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

12
DIGITAL COMMUNICATIONS
UNDER LINEARLY DISTORTIVE
CHANNELS
In our earlier discussion and analysis of digital communication systems, we have made
the rather idealistic assumption that the communication channel introduces no distortion.
Moreover, the only channel impairment under consideration has been additive white Gaussian
noise (AWGN). In reality, however, communication channels are far from ideal. Among
a number of physical channel distortions, multipath is arguably the most serious problem
encountered in wireless communications. In analog communication systems, multipath represents
an effect that can often be tolerated by human ears (as echos) and eyes (as shadows). In
digital communications, however, multipath leads to linear channel distortions that manifest as
intersymbol interferences (ISi). This is because multipath leads to multiple copies of the same
signal arriving at the receiver with different delays. Thus, one symbol pulse is delayed, which
affects one or more adjacent symbols, causing ISi. As we have discussed, ISi can severely
affect the accuracy of the receivers. To combat the effects of ISi due to multipath channels,
we discuss, in this chapter, two highly effective tools: equalization and OFDM (orthogonal
frequency division modulation).
12.1 LINEAR DISTORTIONS OF WIRELESS
MULTIPATH CHANNELS
Digital communication requires that digital signals be transmitted over a specific medium
between the transmitter and the receiver. The physical media (channels) in real world are
analog. Because of practical limitations, however, analog channels are usually imperfect and
can introduce unwanted distortions. Examples of nonideal analog media include telephone
lines, coaxial cables, underwater acoustics, and radio-frequency (RF) wireless channels at
various frequencies. Figure 12.1 demonstrates a simple case in which transmission from a
base station to a mobile unit encounters a two-ray multipath channel: one ray from the line-ofsight
and one from the ground reflection. At the receiver, there are two copies of the transmitted
signal, one of which is a delayed version of the other.
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