A circuit and method form a codeword including parity and message bits, as follows. Each codeword has a first part in a current sequence (e.g. a current OTN-row) that is to be now transmitted and second part spread across multiple past sequences (e.g. previously prepared and transmitted OTN-rows). The codewords are grouped into multiple groups such that each codeword within a group has no bit in common with another codeword in that group. Moreover, each codeword has a bit in common with a different codeword in a different group.