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I am trying to understand the OFDM system. If i am using 16 QAM with 16 subcarriers that means one constellation point mapped to one subcarrier. What will happen to mapping of constellation point to subcarrier when number of subcarrier goes to 8 or 32? Size of QAM will same as 16-QAM.

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The number of subcarriers and the constellation used can be selected independently. Each subcarrier uses the full constellation.

Consider the case of one subcarrier and 16-QAM. The subcarrier takes one of 16 possible quadrature amplitudes, and transmits four bits per symbol. Now extend it to two subcarriers: each of them takes one of 16 amplitudes, independently. Then, one OFDM symbol transmits eight bits (four per subcarrier).

This process can be extended to any number of subcarriers.

UPDATE Here's an example based on a comment below.

The setup is 8 subcarriers with 16-QAM. Then, an OFDM symbol carries $8 \times 4 = 32$ bits. The transmitter then takes 32 bits from the data source, divides them into eight blocks of four bits, and assigns a 16-QAM symbol to each block. This results in eight 16-QAM symbols. Each of these is transmitted by one of the subcarriers.

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  • $\begingroup$ Thanks you so much $\endgroup$ Jun 21, 2022 at 9:21
  • $\begingroup$ Let for modulation we are using 16 QAM i.e. 4 bit per symbol on each subcarrier. now consider if number of subcarriers are 8, then at a perticular time only 8 symbols can load on 8 subcarriers. What about remaining 8 symbols? Are these remaining 8 symbol in buffer and waiting for next time slot? $\endgroup$ Jun 23, 2022 at 4:18
  • $\begingroup$ @PRAVEENYADAV I've updated my answer, please take a look. I hope this example clarifies things for you. $\endgroup$
    – MBaz
    Jun 23, 2022 at 13:22

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