30
14. 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.
15. 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).