An assistant sub-channel to carry a decoding assistant bit, in input bits that are to be encoded, is selected from each of a plurality of non-adjacent segments. Each segment includes a subset of sub-channels with associated reliabilities. Some embodiments also involve grouping the sub-channels into the segments. After a target number of assistant sub-channels have been selected, the input bits are encoded to generate a codeword, and the codeword is transmitted. Assistant sub-channel selection could involve iterations to select assistant sub-channels at each iteration, until at least the target number of assistant sub-channels have been selected.