A satellite communications system is disclosed, the system comprising a ground controller, a decode-and-forward (DF) satellite transponder, and a digital high-power amplifier (HPA). The DF is further comprised of a transponder front end including a digital channelizer and a predistorter. The ground controller is configured to transmit multiple-access frequency-division-multiplexed signals to the DF, where the DF is configured to down-convert the signals received at the transponder front end. The digital channelizer is configured to convert the signals received into a single sample steam and feed the stream through the predistorter into the single digital HPA, wherein the HPA is configured to amplify each input sample of the stream in sample-by-sample fashion and generates a discrete output. The DF is further configured to convert the output of the digital HPA to a continuous time signal and up-convert the output with a main carrier frequency for down-link transmission to multiple-access ground users.