  We investigate theoretically the efficiency of deep-space optical
communication in the presence of background noise. With decreasing average
signal power spectral density, a scaling gap opens up between optimized
simple-decoded pulse position modulation and generalized on-off keying with
direct detection. The scaling of the latter follows the quantum mechanical
capacity of an optical channel with additive Gaussian noise. Efficient
communication is found to require a highly imbalanced distribution of
instantaneous signal power. This condition can be alleviated through the use of
structured receivers which exploit optical interference over multiple time bins
to concentrate the signal power before the detection stage.
