Complexity reduced turbo differential decoding based on layered LDPC decoding

In case of unknown carrier phase and/or imperfect channel estimation in communication systems using QPSK it is well known that phase and channel uncertainty can be resolved by differential modulation at the expense of doubling the bit error ratio. Such a differentially modulated QPSK approach has to be applied in wavelength division multiplexing (WDM) fiber optical communication systems using coherent transmission due to the phase ambiguity of carrier phase estimation (CPE) and potential cycle slips. However, this penalty can be diminished by using an iterative decoding procedure which passes information between the differential decoder and the decoder of a low-density parity-check (LDPC) code, a powerful state-of-the art forward error correction (FEC) code. In addition, it is shown that by applying a layered decoding schedule in the LDPC decoder, the number of required iterations can be reduced by up to 50%.