Page 165 - Fundamentals of Communications Systems

Analog Communications Basics 5.5

(a) Measured time waveform (b) Measured power spectrum
Figure 5.4 An NTSC video signal.

5.2 Analog Transmission
The conventional communication system has a modulator producing a signal
that is transmitted over a channel (a cable or radio propagation) and a demod-
ulator which takes this signal and constructs an estimate of the transmitted
message signal. Figure 5.5 is a block diagram of this system where r c (t)is
1
the output of the channel, Y c (t) is the waveform observed with the receiver ,
and ˆm(t) is the estimate of the transmitted message signal. The noise added
by a radio receiver is usually a combination of signal distortion and additive
interfering noise. It is this noise that makes the job of a communication en-
gineer challenging as this noise makes it impossible to perfectly reconstruct
the transmitted signal. Noise in communication systems will be character-
ized later (see Chapter 11). The job of communication system engineers is
to design and optimize the modulators and demodulators in communication
systems.

x (t) r (t) Y (t)
c
c
c
m(t) Modulator Channel ∑ Demodulator mˆ (t)
Noise
Figure 5.5 An analog communication system block diagram.

1 For clarity this text will try to consistently represent estimates with a caret, random quantities
with capital letters and the deterministic quantities with lower case letters.