FHSS

FHSS is an acronym for frequency-hopping spread spectrum. There are
two types of spread spectrum radio, FHSS and direct-sequence spread
spectrum. FHSS is a transmission technology used in local area wireless
network (LAWN) transmissions where the data signal is modulated with
a narrowband carrier signal that literally hops in a random but predictable
sequence from frequency to frequency. The calculation is a function
of time over a wide band of frequencies. The signal energy is spread
in time domain rather than slicing each element into small pieces in the
frequency domain. This technique reduces interference because a signal
from a narrowband system will only affect the spread-spectrum signal if
both are transmitting at the same frequency at the same time. If this is
synchronized correctly, just one logical channel is supported.

The transmission frequencies are designated by a spreading code. The
receiver must be configured to the same spreading code and must listen
to the incoming signal at the correct time and frequency in order to
receive the signal properly. Federal Communication Commission regulations
require manufacturers to use 75 or more frequencies per transmission
channel with a maximum time spent at a specific frequency during
any single spread at 400 ms.