Downlink Receiver Algorithms for Deep Space Optical Communications

The goal of the Deep Space Optical Communications project at the Jet Propulsion Laboratory is to demonstrate
laser communication links at ranges out to approximately 3 AU. In this paper, we discuss a downlink receiver
concept capable of demodulating optical pulse-position modulated (PPM) waveforms with data rates varying from
approximately 50 kbps up to 265 Mbps, using a range of PPM orders, slot widths, and code rates. The receiver
operates on recorded timestamps corresponding to the times-of-arrival of photons detected by a photon-counting
detector array followed by a commercial time-tagger. Algorithms are presented for slot, symbol, and frame
synchronization as well as parameter estimation. Estimates of link performance are evaluated through Monte-
Carlo simulation for an optical channel that includes optical losses, detector blocking, signal clock dynamics,
and pointing-induced downlink fades. Based upon these simulation results, it is expected that link closure may
be achieved with at least 3 dB of margin under a variety of relevant conditions.