Abstract
We propose a rate-adaptive transmission scheme using variable-rate forward error
correction (FEC) codes with a fixed signal constellation and a fixed symbol rate,
quantifying how achievable bit rates vary with distance in a long-haul fiber system. The
FEC scheme uses serially concatenated Reed–Solomon (RS) codes with hard-decision
decoding, using shortening and puncturing to vary the code rate. An inner repetition
code with soft combining provides further rate variation. While suboptimal, repetition
coding allows operation at very low signal-to-noise ratio (SNR) with minimal increase in
complexity. A rate adaptation algorithm uses the SNR or the FEC decoder input bit-error
ratio (BER) estimated by a receiver to determine the combination of RS-RS and repetition
codes that maximizes the information bit rate while satisfying a target FEC decoder
output BER and providing a specified SNR margin. This FEC scheme is combined here with
single-carrier polarization-multiplexed quadrature phase-shift keying (PM-QPSK) and
digital coherent detection, achieving 100-Gbit/s peak information bit rate in a nominal
50-GHz channel bandwidth. We simulate variable-rate single-channel transmission through
a long-haul system incorporating numerous optical switches, evaluating the impact of
fiber nonlinearity and bandwidth narrowing. With zero SNR margin, achievable information
bit rates vary from 100 Gbit/s at 2000 km, to about 60 Gbit/s at 3000 km, to about 35
Gbit/s at 4000 km. Compared to an ideal coding scheme achieving information-theoretic
limits on an AWGN channel, the proposed coding scheme exhibits a performance gap ranging
from about 5.9 dB at 2000 km to about 7.5 dB at 5000 km. Much of the increase in the gap
arises from the inefficiency of the repetition coding used beyond 3280 km. Rate-adaptive
transmission can extend reach when regeneration sites are not available, helping
networks adapt to changing traffic demands. It is likely to become more important with
the continued evolution toward optically switched mesh networks, which make signal
quality more variable.
© 2010 IEEE
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