In one embodiment, an apparatus is configured to perform mapping an interleaved bit stream into a sequence of 4-level pulse amplitude modulation (PAM4) labels according to a mapping scheme, with a respective PAM4 label including a least significant bit (LSB) and a most significant bit (MSB) respectively corresponding to a bit in the interleaved bit stream; transmitting a sequence of PAM4 symbols generated based on the sequence of PAM4 labels to the ONUs; and wherein, bits from respective one of the N codewords are assigned to LSBs of a first subset of labels, and to MSBs of a second subset of labels, with the first and the second subset of labels being determined based on the interleaving scheme and the mapping scheme and respectively comprising labels located N labels apart in the sequence of PAM4 labels.