Frequency-hopping-spread-spectrum (FHSS)

frequency wave

While direct-sequence spread spectrum (DSSS) can provide higher capacities than frequency-hopping-spread-spectrum (FHSS), it is easily impacted by environmental factors, particularly reflections. As such it is better suited for short point to multipoint implementations or when deployed for long distances, it should be implemented in point to point topologies.

Whereas, FHSS isn’t as easily impacted by RF signals and other environmental factors, and the entity is able to deploy greater numbers of simultaneously active systems in the same geographic area (collocated systems, as compared with DSSS systems.

As an aside, in military situation enemy forces could eavesdrop on, or jam communication, by scanning the frequency spectrum and monitoring for “spikes” in the spectrum, which are indicative of a carrier signal. Once that is determined, they may set their receiver to same carrier frequency, listen in on communications, intercept, or pretend to be part of friendly forces

One technique to mitigate against this is by using (FHSS) frequency-hopping spread spectrum. FHSS is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver.

These radios constantly switch from one frequency to another during the transmission, and they automatically synchronize between radios. If you were trying to listen in and monitored any single frequency during the discussion, as a scanner would, al that would be heard is a microsecond blip of noise. In addition, spread-spectrum signals are highly resistant to deliberate jamming, unless the adversary has knowledge of the sequencing algorithms. This is reminiscent of the capture of the Enigma, German code ciphering machine, with that in hand the Allies were privy to the Axis power communications.

While FHSS is very useful, its traffic is not encrypted, and there are methods to determine the algorithm in use for the sequencing of frequencies. Thus for added security and encryption, in conjunction with FHSS, the military has used: NSA-supplied sequence generators with HAVE QUICK, SINCGARS (Skinner, ).

The NSA was established in 1952 as a highly compartmented secret code-breaking effort undertaken by a handful of military officers and civilians, but the Agency has gradually become an acknowledged government agency responsible for signals intelligence (sigint) (Best). Other encryption devices that the NSA has deployed for the US government are:  KY-5 (Vinson), and in 2012 it deployed “Fishbowl”, a secure VOIP phone.

Finally, there have been many iterations of spread-spectrum; the German military inWW1, and CDMA in recent years. However, as a sinephile staff member pointed out, it was actress Hedy Lamarr and composer George Antheil that received a patent in 1942 for their "Secret Communications System". This early version of frequency hopping used a piano-roll to change among 88 frequencies, and was intended to make radio-guided torpedoes harder for enemies to detect or to jam.

References

Best, R., The National Security Agency: Issues for Congress. Retrieved From http://www.fas.org/irp/crs/RL30740.pdf

Schwartz, S., Frequency Hopping Spread Spectrum (FHSS) vs. Direct Sequence Spread Spectrum (DSSS) in Broadband Wireless Access (BWA) and Wireless LAN (WLAN). Retrieved from http://sorin-schwartz.com/white_papers/fhvsds.pdf

Skinner, J., An Introduction to Frequency--‐Hopping Spread--‐Spectrum (FHSS) Data Communication Techniques. Retrieved from http://macs.citadel.edu/chenm/sigma_xi/Presentation_Skinner.pdf