In n-ary PPM, a single high energy pulse is used to convey M bits of information. In this paper we propose that advantage be taken of the fiber non-linearity such that the n-ary PPM pulse becomes a temporal soliton. We examine the maximum achievable bit rate of such a system and compare it to that of PCM. We show that under ideal conditions (no fluctuations of the input soliton pulse width) n-ary PPM can be used to achieve a higher bit-rate than PCM. However, in a practical situation fluctuations of the input soliton pulse width are translated to variations in the soliton pulse position. We show that this has a severe effect on the error probability of n-ary PPM and that in this situation PCM offers greater immunity. Finally, we consider the implications of the Gordon-Haus effect on the maximum transmission distance of n-ary PPM and show that it is less than that for PCM due to the narrower pulse arrival time window.