A geostationary earth orbit satellite requires high transmitting power (e.g., 20 dBW=100 watts) because the waveform traveling distance can be more than 36,000 km. Thus, it is necessary to operate close to the saturation point in an analog high-power amplifier (HPA), thereby causing undesirable intermodulation and nonlinear impairments when multiple-accesses subband signals of different subcarriers are simultaneously transmitted. This paper considers a future digital HPA instead of an analog HPA. This is because today a digital channelizer can convert the sample stream of multiple-access subband user signals into a single sample stream before an HPA with no overlapping in samples, thereby reducing the intermodulation products significantly. In this paper, we study for the first time a digital channelizer for multiple-access user subband signals combined with a nonlinear HPA and a simple predistorter (PD). Both phase and amplitude predistortion can compensate almost perfectly for the distortion due to the HPA's nonlinear characteristics. Simulation results verify an almost negligible bit error rate (BER) degradation. Therefore, a future satellite communication system using the proposed digital channelizer, PD, and digital HPA can have high-frequency utilization efficiency, multicast, and broadcast capabilities, and gain control for each subchannel.