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ST2115
Scientech Technologies Pvt. Ltd.
20
Spread-spectrum signals for digital communications were originally developed and
used for military communications either (1) to provide resistance to jamming (antijam
protection), or (2) to hide the signal by transmitting it at low power and, thus, making
it difficult for an unintended listener to detect its presence in noise (low probability of
intercept). However, spread-spectrum signals are now used to provide reliable
communications in a variety of civilian applications, including digital cellular
communications and interoffice communications.
In this section we present the basic characteristics of spread-spectrum signals and
assess their performance in terms of probability of error. We concentrate our
discussion on two methods for spreading the signal bandwidth, namely, by direct
sequence modulation and by frequency hopping. Both methods require the use of
pseudorandom code sequences whose generation is also described. Several
applications of spread-spectrum signals are presented.
Model of a Spread-Spectrum Digital Communication System:
The basic elements of a spread spectrum digital communication system are illustrated
in the figure 5 we observe that the channel encoder and decoder and the modulator
and demodulator are the basic elements of a conventional digital communication
system. In addition to these elements, a spread-spectrum system employs two
identical pseudorandom sequence generators, one which interfaces with the modulator
at the transmitting end and the second which interfaces with the demodulator at the
receiving end. These two generators produce a pseudorandom or pseudonoise (PN)
binary-valued sequence, which is used to spread the transmitted signal at the
modulator and to dispread the received signal at the demodulator.
Time synchronization of the PN sequence generated at the receiver with the PN
sequence contained in the received signal is required in order to properly dispread the
received spread-spectrum signal. In a practical system, synchronization is established
prior to the transmission of information by transmitting a fixed PN bit pattern which is
designed so that the receiver will detect it with high probability in the presence of
interference. After time synchronization of the PN sequence generators is established,
the transmission of information commences. In the data mode, the communication
system usually tracks the timing of the incoming received signal and keeps the PN
sequence generator in synchronism.
Interference is introduced in the transmission of the spread-spectrum signal through
the channel. The characteristics of the interference depend to a large extent on its
origin. The interference may be generally categorized as being either broadband or
narrowband (partial band) relative to the bandwidth of the information-bearing signal,
and either continuous in time or pulsed (discontinuous) in time. For example, an
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