Abstract
The most probable error events for a long-haul optical communication
system using sequence estimation are characterized, and the performance using
a modulation code is analyzed. The modulation code is restricted to be a constrained
code that enhances the minimum distance between received signal sequences.
We investigate several constrained codes used in combination with 4- and 8-state
Viterbi algorithms and apply them to a standard noncoherent NRZ system. The
improvement using a rate 0.85 code can be as high as 5 dB relative to an uncoded
system operating at the same symbol rate. If the coded and uncoded systems
are constrained to have the same information rate, a more modest 1 dB improvement
can be obtained for specific fiber distances. We also determine the effect
of the code on the number of states of the Viterbi algorithm and the sensitivity
of the sampling point. Using these codes, the tolerance of the receiver to
sampling errors can be increased by a nearly a factor of two for specific
distances. The performance improvements that these codes can provide make
them attractive candidates when used in conjunction with sequence estimation.
© 2009 IEEE
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