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).
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