Laboratory simulation results of 120 Mbit/s QPSK nonlinear channel modems

This paper reports on the performance of a 120-Mbit/s QPSK modem in an INTELSAT V type band-limited satellite channel with two nonlinear elements (earth station HPA and transponder TWTA) in tandem. The laboratory simulation has been performed in a transponder simulator with a single input multiplexing filter. Some preliminary data have also been included, indicating the performance of the normal configuration of an actual INTELSAT V transponder that includes both input and output multiplexing filters. Initial results of this work confirm previous pulse shaping filter design optimization studies for the nonlinear channel. They also reveal an interesting link equalization phenomenon and approach that was required to achieve optimum bit error rate (BER).