  Following the trends of index modulated (IM) techniques for optical
communications, in the last few years 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 multi-pulse pulse-position
modulation (MPPM) and quadrature amplitude modulation (QAM) in a system called
QAM-MPPM. We present here a new way to demodulate its compound waveform, and,
for the non-turbulent 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. Based on these analytical derivations, 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 with respect to the previously defined one without an
increase in complexity, 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.
