Frequency Hopping Spread Spectrum

[Mike O'Dell]

Robert Dixon's 1976 book is on Amazon
and can be had for between $2-10 or so.
That was the book that was the reference for
AMRAD's SS work in the middle 80s.

Dixon has a newer book with more concentration
on commercial use, but it ain't cheap.

     -mo




On 12/28/13 4:27 PM, James Wolf wrote:
> Frank,
>
> FH is only what it says it is, it does not refer to or define a waveform,
> however a waveform may define FH.  SS would be complimentary to FH and is
> used in 802.11x waveforms as well.  The early frequency hopping HaveQuick
> radios were analog, hopped very slowly and you lost the audio in the dwell
> time when switching to a new frequency, so there were dropouts every time it
> changed frequencies.  I don't recall the dropout time, but I'd guess ~100
> ms, enough to lose at least one syllable.  Current radios can hop thousands
> of times faster.
> As the story goes, a soldier says,"Don't Fire" and what was heard was
> "Fire".
> For Hams, using SSB, (unless you could drop the output power), you would
> need to cut the audio before the VFO "swept" to the next frequency.
> Current radios drop power and use a CVSD modulator/demodulator that
> compresses and then expands the voice to fill in the dwell time gaps and
> make it a seamless voice or data output.  This is done at the digital level.
>
>
> To answer the question, I don't see how one could effectively do it without
> using a data stream or preample in each transmission.  Data in that preample
> is critical to syncing and knowing what the frequency sequence is and if one
> is using FEC, whether or not a retry is needed.
>
> Jim - KR9U
>
> -----Original Message-----
> From: Frank Eliot [mailto:feliot at his.com]
> Sent: Saturday, December 28, 2013 11:33 AM
> To: jbwolf at comcast.net
> Cc: tacos at amrad.org; 'John Donaldson'
> Subject: Re: Frequency Hopping Spread Spectrum
>
> Jim, KR9U/All -
>
> I was using "SS" as short for frequency hopping, which I believe is a form
> of spread spectrum. I think the primary advantage of frequency hopping (FH)
> for hams is for use when one or more of the many frequencies in the sequence
> is covered by QRM. If the narrow-band modulation is analog e.g. voice, then
> an occasional brief dropout would not prevent solid copy. The whole
> discussion assumed a known FH algorithm and a pre-defined set of
> frequencies, as I believe I explained. The question I was addressing was how
> to attain the correct sync, or to quote your message, I wish to figure out a
> cheap way for hams to attain "Only when the correct sync is detected, it
> starts the FH sequence." If you are aware of a simpler way to do that than
> what I suggested in my original message, then that would be useful.
>
> To employ FH where the narrow-band modulation were digital, and in the
> presence of QRM on one or more of the FH frequencies, would require the use
> of a quite robust error correction coding technique to reconstruct the
> dropouts. That might be an interesting experiment to run, once Andre gets an
> on-the-air link set up.
>
> Frank
> W3WAG
>
> On Dec 27, 2013, at 9:53 01PM, James Wolf <jbwolf at comcast.net> wrote:
>
>> All,
>>
>> I didn't see the presentation, but I'm somewhat confused by the semantics.
>> "In this discussion, "SS" I thought might be Spread Spectrum, but it
>> seems to be used for Frequency Hopping -  Maybe I'm missing something.
>> I assume we are talking digital communication?  If so, a preamble
>> containing the sync information, next frequency, etc. is typically
>> used.  To initially start the communications, a known FH algorithm or
>> a pre-defined set of frequencies is used.  Only when the correct sync
>> is detected, it starts the FH sequence.
>> Are we discussing an FM or a SSB/AM mode?
>> Sorry if I'm way behind in the discussion.
>>
>> Jim, KR9U
>>
>> -----Original Message-----
>> From: tacos-bounces+jbwolf=comcast.net at amrad.org
>> [mailto:tacos-bounces+jbwolf=comcast.net at amrad.org] On Behalf Of Frank
>> Eliot
>> Sent: Friday, December 27, 2013 8:26 PM
>> To: John Donaldson
>> Cc: tacos at amrad.org
>> Subject: Re: Frequency Hopping Spread Spectrum
>>
>> John -
>> 	This is an interesting history, but it addresses a different problem
>> from what Andre was proposing. The torpedo application was interested
>> in making it difficult for the enemy to jam or spoof the control
>> signals. It sounds like the torpedo would sync up with the transmitter
>> before launch using a rate offset until sync was achieved. If that was
>> to be the case, then finding a clear channel was not an issue. The
>> question I was addressing was how to sync the receiver to the received
>> SS signal in the real ham world with crowded band conditions.
>>
>> On Dec 27, 2013, at 7:41 49PM, John Donaldson <johnab8yz at verizon.net>
> wrote:
>>> 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 succe
>>> ssful QS
>>> O,
>>>>   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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>>>
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