Modulation
As was discussed in “Modulation” in Chapter 1, modulation can be done by changing the amplitude, phase or frequency of transmitted Radio Frequency (RF) signal. For an OFDM system, the first two methods can be used. Frequency modulation cannot be used because subcarriers are orthogonal in frequency and carry independent information. Modulating subcarrier frequencies would destroy the orthogonality property of the subcarriers; this makes frequency modulation unusable for OFDM systems.
The main design issue of the modulator is the constellation used. Constellation is the set of points that can be transmitted on a single symbol. The used constellation affects several important properties of a communication system; for example, bit error ...
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