What is QAM?

[NetworkExperts]

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

Phase Modulation

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

• 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.

• 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)

• Symbol = 2 bits
• 2 ^ 2 bits = 4
• Constellation Map:

16-QAM

• Symbol = 4 bits
• 2 ^ 4 bits = 16
• Constellation Map:

64-QAM

• Symbol = 6 bits
• 2 ^ 6 bits = 64
• Constellation Map:

256-QAM

• 802.11ac enhancement
• Symbol = 8 bits
• 2 ^ 8 bits = 256
• Constellation Map: