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QAM is a modulation technique where amplitude modulation is performed for two signals, one sine signal and another cosine signal. Since the signals are out of phase, the information in these signals is independent and can be fully recovered.

However, recovering the signals requires knowing the phase of the sender. If the phase is known incorrectly, it is for the receiver possible that the sine signal will be treated as cosine signal and the cosine signal will be treated as minus sine signal. Or worse, the signals can be mixed up.

How do practical implementations solve this problem and have the same phase in the sender and in the receiver?

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One method: -

You have to transmit something in the data that can be unambiguously recognised as a sync. You look to decode this in the receiver then you have acquired sync. It doesn't need to be transmitted very often of course but, until you receive it, you are out of sync. The payload data transmitted must be configured so that it doesn't accidentally transmit the sync.

Another method transmits a marker bit that is different to regular data. This of course means establishing what the relatively slow moving but ever changing ebb and flow of amplitude information is, and looking for a bit that has an amplitude that is clearly different to regular bit amplitudes.

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  • \$\begingroup\$ It can be compared to colour encoding on analog TV for NTSC or PAL : The colorburst pulse ensure phase alignment of the chrominance subcarrier : en.wikipedia.org/wiki/Colorburst \$\endgroup\$
    – TEMLIB
    Mar 12 '17 at 19:44
  • \$\begingroup\$ @TEMLIB yeah good comparison. \$\endgroup\$
    – Andy aka
    Mar 12 '17 at 20:18

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