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238 CHAPTER NINE
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FIGURE 9-10 A symbol space for 64 QAM

A more complex example of this sort of symbol space is 64 Quadrature Amplitude
Modulation (QAM), where 8 bits of data are modulated at the same time. Symbol space
for 64 QAM might have a square structure as shown in Figure 9-10.
The incoming symbol data traces a wild pattern through the 8 8 grid of dots. To a
certain extent, because the symbol data tries to stick to the grid points, the grid has open
areas where the data does not traverse. These open areas look like eyes and are the sub-
ject of the next discussion.

Error Control

Designers of big symbol spaces have to worry about what’s called the open eye.
Remember, when the data clock ticks in the receiver, the received signal should be right
on top of a symbol point. To get there from any other symbol point, it should travel
along a well-known route through symbol space (governed by the shape of the carrier
signal). With a noiseless channel, the trail of the received signal would trace a very nice
set of geometric paths and lots of empty space would be showing on the symbol space,
places where the signal never traverses. These empty spaces are what engineers look for
when they are trying to find the open eye. These spaces are called that because they are
generally formed by two sine waves and have the shape shown in Figure 9-11.
A good engineer can put the communication waveform on an oscilloscope (or other
instrument), look at the eye pattern, and determine the health of the physical layer of
the communication network.