Page 250 - Anatomy of a Robot

09_200256_CH09/Bergren 4/17/03 11:24 AM Page 235
20
-10 -5 0 0 5 10 15 Eb/No (dB) COMMUNICATIONS 235
25
-1
-2
Shannon's
Limit
-1.6 dB -3
-4
-5
-6
Log (BER)
FIGURE 9-5 S/N effect: As the power per bit (Eb/No) goes up, the bit error
rate (BER) goes down.

Eb/No (dB)
0
-10 -5 0 5 10 15 20 25
-1
-2
Shannon's
Limit
-1.6 dB -3
-4
-5

-6
Log (BER)
FIGURE 9-6 A better modulator (the inner curve) can approach the Shannon
limit more closely.

Conversely, if an engineer needs a specific BER (or lower) to make a system work,
this specifies the minimum Eb/No the channel must have. In practice, a perfect
realization of the theoretical Eb/No curve cannot be realized and an engineer
should condition the channel to an Eb/No higher than that theoretically required.

Figure 9-6 shows two BER curves from two different but similar modulation
schemes. These curves show that some modulation schemes are more efficient than oth-
ers. In fact, the entire game of building modulation schemes is an effort to try to