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Generally, when using channel encoding followed by interleaving, that process should be performed before the modulator and the system structure should be like that :

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For me, I only use the conventional encoder for channel coding and then interleaving, but I am wondering, is it possible to use the channel encoder after the digital modulation (I use for example QPSK) ?? .. however the signal after the digital modulation is not binary anymore, but I don't know if that still possible to use the channel encoder and interleaver after digital modulation or not.

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The role of the modulator in your block diagram is to make an analog signal, while in classical communications, the data and the channel coder is discrete. So no, in the sense of your block diagram, that can't work.

Note that you stated trouble because your signal wasn't binary in your approach: Not all channel coders work on binary symbols. For example, Reed-Solomon Codes are used exactly because they work on symbols from a larger field than the binary numbers. (That brings advantages when dealing with burst errors.)

Non-binary LDPC codes exist, and there's other codes, too, that don't use binary numbers. Generally, very little in our modern communications is "forced" to be binary, it just happens so that $\mathbb F^2$ is a very easy-to-deal-with finite field.
From the top of my head, I can't think of a single channel coder type which couldn't exist as a base-3 instead of base-2 algorithm, for example. (Building a good and performant channel decoder might be hard, though.)

TL;DR:

Your modulator needs to stay between your digital processing, which includes the channel coding, and the analog channel, but the notion that "digital == binary" is plain wrong.

Wth the very widely used Reed-Solomon codes you have an excellent counterexample to that, and you might want to look into these codes.

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  • $\begingroup$ Thank you for you quick answer. So, If I want to use conventional coding, I must use it before the modulator. Unless, I can use Reed-Solomon codes (or any other non-binary codes) to be able to use it after the modulator. Is what I understood right? $NP$: Modulator, I mean digital modulation (for me I use QPSK). I will modify it in the question. $\endgroup$
    – Fatima_Ali
    May 11, 2020 at 14:00
  • $\begingroup$ NO. you didn't read closely. Your modulator is always the last thing before the channel. full stop. $\endgroup$
    – Marcus Müller
    May 11, 2020 at 14:31
  • $\begingroup$ what you mean is a "symbol mapper". and sure, you can remap your data to symbols as much as you like. $\endgroup$
    – Marcus Müller
    May 11, 2020 at 14:32

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