The sensitivity of a high-rate photon-counting optical communications link depends on the performance of the photon counter used to detect the optical signal. In this paper, we focus on ways to reduce the effect of blocking, which is loss due to time periods in which the photon counter is inactive following a preceding detection event. This blocking loss can be reduced by using an array of photon counting detectors or by using photon counters with a shorter inactive period. Both of these techniques for reducing the blocking loss can be employed by using a multi-element superconducting nanowire single-photon detector. Two-element superconducting nanowire single-photon detectors are used to demonstrate error-free photon counting optical communication at data rates of 781 Mbit/s and 1.25 Gbit/s.
Free-space laser communications systems experience fading due to quasi-static pointing error and tracking error
that impairs communications performance. Under a block fading model we show that using forward error
correcting codes and interleaving - a simple non-mechanical data processing technique - much of the harmful
effect of tracking error on communications performance can be removed. Using the concepts of fading capacity and
outage capacity, we provide analytical tools that quantify the effects of fading on communications performance
with and without interleaving. A link budget based on this analysis includes three loss terms due to pointing
and tracking error: 1) static loss, which is primarily a function of pointing error; 2) capacity loss, which is the
power difference between fading capacity and fade-free capacity; and 3) finite interleaver loss, which is the power
difference between the probability of outage curve and fading capacity. Assuming pulse-position modulation and
a Poisson channel, we derive closed-form solutions for the probability of outage of interleaved systems using a
Gaussian beam with circularly symmetric Gaussian tracking error.
We develop a coding method called binary shaping that generates
a binary channel input of arbitrary duty-cycle with Bernoulli
statistics, which is useful for laser communications using generalized
on-off keying. We couple binary shaping with a serially concatenated
turbo code for forward error correction, and show by simulation that
the performance of this scheme with iterative decoding approaches the
capacity of generalized on-off keying on the Poisson channel for
medium duty cycles, significantly exceeding the performance of
pulse-position modulation (PPM) in this regime. Analogous to trellis
shaping for the AWGN channel, binary trellis shaping utilizes a
convolutional shaping code to which we apply the Viterbi algorithm at
the encoder to minimize Hamming weight. We show how low-duty-cycle
communications is a special case of information embedding and discuss
how binary shaping relates to information embedding methods.
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