Frequency Hopping Spread Spectrum Radio

 
Frequency Hopping Spread Spectrum Radio
Frequency hopping works very much as its name implies. It modulates the data signal with a
carrier signal that hops from frequency to frequency as a function of time over a wide band of
frequencies (see Figure 2.5). An IEEE 802.11 frequency-hopping radio, for example, will hop
the carrier frequency over the 2.4GHz frequency band between 2.4GHz and 2.483GHz A hopping code determines the frequencies the radio will transmit and in which order. To
receive the signal properly, the receiver must be set to the same hopping code and listen to the
incoming signal at the right time and correct frequency. FCC regulations require manufacturers
to use 75 or more frequencies per transmission channel with a maximum dwell time (the time
spent at a particular frequency during any single hop) of 400ms. If the radio encounters interference
on one frequency, it will retransmit the signal on a subsequent hop on another frequency The frequency hopping technique reduces interference because an interfering signal from a
narrowband system will affect the spread spectrum signal only if both are transmitting at the
same frequency at the same time. Thus, the aggregate interference will be very low, resulting
in few or no bit errors.
It is possible to have operating radios use spread spectrum within the same frequency band and
not interfere, assuming they each use a different hopping pattern. While one radio is transmitting
at one particular frequency, the other radio is using a different frequency. A set of hopping
codes that never uses the same frequencies at the same time is considered orthogonal. The
FCC’s requirement for the number of different transmission frequencies allows frequencyhopping
radios to have many non-interfering channels.