Differential pulse-position modulation for power-efficient optical communication

In this paper, we examine the use of differential pulse-position modulation (DPPM) for optical communication systems using intensity modulation with direct detection in the presence of additive white Gaussian noise. We present expressions for the error probability and power spectral density of DPPM. We show that for a given bandwidth, DPPM requires significantly less average power than pulse-position modulation (PPM). We also examine the performance of DPPM in the presence of multipath intersymbol interference (ISI). We find that the ISI penalties incurred by PPM and DPPM exhibit very similar dependencies upon the channel rms delay spread. We discuss the use of chiprate and multichip-rate equalization to combat ISI. Finally, we describe potential problems caused by the nonuniform bit-rate characteristic of DPPM, and we propose several solutions.