Frequency Hopping Spread Spectrum

[John Donaldson]

Frank,
     SS technology goes back to WWII. A famous Actress came up with SS 
as a way to control torpedoes and keep the enemy from
jamming the RF control signals. Synchronization was done with coded 
paper tape loop. the torp would listen at the freq dictated by the tape 
and wait for the transmitter to catch up and then both would then start 
cycling thru the paper tape loop. The Navy did not use the system 
because they were not convinced that this Actress could be that smart 
and was trying to fool the Navy. Turns out she also held a Engineering 
Degree, so knew what she was talking about. LOL

John Donaldson
AB8YZ

On 12/26/2013 7:30 PM, Frank Eliot wrote:
> 	Earlier this month, Andre Kesteloot gave an interesting talk on frequency hopping, and demonstrated a transmitter driver he had designed. One aspect of the talk really interested me. I had often wondered how commercial SS systems attained their initial synchronization. I had assumed that there must be some characteristic that enabled them to sync up quickly either periodically or on each transmission. I couldn't figure out what it was, but just assumed it must be easy. In the meeting discussion, it was brought out that there is no magic, and that they do it simply by doing some parallel processing that is probably not practical for hams. Much of the evening's discussion centered on how hams could sync a frequency hopping system. For discussion in what follows, assume, as Andre did, 127 points per cycle from a shift register, known frequency channelization, and a one second cycle.
> 	Andre's proposal for synchronization was to start off a transmission with the required NB FCC station ID on say, frequency one, with the timing reference being a transmitted pulse or mike button release. Then the QSO could continue in SS mode. This would be cool, and a great generator of bragging rights, but it seems to me that it wouldn't be very useful in the wild. The reason is simply that, if we are confident that frequency channel one, or any other for that matter, is clear of QRM so that we can be heard sending this initial timing signal, then we might as well simply sit on that NB channel and not bother with frequency hopping. Frequency hopping yields no processing gain in the absence of QRM. Its sole advantage, I believe, is in the case where some of the frequency channels are blocked by QRM. In that case, if you are transmitting information that is still usable if brief snippets of the received time series are blanked out, then frequency hopping will yield a successful QSO,
>    whereas if you had unfortunately chosen one of those blocked channels for your communication, you would be out of luck, and never get started.
> 	It seems to me that the best, and maybe cheapest, way to establish sync while taking advantage of the SS processing gain, would be for the receiving station to temporarily offset his shift register clock a little from its one-second cycle, and then either listen for the transmitted sub-audible tone, as Andre suggested, or simply watch the received S-meter that had a longish time constant. When the receiver clock walks into sync, the tone will be detected, or the S-meter should jump up to indicate sync.
> 	I have thought a lot about a related synchronization problem in connection with my work on coherent detection. This goes in a different direction than frequency hopping. Its goal is to achieve significant S/N processing gain on an open channel by pushing to very narrow band transmission. Coherent detection requires that the carrier phase as received be known at the receiving site. I had to figure out a way to do that cheaply, but be able to establish that synchronization under the same very weak signal conditions that I wanted to communicate with after sync was achieved. I proposed such a method at the AMRAD meeting a few months ago.
> 	In summary, I think that whenever a transmission format is proposed that attempts to overcome a limitation such as QRM, QRN, etc., then a necessary requirement when suggesting it for ham use is that if the protocol must employ an initialization procedure before communication starts, then the only fair way to advertise the protocol is to define the robustness of a "system" that employs both a synchronization step and a communication step as that of the poorer performer of the two. If the setup protocol requires a big signal or choice of a clear channel in a crowded band, it seems like the wrong way to go for hams.
>
> 73, Frank
> W3WAG
>
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