12.4 Application to Digital Communications 733
Baseband and Band-Pass Communication Systems
A baseband signal can be transmitted over a pair of wires (like in a telephone), coaxial cables, or
optical fibers. But a baseband signal cannot be transmitted over a radio link or a satellite because this
would require a large antenna to radiate the low-frequency spectrum of the signal. Thus, the signal
spectrum must be shifted to a higher frequency by modulating a carrier by the baseband signal.
This can be done by amplitude and by angle modulation (frequency and phase). Several forms are
possible.
nExample 12.8
Suppose the binary signal 01001101 is to be transmitted over a radio link using AM and FM
modulation. Discuss the different band-pass signals obtained.
Solution
The binary message can be represented as a sequence of pulses with different amplitudes. For
instance, we could represent the binary digit 1 by a pulse of constant amplitude, and the binary
0 is represented by switching off the pulse (see the corresponding modulating signalm 1 (t)in
Figures 12.10(a) and 12.10(b)). Another possible representation would be to represent the binary
digit 1 with a positive pulse of constant amplitude, and 0 with the negative of the pulse used for 1
(see the corresponding modulating signalm 2 (t)in Figures 12.10(c) and 12.10(d)).
In AM modulation, if we usem 1 (t)to modulate a sinusoidal carrier cos( 0 t)we obtain the
amplitude-shift keying (ASK) signal shown in Figure 12.10(a). On the other hand, when using
m 2 (t)to modulate the same carrier we obtain a phase-shift keying (PSK) signal shown in
Figure 12.10(c). In this case, the phase of the carrier is shifted 180oas the pulse changes from
positive to negative.
Using FM modulation, the symbol 0 is transmitted using a windowed cosine of some frequency
c 0 and the symbol 1 is transmitted with a windowed cosine of frequencyc 1 resulting in
frequency-shift keying (FSK). The data are transmitted by varying the frequency. In this case it
is possible to get the same modulated signals for bothm 1 (t)andm 2 (t). The modulated signals are
shown in Figure 12.10(b and d).
The ASK, PSK, and FSK are also known as BASK, BPSK, and BFSK, respectively, by adding the word
“binary” (B) to the corresponding amplitude-, phase-, and frequency-shift keying. n
12.4.2 Time-Division Multiplexing
In a telephone system, multiplexing enables multiple conversations to be carried across a single
shared circuit. The first multiplexing system used was frequency-division multiplexing (FDM), which
we covered in Chapter 6. In FDM an available bandwidth is divided among different users. In the
case of voice communications, each user is allocated a bandwidth of 4 KHz, which provides good
fidelity. In FDM, a user could use the allocated frequencies all of the time, but the user could not go
outside the allocated band of frequencies.
