Could you please tell me what would be the benefit of using Raw BER (or uncoded BER) in comparison to BER?
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Well, that depends on what you want to do with things. You typically see uncoded BER in observations of the behaviour of the physical layer transmission (because there, it is a good benchmark for how well the system behaves), where as post-decoder BER is relevant if you care about the data transmission end-to-end.
Obviously, if you want to know how well your channel decoder works, you'd need to know both: the BER of (soft)bits coming out of the (symbol) decision process (going into the decoder), and of bits coming out of the decoder.
The hard facts being said, maybe something to broaden your horizon:
Note that this logical cut between the bits you get out of deciding which symbol you received, and the bits you get after applying your channel decoder, if you look at it from a theoretical perspective, is a rather arbitrary cut:
Ungerböck & co will tell you that it's sensible and feasible to consider "FEC coding and mapping to physical layer symbols" as a single step, "encoding" (and that's why that school of though unifies constellations and what would classically be channel codes in one logical hierarchy). The keyword you'd be looking for would be Bit-Coded Modulation. Specifically Bit-Interleaved Coded Modulation is a practical scheme which can be used to implement this basic idea:
The complete process of mapping bits to waveforms should be considered the encoding step in FEC, and mapping the waveform back to bits should be considered decoding, and that the symbol decider shouldn't be separate from the information flow in the decoder.
You'll find a couple of papers on BICM applied to 5G systems as well, if 5G FEC research is your thing.