By main points, I mean the ideal constellation points (which can be seen in the transmitter before OFDM modulator using a constellation visualizer). For 4-QAM, these points are 1+j,1-j,-1+j, and -1-j. Now if at the receiver a point was received as -0.5-j1.5, the OFDM demodulator decides it is -1-j and therefore, shows it as -1-j. This is what I meant by “deciding the place of each point based on which of these main points each constellation point is closest to”. Let me know if this explanation helps.
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