Quadrature Amplitude Modulation


Quadrature amplitude modulation, or QAM, is a big name for a relatively simply technique. It is simply a combination of amplitude modulation and phase shift keying.

Let's just dive right in. We'll use a signal that is transmitting at 3600 bps, or 3 bits per baud. This means that we can represent 8 binary combinations.

We'll use 2 measures of amplitude, 1 and 2, just we did before. We'll also use 4 possible phase shifts, like we did before. Combining the two, we have 8 possible waves that we can send. How convenient. :)

First step is to generate a table to show us which waves correspond to which binary combination. This can basically be done at random, although modem manufacturers have agreed on standards.

Bit value Amplitude Phase shift
000 1 None
001 2 None
010 1 1/4
011 2 1/4
100 1 1/2
101 2 1/2
110 1 3/4
111 2 3/4

Let's encode a big bit stream:

001010100011101000011110

First, we break it up into 3-bit triads:

001-010-100-011-101-000-011-110

Now all we have to do is figure out what the resulting signal should look like. Remember that we shift each wave relative to the wave before it!

(These values are correct.)

This is the technique most often used. You can imagine how many different amplitudes and phase shifts are needed for 28.8 kbps, which uses 24 bits per baud!