On the effect of uplink noise on a nonlinear digital satellite channel

The effect of uplink noise in a 60 Mb/s QPSK system operating over a 40 MHz bandlimited nonlinear channel was measured on a satellite simulator, and computer simulated using a modified Monte-Carlo simulation technique. Simulations and measurements show that uplink and downlink noise have the same effect on the bit error rate if the uplink noise and the signal occupy the same bandwidth at the input to the nonlinearity. This result holds for both hard-limited and soft-limited channels. If the QPSK signal is unfiltered before the nonlinearity, then the uplink noise is suppresed by as much as 3 dB.