B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

372 PRINCIPLES OF DIGITAL DATA TRANSMISSION
Fi g ure 7.29
Eye diagrams of
a 4-ary PAM
signaling system
using a
raised-cosine
pulse with roll-off
factor 0.5:
(a) over two
symbol periods
2T b with time
offset T h /2;
(6) without time
offset.
Sampling and decision instant
(a)
Sampling and decision instants
(b)
Eye diagrams can also be generated for M -ary PAM by using the same method used for
binary modulations. Because of multilevel signaling, the eye diagram should have M levels
at the optimum sampling instants even when ISI is zero. Here we generate the practical eye
diagram example for a four-level PAM signal that uses the same cosine roll-off pulse with
roll-off factor r = 0.5 that was used in the eye diagram of Fig. 7.27. The corresponding eye
diagrams with time offsets of Tb /2 and O are given in Fig. 7.29a and b, respectively. Once
again, no ISi is observed at the sampling instants. The eye diagrams clearly show four equally
separated signal values without ISi at the optimum sampling points.
7.8 DIGITAL CARRIER SYSTEMS
Thus far, we have discussed baseband digital systems, where signals are transmitted directly
without any shift in frequency. Because baseband signals have sizable power at low frequencies,
they are suitable for transmission over a pair of wires and coaxial cables. Much of the modem
communication is conducted this way. However, baseband signals cannot be transmitted over a
radio link or satellites because this would necessitate impractically large antennas to efficiently
radiate the low-frequency spectrum of the signal. Hence, for these applications, the signal
spectrum must be shifted to a high-frequency range. A spectrum shift to higher frequencies
is also required to transmit several messages simultaneously by sharing the large bandwidth
of the transmission medium. As seen in Chapter 4, the spectrum of a signal can be shifted
to a higher frequency by applying the baseband digital signal to modulate a high-frequency
sinusoid (caJTier).
In transmitting and receiving digital carrier signals, we need a modulator and demodulator
to transmit and receive data. The two devices, modulator and demodulator are usually packaged
in one unit called a modem for two-way (duplex) communications.
7.8.1 Basic Binary Carrier Modulations
There are two basic forms of carrier modulation: amplitude modulation and angle modulation.
In amplitude modulation, the carrier amplitude is varied in proportion to the modulating signal
(i.e., the baseband signal). This is shown in Fig. 7 .30. An unmodulated carrier cos W e t is shown