HD- FEC stands for Hard Decision Forward Error Correction. It is the classic forward error correction (FEC) with the two methods Generic FEC (G-FEC) and Enhanced FEC (E-FEC). Another method is Soft Decision FEC( SD-FEC), which is used in optical transmission technology.
The difference between HD-FEC and SD-FEC lies in their algorithms and the number of bits used for decoding. HD decoders make a decision between 0 and 1. The output knows only two states, 0 and 1. It is different with Soft Decision FEC (SD-FEC), whose algorithm receives additional information relevant to the decision and distinguishes between several different output levels. As a result, the SD-FEC technique has a higher net coding gain (NCG) and a higher quality factor, which is reflected in larger bridgeable repeater-less distances. Only the header of SD-FEC is more extensive than that of the HD-FEC method.