  Protograph-based, off-the-shelf low-density parity-check (LDPC) codes are
optimized for higher-order modulation and quantized sum-product decoders. As an
example, for the recently proposed LDPC code from the upcoming IEEE 802.3ca
standard for passive optical networks (PONs), an optimized mapping of the bit
channels originating from bit-metric decoding to the protograph variable nodes
gains 0.4 dB and 0.3 dB at a bit-error rate of 1e-6 for shaped and uniform
signaling, respectively. Furthermore, the clipping value for a quantized
sum-product LDPC decoder is optimized via discretized density evolution.
