Frequency hopping is the simpler of the two
spread-spectrum techniques. A frequency synthesizer is used to generate a
carrier in the ordinary way.
There is one difference, however: instead
of operating at a fixed frequency, the synthesizer changes frequency many times
per second according to a preprogrammed sequence of channels.
This sequence is known as a pseudo-random
noise (PN) sequence because, to an outside observer who has not been given the
sequence, the transmitted frequency appears to hop about in a completely random
and unpredictable fashion.
In reality, the sequence is not random at
all, and a receiver which has been programmed with the same sequence can easily
follow the transmitter as it hops and the message can be decoded normally.
Since the frequency-hopping signal
typically spends only a few milliseconds or less on each channel, any
interference to it from a signal on that frequency will be of short duration.
If an analog modulation scheme is used for voice, the interference will appear
as a click and may pass unnoticed.
If the spread-spectrum signal is modulated
using digital techniques, an errorcorrecting code can be employed that will
allow these brief interruptions in the received signal to be ignored, and the
user will probably not experience any signal degradation at all. Thus reliable
communication can be achieved
in spite of interference.
Sample Problem
A frequency-hopping spread-spectrum system
hops to each of 100 frequencies
every ten seconds. How long does it spend
on each frequency?
SOLUTION
The amount of time spent on each frequency
is
t = 10 seconds/100 hops
= 0.1 second per hop
If the frequency band used by the
spread-spectrum system contains known sources of interference, such as carriers
from other types of service, the frequency-hopping scheme can be designed to
avoid these frequencies entirely.
Otherwise, the communication system will
degrade gracefully as the number of interfering signals increases, since each
new signal will simply increase the noise level slightly.
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