What is QAM?

This blog is for anyone wanting to learn about QAM. It is written to help depict and describe aspects of QAM in a way that is hopefully easy to grasp.

What is Quadrature Amplitude Modulation?
Quadrature Amplitude Modulation, QAM is a form of modulation that is a combination of phase modulation and amplitude modulation. The QAM scheme represents bits as points in a quadrant grid know as a constellation map.
Constellation Map

Is a graph of the phase and amplitude modulation points in a given modulation scheme.
Shown below is the constellation map of 16-QAM

16-QAM

Phase Modulation

Represents bits by changing the angle of a wave.
An example of Phase Modulation is QPSK.

angle

As seen above, QPSK can have four different phase changes as four different angles.
Is the angle of the constellation point.

Amplitude Modulation

Represents bits by changing the amplitude of a wave.
It is the change in distance from 0,0 to the constellation point.
Could also be described as the hypotenuse of the right angle formed from the x-axis and the constellation point.

amplitude

As seen above. QPSK does not have aplitude modulation. At each of the 4 states the aplitude is the same.

How does QAM work with Wi-Fi?

Depending on the modulation scheme, more bits can be represented at a given constellation point. Consequently, a higher signal quality must be present to distinguish between constellation points when using high modulation schemes.

Think of the different schemes as different languages.
The higher the modulation the more complex the language.
The complex languages become difficult to understand while walking further and further away from a Wi-Fi radio.
Complexity needs to be lowered. This results in a lower data rate.
This is why lower signal quality usually equals slower speed.

QPSK (Sometimes referred to as 4-QAM)