32 lines
1.5 KiB
Plaintext
32 lines
1.5 KiB
Plaintext
= 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). Thus each sample is reprsented as a complex
|
|
number, and the IQ or constillation diagram can be thought of as a complex
|
|
plane.
|