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How can you distinguish between two quadratures in QAM when the phase is not known at the receiver? For example, if the quadratures are sin and cos, how can the receiver tell which one is which, since both look the same except for the phases?

I've found the idea of random-phase carrier but I'm not sure if this is related to this question, or how it can be used if it is.

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You would need to recover some timing information from the transmitted signal.

A common way is to transmit some sort of preamble before the rest of the data in your frame that is transmitted. The preamble would contain known symbols, so it could be used for multiple purposes, including channel estimation and timing recovery (also known as synchronization).

Once you can recover timing, e.g., from using the received preamble, you can attempt to demodulate your QAM symbols. And besides the phase information that you recover, you also want to avoid any difference in carrier frequency of the signal coming in to the receiver, and the oscillator in the receiver. That's where carrier frequency offset estimation comes into the picture too.

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You need to synchronize phase first, otherwise QAM decision is not possible.

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  • $\begingroup$ Thanks, how is this done if the transmitter and receiver are far from each other? Eg, how does your phone synchronise with the tower? $\endgroup$
    – RM1524
    Commented Jun 7, 2020 at 21:13
  • $\begingroup$ A phone doesn't do QAM on its own (it might do QAM within a multicarrier or spread-spectrum system). Anyway, you always use some synchronization method that fits your application. THere's very many synchronization methods, and which one you use really depends on many factors, so I can't answer that in a comment. $\endgroup$ Commented Jun 7, 2020 at 21:17

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