According to example embodiments, the OLT includes at least one processor and at least one memory including computer program code, the at least one memory and computer program code configured to, with the at least one processor, cause the OLT to perform grouping data units for the ONUs in groups, wherein a group includes data units for a subset of the ONUs, encoding the groups of data units into respective sets of at least one codeword, interleaving the at least one codeword of the respective sets thereby obtaining a stream of codewords, and sequentially transmitting the stream of codewords to the ONUs.