FHSS is used to avoid interference, to prevent eavesdropping, and to enable code-division multiple access (CDMA) communications.
[1] FHSS offers four main advantages over a fixed-frequency transmission: Spread-spectrum signals are highly resistant to deliberate jamming unless the adversary has knowledge of the frequency-hopping pattern.
Military radios generate the frequency-hopping pattern under the control of a secret Transmission Security Key (TRANSEC) that the sender and receiver share in advance.
While providing no extra protection against wideband thermal noise, the frequency-hopping approach reduces the degradation caused by narrowband interference sources.
[6] The German military made limited use of frequency hopping for communication between fixed command points in World War I to prevent eavesdropping by British forces, who did not have the technology to follow the sequence.
[7] Jonathan Zenneck's book Wireless Telegraphy was originally published in German in 1908, but was translated into English in 1915 as the enemy started using frequency hopping on the front line.
During World War II, the US Army Signal Corps was inventing a communication system called SIGSALY, which incorporated spread spectrum in a single frequency context.
[11] Frequency-hopping ideas may have been rediscovered in the 1950s during patent searches when private companies were independently developing direct-sequence Code Division Multiple Access, a non-frequency-hopping form of spread-spectrum.
[citation needed] In 1957, engineers at Sylvania Electronic Systems Division adopted a similar idea, using the recently invented transistor instead of Lamarr's and Antheil's clockwork technology.
[9][dubious – discuss] In 1962, the US Navy utilized Sylvania Electronic Systems Division's work during the Cuban Missile Crisis.
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.