0195136047.pdf

652 SIGNAL PROCESSING

receiver of the communication system, or from interference encountered in transmission as in

the case of radio-signal transmission. The effect of signal attenuation is to reduce the amplitude

of the desired signals(t) and, thus, to render the information-bearing signal more vulnerable to

additive noise.

Signal attenuation in many channels can be offset by using amplifiers to boost the signal

level during transmission. However, the amplifier also introduces additive noise in the process

of amplification, thereby corrupting the signal. The additional noise must also be taken into

consideration in the design of the communication system.

Any conductive two-terminal device is characterized generally as lossy, with some resistance

R. A resistor, which is at a temperatureτabove absolute zero, contains free electrons that exhibit

random motion and, thus, result in a noise voltage across the terminals of the resistor. Such a noise

voltage is known asthermal noise.If the noise is introduced primarily by electronic components

and amplifiers at the receiver, it may be characterized as thermal noise.

In general, any physical resistor or lossy device can be modeled by a noise source in series

with a noiseless resistor, as shown in Figure 14.3.4. The noise source is usually characterized as a

sample function of a random process. Since random processes involving probability and random

variables are outside the scope of this text, we will resort to simpler explanations. Figure 14.3.5

(a) (b)

RR

n(t)

Figure 14.3.4Physical resistor or lossy device.(a)Noiseless resistor.(b)Noise-

less resistor in series with a noise source.

n(t)

nrms

f

(^01012) Hz
(a)
(b)
Noise power spectrum
η
0 t
Figure 14.3.5Thermal or white noise.(a)Typical waveform.(b)Typical power spectrum.