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HD-FEC & SD-FEC differences

DefinitionDecoding 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.
ApplicationGenerally 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 RequirementADC(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.
specificationgeneral FEC per [ITU-T G.975];super FEC per [ITU-T G.975.1].vendor specific
typical schemeConcatenated RS/BCHLDPC(Low density parity check),TPC(Turbo product code)
redundancy ratiogenerally 7%around 20%
NCGabout 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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