0195136047.pdf

14.3 INTERFERENCE AND NOISE 651

Amplitude

spectrum

Signal

Interference

f

f 0

(a)

0

|H(f)|

f

f 0

(a)

0

Figure 14.3.2(a) Composite am-

plitude spectrum including single-

frequency interference.(b)Amplitude

ratio of notch filter.

Noise

In any communication system there are usually two dominant factors that limit the performance
of the system:

1. Additive noise,generated by electronic devices that are used to filter and amplify the
   communication signal.


2. Signal attenuation,as the signal travels through a lossy channel.

A simple mathematical model of a channel with attenuation and additive noise is shown in
Figure 14.3.3. If the transmitted signal iss(t), the received signal is given by

r(t)=αs(t)+n(t) (14.3.3)

whereα<1, andn(t) represents an additive random noise process corrupting the transmitted
signal. Physically, the additive noise may arise from electronic components and amplifiers at the

Channel

Noise n(t)

Transmitted

signal s(t)

Received signal

r(t) : α s(t) + n(t)

Attenuation α

Figure 14.3.3Mathematical model of channel with attenuation and additive noise.