Optimum digital modulation for nonlinear FDMA uplink and a regenerative satellite on-board processor

A number of linear, digital modulation techniques are evaluated for FDMA access to a regenerative, satellite repeater. The new aspect of the research is in considering a regenerative rather than an amplifying satellite transponder. QPSK, OQPSK, both with Nyquist pulse shaping, and MSK are the modulation techniques considered. A nonlinear, high power amplifier (HPA) is included in the user terminal for the FDMA uplink access to the regenerative satellite repeater. The multi-carrier demodulation algorithm on-board the satellite is based on the transmultiplexer algorithm as all uplink signals are assumed to have a fixed data rate. The performance criteria used in the study are power efficiency, bandwidth efficiency, and the complexity of the transmultiplexer algorithm for on-board processing, all of which vary relative to the modulation technique considered. If the HPA is linear we find that QPSK is the best modulation. When the HPA is nonlinear, however, it is the worst technique and OQPSK is best. The performance analysis is based on a full computer simulation of 8 uplink carriers being processed by the transmultiplexer algorithm. The downlink would be based on TDM access but is not considered in the paper.