the emitter´s power amplifier, from which the design of an efficient amplitude and phase predistorter is straightforward. An advantageous tradeoff between complexity and processing time is achieved by applying predistortion at intermediate frequency. In addition, the proposed solution is independent of the frequency of the output radio frequency signals. A predistorter has been implemented in a field-programmable gate array and applied to a real gap filler. Experimental results which show the significant improvement achieved in the quality of real DVB-T signals, as well as the low complexity and low processing delay associated to the proposed approach, are presented.