There's another major factor. I suppose it falls under the encoding. Frequency stability. In a certain sense, having an extremely precise oscillator at both the receiver and the transmitter, is the same thing as just having a better more frequency-stable antenna, or less noise in the channel (because you know what the signal you're listening for should look like).

I'm no physicist here so take this with a major grain of salt. I think the limit might ultimately arise from the uncertainty principle? Eventually the signal becomes so weak that measuring it, overwhelms the signal. This is why the receiver of space telescopes is cooled down with liquid helium. The thermally-generated background RF noise (black bodies radiate right down into the radio spectrum) would drown everything else out otherwise.

Along those lines, while I'm still not quite sure where the limit is, things become discrete at the micro level, and the smallest possible physical state change appears to be discrete in nature: https://en.wikipedia.org/wiki/Landauer%27s_principle Enough work physically must occur to induce a state change of some kind at the receiver, or no communication can occur. (But this interpretation is disputed!)