Combined constrained code and LDPC code for long-haul fiber-optic communication systems

In long-haul fiber-optic communication systems, the system performance is affected adversely by both severe physical impairments and amplified spontaneous emission (ASE) noise. Constrained coding, which avoids waveforms in the transmitted signal that are more likely to be detected incorrectly, has been proved to be an effective approach to suppress some physical impairments. However, the constrained coding schemes proposed in the literature are limited to the suppression of only some "resonant" sequences and their performance is evaluated in the absence of ASE noise. Various error correction codes also have been developed to reduce errors due to ASE noise but their performance is very vulnerable to the strong nonlinear impairments. This paper develops a novel concatenation scheme with the inner code being a constrained code based on Total Impairment Extent Rank (TIER) and the outer code being a low-density parity-check (LDPC) code. The TIER code ranks the bit patterns by order of all physical impairments imposed on them and constrains the bit patterns with large physical impairments. It is based on a discrete-time analytical model of physical impairments in long-haul fiber-optic communication systems. Compared with the current constrained codes, the TIER code offers more flexibility and better effectiveness. The TIER-LDPC concatenation scheme combines the strength of the TIER code in correcting errors due to physical impairments and that of the LDPC code in correcting memoryless errors due to ASE noise. Simulation results in the presence of ASE noise show that the TIER-LDPC concatenation scheme performs significantly better than the LDPC code alone in the case of severe impairments.