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12. What is DSSS? What is FHSS? And what are their
differences?
Frequency-hopping spread-spectrum (FHSS) uses a
narrowband carrier that changes frequency in a pattern that is
known to both transmitter and receiver. Properly synchronized,
the net effect is to maintain a single logical channel. To an
unintended receiver, FHSS appears to be short-duration
impulse noise. Direct-sequence spread-spectrum (DSSS)
generates a redundant bit pattern for each bit to be transmitted.
This bit pattern is called a chip (or chipping code). The longer
the chip is, the greater the probability that the original data can
be recovered. Even if one or more bits in the chip are
damaged during transmission, statistical techniques
embedded in the radio can recover the original data
without-the need for retransmission. To an unintended receiver,
DSSS appears as low power wideband noise and is rejected
(ignored) by most narrowband receivers.
13. What is Spread Spectrum?
Spread Spectrum technology is a wideband radio frequency
technique developed by the military for use in reliable, secure,
mission-critical communication systems. It is designed to trade
off bandwidth efficiency for reliability, integrity, and security. In
other words, more bandwidth is consumed than in the case of
narrowband transmission, but the trade off produces a signal
that is, in effect, louder and thus easier to detect, provided that
the receiver knows the parameters of the spread-spectrum
signal being broadcast. If a receiver is not tuned to the right
frequency, a spread –spectrum signal looks like background
noise. There are two main alternatives, Direct Sequence
Spread Spectrum (DSSS) and Frequency Hopping Spread
Spectrum (FHSS).
