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I'm reading about amplitude shift keying, where 2^n assigned amplitudes of a carrier map to n bits per symbol. Unfortunately, most of the introductory material only covers OOK, where n=1 and decoding is trivial.

For an ASK system where n > 1, i.e. a system where the amplitude can be modulated by multiple levels and not simply on/off, how would a demodulator account for attenuation? In other words, how would a demodulator account for the ambiguity present where a low level could actually be a low level, or could be a high level that's simply a weak signal? Would one need to have heuristics, or maybe an occasional checksum that would only match if the inferred attenuation is correct?

Reinderien
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  • It doesn't, inherently. A specific ASK system might, using any of your suggested techniques or others including AGC, adapted to its specific requirements (e.g. fading or multipath on the channel). Designing such a system in response to a vague question would be too broad for an answer. –  Jan 02 '18 at 16:30

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The reason you state is why ASK with more than two levels is not often used except in controlled fixed installations.

One common way is to enforce that the maximum amplitude is sent regularly. There are various ways to enforce this, like a fixed bit every N bits, bit stuffing, etc. Some of these means can also provide some out of band information, like framing of bytes, packets, etc.

The receiver keeps track of the maximum amplitude recently received, and interprets the current received amplitude relative to that.

Olin Lathrop
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