Editing Frequency-hopping spread spectrum (section)

== Variations ==
'''Adaptive frequency-hopping spread spectrum''' ('''AFH''') as used in [[Bluetooth]] improves resistance to [[co-channel interference|radio frequency interference]] by avoiding crowded frequencies in the hopping sequence. This sort of adaptive transmission is easier to implement with FHSS than with [[DSSS]].

The key idea behind AFH is to use only the "good" frequencies and avoid the "bad" ones—those experiencing [[frequency selective fading]], those on which a third party is trying to communicate, or those being actively jammed. Therefore, AFH should be complemented by a mechanism for detecting good and bad channels.

But if the radio frequency interference is itself dynamic, then AFH's strategy of "bad channel removal" may not work well. For example, if there are several colocated frequency-hopping networks (as Bluetooth [[Piconet]]), they are mutually interfering and AFH's strategy fails to avoid this interference.

The problem of dynamic interference, gradual reduction of available hopping channels and backward compatibility with legacy Bluetooth devices was resolved in version 1.2 of the Bluetooth Standard (2003). Such a situation can often happen in the scenarios that use [[unlicensed spectrum]].

In addition, dynamic radio frequency interference is expected to occur in the scenarios related to [[cognitive radio]], where the networks and the devices should exhibit [[frequency-agile]] operation.

[[Chirp#Chirp modulation|Chirp modulation]] can be seen as a form of frequency-hopping that simply scans through the available frequencies in consecutive order to communicate.

Frequency hopping can be superimposed on other modulations or waveforms to enhance the system performance.