Layered LDPC decoding for turbo-differential decoding in presence of cycle slips in optical communications

In optical communication systems using wavelength division multiplexing, the transmission link can no longer be described as an AWGN channel as the co-propagating signals influence each other due to the nonlinearity of the fiber. As a consequence of the resulting phase noise, frequent cycle slips may occur in the carrier phase estimation which can be counteracted by the use of differential modulation. To mitigate random transmission errors, state-of-the-art Low-Density Parity-Check (LDPC) codes are for large coding gains. The differential decoding penalty due to error doubling can be reduced by iterative demodulation and decoding. For AWGN channels, this penalty is almost entirely recoverable. However, cycle slips induce contradictions of the received channel sequence and the a-priori information from the LDPC decoder in the differential demodulation process leading to non-negligible error floors. We show that using so-called layered LDPC decoding, the number of required iterations for iterative demodulation and decoding can be reduced and the sensitivity towards cycle slips can be improved.