Performance Analysis of QAM-MPPM in Turbulence-Free FSO Channels: Accurate Derivations and Practical Approximations

Following the trends of index modulated (IM) techniques applied to optical communications, several new waveform proposals have been made, aiming at conveying a higher density of information by driving different signal properties. One of these proposals mixes multipulse pulse-position modulation (MPPM) and quadrature amplitude modulation (QAM). We present here a new way to demodulate it, and, for the nonturbulent free space optical (FSO) channel, we provide accurate analytical expressions for its error probabilities, both in the case of the traditional and the new detector. We also provide simplified expressions for the estimation of the error probabilities. We show that the new detector offers a gain of some tenths of dB in signal-to-noise ratio (SNR) with respect to the previously defined one, and that our error probability estimations are more accurate than previously published results. To the best of our knowledge, this work is the first to provide simulation results validating the study of the error probability performance of QAM-MPPM.