  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.
