The simplest form of QAM is in fact the QPSK symbol set, which can be
viewed as two quadrature amplitude modulated carriers, with amplitude levels of +A and
-A. Increasing the number of amplitude levels on each carrier to 4, for example,
+/-A, +/-3A, gives 16 possible combinations of symbols at the transmitter output,
each equally spaced on the constellation diagram, and each represented by a unique
amplitude and phase.
Pulse shaping is performed by filtering the multi-level baseband input symbol streams in
exactly the same manner as would be used for a binary ASK waveform.
The modulator is again making use of orthogonality of the sine and cosine
carriers to allow independent detection of the two M-ary ASK data streams at the receiver.
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