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?

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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