96 research outputs found
Optimized Bit Mappings for Spatially Coupled LDPC Codes over Parallel Binary Erasure Channels
In many practical communication systems, one binary encoder/decoder pair is
used to communicate over a set of parallel channels. Examples of this setup
include multi-carrier transmission, rate-compatible puncturing of turbo-like
codes, and bit-interleaved coded modulation (BICM). A bit mapper is commonly
employed to determine how the coded bits are allocated to the channels. In this
paper, we study spatially coupled low-density parity check codes over parallel
channels and optimize the bit mapper using BICM as the driving example. For
simplicity, the parallel bit channels that arise in BICM are replaced by
independent binary erasure channels (BECs). For two parallel BECs modeled
according to a 4-PAM constellation labeled by the binary reflected Gray code,
the optimization results show that the decoding threshold can be improved over
a uniform random bit mapper, or, alternatively, the spatial chain length of the
code can be reduced for a given gap to capacity. It is also shown that for
rate-loss free, circular (tail-biting) ensembles, a decoding wave effect can be
initiated using only an optimized bit mapper
Performance Prediction of Nonbinary Forward Error Correction in Optical Transmission Experiments
In this paper, we compare different metrics to predict the error rate of
optical systems based on nonbinary forward error correction (FEC). It is shown
that the correct metric to predict the performance of coded modulation based on
nonbinary FEC is the mutual information. The accuracy of the prediction is
verified in a detailed example with multiple constellation formats, FEC
overheads in both simulations and optical transmission experiments over a
recirculating loop. It is shown that the employed FEC codes must be universal
if performance prediction based on thresholds is used. A tutorial introduction
into the computation of the threshold from optical transmission measurements is
also given.Comment: submitted to IEEE/OSA Journal of Lightwave Technolog
Terminated and Tailbiting Spatially-Coupled Codes with Optimized Bit Mappings for Spectrally Efficient Fiber-Optical Systems
We study the design of spectrally efficient fiber-optical communication
systems based on different spatially coupled (SC) forward error correction
(FEC) schemes. In particular, we optimize the allocation of the coded bits from
the FEC encoder to the modulation bits of the signal constellation. Two SC code
classes are considered. The codes in the first class are protograph-based
low-density parity-check (LDPC) codes which are decoded using iterative
soft-decision decoding. The codes in the second class are generalized LDPC
codes which are decoded using iterative hard-decision decoding. For both code
classes, the bit allocation is optimized for the terminated and tailbiting SC
cases based on a density evolution analysis. An optimized bit allocation can
significantly improve the performance of tailbiting SC codes codes over the
baseline sequential allocation, up to the point where they have a comparable
gap to capacity as their terminated counterparts, at a lower FEC overhead. For
the considered terminated SC codes, the optimization only results in marginal
performance improvements, suggesting that in this case a sequential allocation
is close to optimal.Comment: This paper has been accepted for publication in the IEEE/OSA Journal
of Lightwave Technolog
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