Experimental Demonstration of Simultaneous Compensation of Polarization Mode Dispersion and Fiber Nonlinearities by LDPC-Coded Turbo Equalization

In recent years LDPC codes have gained significant interest in the area of optical communication systems due to their capacity-approaching performance, high coding gain and low decoding-complexity. We describe the construction principles of high-rate, high-girth, quasi-cyclic LDPC codes and present an LDPC-coded turbo equalization scheme suitable for simultaneous mitigation of multiple transmission impairments. The equalization scheme is based on the maximum a posteriori probability detection, based on Bahl Cocke Jelinek Raviv (BCJR) algorithm that employs the conditional density probability functions of the channel to calculate the initial log-likelihood ratios for the LDPC decoder. To optimize the code performance extrinsic information transfer charts are used. We then investigate and evaluate the performance of the proposed scheme in the presence of polarization mode dispersion (PMD), fiber nonlinearities and chromatic dispersion for 10 Giga symbols/s transmission system and various modulation formats including NRZ and polarization-multiplexed BPSK with both direct and coherent detection. LDPC codes of rates 0.8, 0.9 and 0.95 are evaluated. Experiments with and without chromatic dispersion compensation are conducted.