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Doesn't symbols in QAM modulation add destructively as they are symmetric about the origin (equal amplitude and opposite phase) about the I axis?

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  • \$\begingroup\$ What do you mean? Simplest QAM constellation you can think is 2 bits for 4 symbols, all are sine waves at same amplitude, but 4 different phases. \$\endgroup\$
    – Justme
    Feb 2 at 6:55
  • \$\begingroup\$ Sure... Please provide the context where adding in this way is used. I can't imagine a reason to do this. \$\endgroup\$
    – RussellH
    Feb 2 at 9:45
  • \$\begingroup\$ The symbols in constellation mathemathically sums like how vectors are summing. \$\endgroup\$ Feb 2 at 20:41

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Yes if you happen to have a circuit which combines 2 different modulated data signals by adding them together. The result can sometimes be 0 volts longer than only just at the zero crossing and that situation lasts as long as the signals have same amplitudes and opposite phases. To make it happen the signals must have the same carrier frequency and data symbol rate and they must be in sync with each other.

It's not clear at least for me why one wants to build a circuit which adds together 2 modulated data signals which jam each other. It's not uncommon to add together signals which have different frequencies. It happens all the time and nearly everywhere in the air.

Radiowaves in the air do not affect each other. In a case where 2 radiowaves in the same place have the same frequency the cancellation has a possibility to happen in antennas.

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  • \$\begingroup\$ In OFDM system, when multiple carriers are multiplexed and when there is ISI, then would the symbols destructively add in time domain to cancel out each other?. \$\endgroup\$ Feb 2 at 10:21
  • \$\begingroup\$ @AnanthTenappan no need to cancel. ISI (intersymbol interference) causes harm as soon as the signal changes from perfect just as much as it's needed to make a wrong decision "of what symbol it might be". And that amount can be much less than total cancel during a symbol. \$\endgroup\$
    – oneprivate
    Feb 2 at 18:35

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