Page 252 - Anatomy of a Robot

09_200256_CH09/Bergren 4/17/03 11:24 AM Page 237
COMMUNICATIONS 237
When the data clock ticks, we sample the position of the received signal in symbol
space. Suppose we receive a symbol whose amplitude is a little low but has a very clear
phase. It might map into the following point shown in Figure 9-8.
To decide on which symbol is received, we put a decision grid into symbol space, as
shown in Figure 9-9. The decision grid makes the decision quickly, and the symbol is
resolved to be 01.
It’s clear that we do not want symbols to be too close together in symbol space.
Modulation schemes are designed to minimize the probability that symbols will be too
close or that the peculiarities of the channel will cause one symbol to be mistaken for
another.

10

X

00 X X 11
?
X

01

FIGURE 9-8 Classifying a recently received symbol that is shown as “?”

10
Decision
X Grid
? = "01"

00 X X 11

?
X

01

FIGURE 9-9 A hard decision grid classifies the received symbol as 01.