Abstract
We describe a method to estimate the capacity limit of fiber-optic communication systems
(or “fiber channels”) based on information theory. This paper is divided
into two parts. Part 1 reviews fundamental concepts of digital communications and information
theory. We treat digitization and modulation followed by information theory for channels both
without and with memory. We provide explicit relationships between the commonly used
signal-to-noise ratio and the optical signal-to-noise ratio. We further evaluate the
performance of modulation constellations such as quadrature-amplitude modulation, combinations
of amplitude-shift keying and phase-shift keying, exotic constellations, and concentric rings
for an additive white Gaussian noise channel using coherent detection. Part 2 is devoted
specifically to the ``fiber channel.' We review the physical phenomena present in
transmission over optical fiber networks, including sources of noise, the need for optical
filtering in optically-routed networks, and, most critically, the presence of fiber Kerr
nonlinearity. We describe various transmission scenarios and impairment mitigation techniques,
and define a fiber channel deemed to be the most relevant for communication over
optically-routed networks. We proceed to evaluate a capacity limit estimate for this fiber
channel using ring constellations. Several scenarios are considered, including uniform and
optimized ring constellations, different fiber dispersion maps, and varying transmission
distances. We further present evidences that point to the physical origin of the fiber
capacity limitations and provide a comparison of recent record experiments with our capacity
limit estimation.
© 2010 IEEE
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