## Abstract

We propose a rate-adaptive optical transmission scheme using variable-size
constellations at a fixed symbol rate and variable-rate forward error correction
(FEC) codes with soft-decision decoding (SDD), quantifying how achievable bit
rates vary with transmission distance. The scheme uses outer Reed–Solomon
codes and inner extended irregular repeat-accumulate low-density parity-check
(LDPC) codes to vary the code rate, combined with single-carrier
polarization-multiplexed *M*-ary quadrature amplitude modulation
with variable *M* and digital coherent detection. LDPC codes are
decoded iteratively using belief propagation. Employing
$M=4,8,16$, the scheme achieves a maximum bit rate of
200 Gbit/s in a nominal 50-GHz channel bandwidth. A rate adaptation
algorithm uses the signal-to-noise ratio (SNR) or the FEC decoder input
bit-error ratio (BER) estimated by a receiver to determine the FEC code rate and
constellation size that maximize the information bit rate while yielding a
target FEC decoder output BER and a specified SNR margin. We simulate
single-channel transmission through long-haul fiber systems with or without
inline chromatic dispersion compensation, incorporating numerous optical
switches, evaluating the impact of fiber nonlinearity and bandwidth narrowing.
With zero SNR margin, we achieve bit rates of 200/100/50/20 Gbit/s over
distances of 960/2800/4400/9680 km and 1920/4960/8160/19,360 km in
dispersion-compensated and -uncompensated systems, respectively, corresponding
to an increase of about 50% in reach compared to a reference system that
uses a hard-decision FEC scheme. Compared to an ideal coding scheme, the
proposed scheme exhibits a performance gap ranging from about 4.0 dB at
960 km to 2.7 dB at 9680 km in compensated systems, and
from about 3.9 dB at 1920 km to 2.9 dB at 19,360 km
in uncompensated systems. Observed performance gaps are about 2.5 dB
smaller than for the reference hard-decision FEC scheme, close to the
improvement expected when using SDD.

© 2012 OSA

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