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632 Chapter 12 APPLICATIONS IN COMMUNICATIONS
12.8 SPREAD-SPECTRUM COMMUNICATIONS
Spread-spectrum signals are often used in the transmission of digital data
over communication channels that are corrupted by interference due to
intentional jamming or from other users of the channel (e.g., cellular telephones
and other wireless applications). In applications other than communications,
spread-spectrum signals are used to obtain accurate range
(time delay) and range rate (velocity) measurements in radar and navigation.
For the sake of brevity we shall limit our discussion to the use of
spread spectrum for digital communications. Such signals have the characteristic
that their bandwidth is much greater than the information rate
in bits per second.
In combatting intentional interference (jamming), it is important to
the communicators that the jammer who is trying to disrupt their communication
does not have prior knowledge of the signal characteristics. To
accomplish this, the transmitter introduces an element of unpredictability
or randomness (pseudo-randomness) in each of the possible transmitted
signal waveforms, which is known to the intended receiver, but not to the
jammer. As a consequence, the jammer must transmit an interfering signal
without knowledge of the pseudo-random characteristics of the desired
signal.
Interference from other users arises in multiple-access communications
systems in which a number of users share a common communications
channel. At any given time a subset of these users may transmit information
simultaneously over a common channel to corresponding receivers.
The transmitted signals in this common channel may be distinguished
from one another by superimposing a different pseudo-random pattern,
called a multiple-access code, ineach transmitted signal. Thus a particular
receiver can recover the transmitted data intended for it by knowing the
pseudo-random pattern, that is, the key used by the corresponding transmitter.
This type of communication technique, which allows multiple users
to simultaneously use a common channel for data transmission, is called
code division multiple access (CDMA).
The block diagram shown in Figure 12.17 illustrates the basic elements
of a spread-spectrum digital communications system. It differs
FIGURE 12.17
Basic spread spectrum digital communications system
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