The effects of nonlinear distrotion on the error performance of a 16-QAM signalling scheme, operating in a typical satellite channel, are investigated by means of a computer simulation program. Additive white Gaussian noise is present on both input and output of the TWT amplifier. The results obtained are in good agreement with those obtained theoretically in other works, and show that the noise presence on the up link causes severe system performance degradation. Moreover, the behaviour of a nonlinear system in the presence of up-link cochannel interference is examined. The results clearly show that the symbol error probability depends strongly on the symbol timing difference between the desired and the interfering signal, as well as on the phase difference of their carriers especially for low signal to interference ratios.