Page 270 - Anatomy of a Robot

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COMMUNICATIONS 255
by a definitive amount based on the coding strength. The operator of the communica-
tion link can select a code and decrease the input data by just the right amount so the
coding expansion fills up the channel bandwidth.
Remember, the goal of this type of encoding is to organize the input data with rec-
ognizable patterns so the decoder can determine if the channel noise has altered it. The
technique generally used to accomplish this is to sacrifice some of the symbol positions
in the symbol constellation.
Suppose, for example, that the symbol constellation looks like Figure 9-10 showing
64 QAM, which we’ve seen before. In the case of QAM that is not encoded, all transi-
tions are possible. The signal can move from any X mark to any other X mark to signal
6
the transmission of 6 more bits (2 64). But it is possible to restrict the possible tran-
sitions in a recognizable way. If, for instance, it was only possible to jump from one X
mark to just 32 other X marks, then only 5 bits would be transmitted by the transition
5
(2 32). The data rate would be cut in half, but the signal would have to follow a dis-
tinct set of rules that would be known to the decoder. The decoder would then be in a
better position to detect errors by the means, outlined earlier.
One other technique for restricting the transitions the symbols can make is to liter-
ally provide extra symbol positions. Consider, for the moment, a 16 QAM system with
the symbol constellation shown in Figure 9-13.
It’s possible to double the number of symbol positions to make a 32 QAM system
and to double them again to make a 64 QAM system. The symbols in the 32 QAM sys-
tem can be arranged in any geometric arrangement but are best packed into an approx-
imation of a circle (see Figure 9-14).

FIGURE 9-13 16 QAM