PPM demodulation: On approaching fundamental limits of optical communications

Abstract — We consider the problem of demodulating M-ary optical PPM (pulse-position modulation) waveforms, and propose a structured receiver whose mean probability of symbol error is smaller than all known receivers, and approaches the quantum limit. The receiver uses photodetection coupled with optimized phase-coherent optical feedback control and a phase-sensitive parametric amplifier. We present a general framework of optical receivers known as the conditional pulse nulling receiver, and present new results on ultimate limits and achievable regions of spectral versus photon efficiency tradeoffs for the single-spatial-mode pure-loss optical communication channel. I. COMMUNICATIONS UNDER THE POISSON REGIME As per the semi-classical theory of photodetection, the out-put of a photodetector (e.g., a PIN diode) is the superposition of two statistically independent Poisson point processes: on