Universal Decoder for Variable Duty-Cycle Optical Communications

Efficient use of a deep-space optical communications channel requires changes to the duty cycle of the modulation as the signal and noise powers change. This can be facilitated by modulating the signal with pulse-position-modulation (PPM), supporting multiple PPM orders. To implement iterative demodulation, which is required by certain error-correction-codes to obtain near-capacity performance, would nominally require a distinct hardware implementation for each PPM order. In this paper we describe a method to utilize a single hardware implementation of an iterative demodulator for any PPM order. The method may be applied to any coded modulation that utilizes iterative demodulation and maps to multiple modulation orders.