Some information about FHSS radios (probably ED devs already know, just in case to help out :) ), some resources too :) !

[MrWolf]

FHSS it's basically used in all types of systems, most commonly Bluetooth, some 802.11 standards, and military/nontrackable comms. It's noise/interfence resistant as other SS (Spread Spectrum) techniques, like DSSS (Direct Sequence Spread Spectrum) (i.e CDMA for DSSS) etc. FHSS doesn't provide security, it just makes the frequency use random so jamming systems cannot know which band to jamm, and to overall reduce the noise/interference. But if "enemy" know your hop pattern it can listen to the comms, maybe not jam it because the speed of the hops. FHSS still requires encryption for full security.

 

Well let's go to the point:

FHSS uses frequency hoping along time to create an spread spectrum modulation. This means that electronic systems designed to work with this transmision scheme need to jump their carrier frequencies every x times, and because real systems aren't ideal this takes some time to perform.

This time that the electronic systems takes to jump between carriers it's translated on abrupt periodical noise pulses that can be heard on the received signal (for example audio). As far as i know this only applies to analog (audio for example) systems; and on digital systems if the "hop time" wasn't enough symbol periods of length ( enough symbol periods regarding the codeword set used and depending on the, forward error correction mechanism, etc.) to make the codeword that was being transmitted unrecoverable, the system should be capable of recovering the data lost and make it unnoticable by a human, or even approximate what the audio sounded like (like a phone call when because of bad channel condition you can hear someone talking for a moment with a robotic voice).

This could be a good addition to our aircraft radios: old school (or modern aircraft using old comms) aircraft having analogic comms FHSS and having this "vintage" noise over the audio.

And modern aircraft using digital systems should have more crisp, good quality sound, when using for example MIDS.

 

Examples for FHSS, analog, or no "hop time" recovery:

 

 

 

Example FHSS with "hop time" recovery, notice no periodic noise (the bad quality of audio it's because the mic):

 

Thanks