SPREAD SPECTRUM
characteristics of any spread spect ~um system. Input
is fed into a channel encoder that produces an analog signal with a relatively
narrow bandwidth around some center frequency. This signal is further modulated
using a sequence of digits known as a spreading code or Sf reading sequence.
Typically, but not always, the spreading code is generated by a pseudonoise, or
pseudorandom number, generator. The effect of this modula :ion is to increase
significantly the bandwidth (spread the spectrum) of the sigral to be transmitted.
On the receiving end, the same digit sequence is used 10 demodulate the
spread spectrum signal. Finally, the signal is fed into a channel jecoder to recover
the data.
Several things can be gained from this apparent waste of Sl >ectrum:
• We can gain immunity from various kinds of noise and n .ultipath distortion.
The earliest applications of spread spectrum were militar: " where it was used
for its immunity to jamming.
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