An expression is derived for the symbol error probability (SEP) of coherentM-ary phaseshift-keyed (PSK) signals (M> 4) transmitted through a bandlimited nonlinear channel. The bandpass nonlinearity (BPNL) considered exhibits AM/AM and AM/PM distortions. Additive white Gaussian noise (AWGN) on both up- and downlinks, and intersymbol interference (ISI) on up-link only have been taken as system impairments. SEPs are computed for an onboard TWTA and a phase-compensating receiver. The use of a softlimiter lineariser (SL-LRZ) to compensate the nonlinearities is also presented.