Last Updated: August 16, 2025
2 min read
| Items | HD-FEC | SD-FEC |
|---|---|---|
| Definition | Decoding based on hard-bits(the output is quantized only to two levels) is called the “HD(hard-decision) decoding”, where each bit is considered definitely one or zero. | Decoding based on soft-bits(the output is quantized to more than two levels) is called the “SD(soft-decision) decoding”, where not only one or zero decision but also confidence information for the decision are provided. |
| Application | Generally for non-coherent detection optical systems, e.g., 10 Gbit/s, 40 Gbit/s, also for some coherent detection optical systems with higher OSNR | coherent detection optical systems, e.g., 100 Gbit/s,400 Gbit/s. |
| Electronics Requirement | ADC(Analogue-to-Digital Converter) is not necessary in the receiver. | ADC is required in the receiver to provide soft information, e.g., coherent detection optical systems. |
| specification | general FEC per [ITU-T G.975];super FEC per [ITU-T G.975.1]. | vendor specific |
| typical scheme | Concatenated RS/BCH | LDPC(Low density parity check),TPC(Turbo product code) |
| complexity | medium | high |
| redundancy ratio | generally 7% | around 20% |
| NCG | about 5.6 dB for general FEC;>8.0 dB for super FEC. | >10.0 dB |
| Example(If you asked your friend about traffic jam status on roads and he replies) | maybe fully jammed or free | 50-50 but I found othe way free or less traffic |
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