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= Quadrature Amplitude Modulation =
Quadrature Amplitude Modulation
Quadrature Amplitude Modulation or QAM is a type of digital modulation. We can
modulate two sin waves that are pi / 2 out of phase and add them together.
When you do this, the resulting signal's phase and amplitude relies on the
input signals. Because it is known that they are out of phase by pi / 2, we
know we can seperate them again.
The condition of being out of phase by pi / 2 is known as quadrature.
QAM is used in serveral digital modulation schemes including 802.11 and GOES.
== Constellation diagram ==
QAM is often shown on a constellation diagram. This is often a circle with points
around a center. The angle of the point, mesured counterclockwise from the horizontal
axis, represents the phase shift of the carrier wave with respect to the reference.
The distance from the center idicates the amplitude of the signal.
For a set baud rate, an incoming signal will hold a phase and amplitude,
occuping a sample. This combination maps to a finite number of sybmols on the
constallation diagram representing some combination of bits. For a QAM signal
the amplitude may vary, however for QFM or QPM signal, however, this is not
true.
These are also often called "IQ" diagrams because they consist of a horizontal
I and veritcal Q axis. The I axis reprsents a cosin "in phase (with the
carrier)" signal. The Q represents a sine wav shifted by pi/2 from the carrier
(aka the _quadrature_ carrier).