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HD-FEC & SD-FEC differences

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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