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What is QAM?

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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)

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

4-QAM

16-QAM

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

16-QAM

64-QAM

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

64-QAM

256-QAM

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

256-QAM