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Spread-Spectrum Communications 633
from a conventional digital communications system by the inclusion of
two identical pseudo-random pattern generators, one that interfaces with
the modulator at the transmitting end and the second that interfaces with
the demodulator at the receiving end. The generators generate a pseudorandom
or pseudo-noise (PN) binary-valued sequence (±1’s), which is
impressed on the transmitted signal at the modulator and removed from
the received signal at the demodulator.
Synchronization of the PN sequence generated at the demodulator
with the PN sequence contained in the incoming received signal is
required in order to demodulate the received signal. Initially, prior to the
transmission of data, synchronization is achieved by transmitting a short
fixed PN sequence to the receiver for purposes of establishing synchronization.
After time synchronization of the PN generators is established,
the transmission of data commences.
12.8.1 PROJECT 12.8: BINARY SPREAD-SPECTRUM
COMMUNICATIONS
The objective of this project is to demonstrate the effectiveness of a PN
spread-spectrum signal in suppressing sinusoidal interference. Let us consider
the binary communication system described in Project 12.7, and let
us multiply the output of the modulator by a binary (±1) PN sequence.
The same binary PN sequence is used to multiply the input to the demodulator
and thus to remove the effect of the PN sequence in the desired
signal. The channel corrupts the transmitted signal by the addition of a
FIGURE 12.18 Block diagram of binary PN spread-spectrum system for simulation
experiment
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