Mitigation of linear and nonlinear impairments in high-speed optical networks by using LDPC-coded turbo equalization

We study a turbo equalization scheme based on low-density parity-check (LDPC) coded turbo equalization (TE). This scheme is suitable for simultaneous: (i) suppression of intra-channel nonlinearities, (ii) chromatic dispersion compensation, and (iii) polarization-mode dispersion (PMD) compensation. LDPC coding is based on large girth (g ges 8) block-circulant codes, and maximum a posteriori probability (MAP) equalizer is based on Bahl-Cocke-Jelinek-Raviv (BJCR) algorithm. The ultimate channel capacity limits, assuming an independent identically distributed (i.i.d.) source are reported as well. In the presence of intrachannel nonlinearities the LDPC-coded TE provides almost 12 dB improvement over BCJR equalizer at BER of 10^-8. For an NRZ system operating at 10 Gb/s with residual dispersion of 11200 ps/nm and for differential group delay of 50 ps, the LDPC-coded TE is only 1 dB away from the i.i.d channel capacity. The efficiency of LDPC-coded TE in PMD compensation is demonstrated experimentally, with decoding performed off line.