Communication Systems/Quadriture Amplitude Modulation

The quadrature amplitude modulation (QAM) system of modulation is the most popular M-ary scheme.

Definition

Let us say that we have 2 carrier waves. One is a sine wave, and the other is a cosine wave. Since these two waves are orthogonal, we can use them simultaneously, without losing the information of either. If both waves have the same frequency, f, we can write out the equation for a generic symbol, s:

$s(t)=A_k\sin (ft)+B_k\cos (ft)$

In this way, we can create multiple symbols by simply changing the values of A and B. This equation can be broken up into two parts:

• $A_k\sin (ft)$ Which is called the "in-phase" component of the equation.
• $B_k\cos (ft)$ Which is called the "quadrature" component of the equation.

An equation which is written as a sum of a sine plus a cosine is said to be in "quadrature form". If we combine the two components into a single waveform as such:

$s(t)=\sqrt{A_k^2+B_k^2}\cos (ft+{\tan }^{-1}(B_k/A_k))$

This form is called the "Polar Form" of the equation.

Constellation Plots

If we make a graph with the X axis being the values for A, and the Y axis being the values for B, we get what is called a "Constellation Plot". If A and B have discrete values, then the constellation plot will show dots at points that correspond to values for A and B coordinates. It is called a constellation plot because the layout of the different points can look very similar to the layout of stars in the sky.

On a constellation plot, we can see a number of points:

1. The further apart the points are, the less likely they are to be mixed up
2. The closer the points are to the origin, the less power it takes to send.

1. The more points there are, the faster the data rate (bit rate) at a fixed symbol rate.
2. The fewer points there are, the simpler and cheaper the hardware necessary to distinguish between them.

For these two reasons, there is no single "best" constellation plot, but it is up to the engineer to pick the points that are best for the system. By placing the points manually, the engineer is able to make trade-offs between the power of the system, and the number of bits per symbol (and therefore the bitrate).