Monday, 4 February 2013

FREQUENCY HOPPING SPREAD SPECTRUM

FREQUENCY HOPPING SPREAD SPECTRUM


With frequency hopping spread spectrum (FHSS), the signal is broac cast over a
seemingly random series of radio frequencies, hopping from frequency t) frequency
at fixed intervals. A receiver, hopping between frequencies in synchron zation with
the transmitter, picks up the message. Would-be eavesdroppers hear on ly unintelligible
blips. Attempts to jam the signal on one frequency succeed only It knocking
out a few bits of it.
Basic Approach
Figure 7.2 shows an example of a frequency hopping signal. A number of channels
are allocated for the FH signal. Typically, there are 2k carrier frequencie s forming 2k
channels. The spacing between carrier frequencies and hence the w: dth of each
channel usually corresponds to the bandwidth of the input signal. The transmitter
operates in one channel at a time for a fixed interval; for example, the: EEE 802.11
wireless LAN standard uses a 300-ms interval. During that interval, son e number of
bits (possibly a fraction of a bit, as discussed subsequently) is trans nitted using
some encoding scheme. The sequence of channels used is dictated by a spreading
code. Both transmitter and receiver use the same code to tune into a sequence of
channels in synchronization.






































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