We identify theoretical limits on the photon information efficiency (PIE) of a deep-space optical communication link constrained by the average signal power and operated in the presence of background noise. The ability to implement a scalable modulation format, Geiger-type direct photon counting detection, and complete decoding of detection events are assumed in the analysis. The maximum attainable PIE is effectively determined by the background noise strength and it exhibits a weak, logarithmic dependence on the detected number of background photons per temporal slot.
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