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The error performance of 16-QAM schemes operating in nonlinear satellite channels where additive white Gaussian noise (AWGN) appears on the down link has already been evaluated. However, it is well known that in realistic nonlinear satellite transmission systems, additive channel noise appears on both the up- and the downlink; and indeed it is mostly the presence of uplink Gaussian channel noise that creates severe system performance degradation. A theoretical analysis is carried out by formulating upper and lower bounds of the 16-QAM system error performance, considering that Gaussian channel noise appears at both the input (uplink) and the output (downlink) of the space bandpass nonlinearity (BPNL). This creates AM/AM distortion and AM/PM conversion products, whose origin may be demonstrated by quadrature modelled nonlinear processing of the signal and the uplink Gaussian noise. The analysis described offers a technique for evaluating M-ary QAM system performance appraisals in a complex nonlinear satellite environment.