Experimental study of non-binary LDPC coding for long-haul coherent optical QPSK transmissions

The performance of rate-0.8 4-ary LDPC code has been studied in a 50GHz-spaced 40Gb/s DWDM system with PDM-QPSK modulation. The net effective coding gain of 10dB is obtained at BER of 10−6. With the aid of time-interleaving polarization multiplexing and MAP detection, 10,560km transmission over legacy dispersion managed fiber is achieved without any countable errors. The proposed nonbinary quasi-cyclic LDPC code achieves an uncoded BER threshold at 4×10−2. Potential issues like phase ambiguity and coding length are also discussed when implementing LDPC in current coherent optical systems.