"Hedy Lamarr style" - Frequency Hopping

[MichaelLAX]

Does anyone have any experience with this feature on GMRS?

I notice that on Randy's YouTube review of the Radioddity GM-30, he said he could not get that feature (Menu Item 39) to turn on, and a friend I have met on this website, also confirms he cannot turn on this feature on his GM-30.

I have the clone Pofung P15UV and I can indeed turn it on, Hopping RX (Menu Item 39), but so far I have no one to test it with.

Both the Radioddity and the Pofung Manual describes this function briefly (although the Pofung Manual incorrectly claims it is Menu Item 40): 

 

[Lscott]

The questions I have are first is this even legal on GMRS? Second with the small number of channels available just how effective would it even be? I don't think you would want to be doing this on the repeater input channels. The low power FRS ones are sort of useless which leaves just channels 1 to 7 to use.

The frequency hopping radios I've heard about are digital, and digital voice is not allowed on GMRS anyway.

[MichaelLAX]

The Radioddity GM-30 and Pofung P15UV are clearly not digital voice, so your concern is misplaced on that issue.

Acknowledging your second concern, I think you mistakenly left off the repeater output channels, which are clearly available for simplex operations, too. Fifteen channels to use for frequency hopping is more than enough.

As to your first concern, do you have any information that it is not allowed under the FCC Rules? Why would you assume otherwise?

[MichaelLAX]

I just LOVE this comment on another thread: 

On 7/22/2021 at 12:53 PM, generalpain said:

There are actually 22 GMRS frequencies. How did you even pass the FCC GMRS exam?

[Radioguy7268]

Just because the FCC rules do not specifically prohibit an intended use, does not mean that the rules allow that use.

Please tell me how you expect to monitor the frequency to determine if it is clear to transmit on - before your radio automatically "hops" to the next channel? Do you just assume that the entire GMRS band is clear and available for your sole use in the area of your operation? Or does the radio go ahead and transmit over top of any current users, with the idea that since you'll be hopping to another channel in just a moment, the brief interference is annoying but acceptable?

If your fancy frequency hopping radio actually does monitor prior to transmit - how does the radio you're talking to know what frequency it should be moving to in the event that the intended channel it was supposed to hop to is now occupied?

Also, once you're done your conversation, are you going to manually go back and clearly identify by callsign on each one of the channels which your radio just transmitted on? Or, does your fancy Wouxun/Radioditty/Pofung take care of identifying for you?

If you can't answer those questions, I don't see how you could say that the FCC allows that use. The radio may carry a "Type 95 Certification" label, but that doesn't mean that the FCC actually tested the feature in question and approved it for GMRS use.

 

[MichaelLAX]

Thank you for your thoughtful questions:

I wouldn't consider a $30 radio "fancy," but your mileage may differ.

The technology checks to be sure the frequency is clear before transmitting on it and yes, I plan to "ID" at the end of every transmission.

It is a small price to pay to be able to use this feature!

How does it know?  That is the essence of the patent granted to Hedy Lamar (now expired) for her WWII invention! 

As to your statement:

Quote

If you can't answer those questions, I don't see how you could say that the FCC allows that use.

I don't believe I ever affirmatively said that the FCC rules do allow that use.  I asked Lscott why he assumed otherwise.

[Radioguy7268]

Heddy envisioned using a player piano roll with pre-programmed punchouts that would determine the cycle of hops. A pseudo random code at best. Heddy assumed that any interference on a given frequency would be overcome by hopping to the next channel. She wasn't trying to avoid creating interference, she was trying to deal with interference by getting the most information through, and providing a moving frequency target for the enemy jammers.

Transmitting your ID on the last channel transmitted on does not meet the FCC rules as I read them. You would need to transmit ID on each and every channel you transmitted on during the prior 15 minutes. Without hopping during said ID.

 

[MichaelLAX]

5 minutes ago, Radioguy7268 said:

Transmitting your ID on the last channel transmitted on does not meet the FCC rules as I read them. You would need to transmit ID on each and every channel you transmitted on during the prior 15 minutes. Without hopping during said ID.

 

I said I would ID on EVERY transmission; I do not believe it hops during transmission.

[MichaelLAX]

7 minutes ago, Radioguy7268 said:

Heddy envisioned using a player piano roll with pre-programmed punchouts that would determine the cycle of hops. A pseudo random code at best...

Amazing how technology has progressed in 75 years! 

[Lscott]

5 hours ago, MichaelLAX said:

Thank you for your thoughtful questions:

I wouldn't consider a $30 radio "fancy," but your mileage may differ.

The technology checks to be sure the frequency is clear before transmitting on it and yes, I plan to "ID" at the end of every transmission.

It is a small price to pay to be able to use this feature!

How does it know?  That is the essence of the patent granted to Hedy Lamar (now expired) for her WWII invention! 

As to your statement:

I don't believe I ever affirmatively said that the FCC rules do allow that use.  I asked Lscott why he assumed otherwise.

It wasn’t clear so I was questioning it.

I would have to go and double check on this point but even on the Ham bands the FCC specified the spreading code was allowed at one time. That might have changed over the years.

https://www.law.cornell.edu/cfr/text/47/97.311

https://files.tapr.org/tech_docs/Ham_Ethernet_GBPPR.pdf

I have a book on my shelf written by TAPR, Tucson Amateur Packet Radio Corporation, with the title of “Spread Spectrum Update - tales from the rebel alliance”. The book covers spread spectrum technologies and some of the basic math behind it. This also includes issues with interference etc. The book was published in 1998 so a few things have likely changed since then. It’s primarily aimed towards Ham radio of course.

[MichaelLAX]

Thank you for those two articles, but I do not read anything in there that would indicate that Frequency Hopping is prohibited for GMRS.

I would suggest that I could have a pre-arranged chart of random GMRS channels with another user and at the end of each transmission, just before I identified, I would say: "Channel X" where "X" is the channel listed on our pre-arranged sheet, but coded for any other listeners.

And then we would move to that next channel.

It seems that the Frequency Hopping feature just implements that scheme automatically.

Just a guess on my part, since I do not have a 2nd HT to try it out.

[Lscott]

The permitted emission types for GMRS are listed here:

https://www.law.cornell.edu/cfr/text/47/95.1771

The FCC emission designations are listed here:

https://www.electronics-notes.com/articles/radio/modulation/itu-radio-emission-designators.php

And yes the FCC does in fact use the ITU emission designations as do many countries.

Note under "Character 5 details of multiplexing" letter indicator "C". This does not appear in any of the permitted emission designations for GMRS. Depending on which source you consult FHSS, frequency hopping spread spectrum, could also be designated as an "X" in the emission type, a type not covered by any of the other types. This also does not appear in the permitted emission designations for GMRS either.

Then the very name "spread spectrum" of course implies the signal is spread out over a range of frequencies. The permitted bandwidths on GMRS are only 20Khz and 12.5KHz. Having a signal hopping around many times per second would be effectively a wide band signal exceeding the permitted bandwidth. Again, IMHO, I think this would exclude the use of FHSS on GMRS. 

If somebody has a different opinion that's what the form is here for, a friendly discussion and debate.

[MichaelLAX]

I’ll read the link later, but I think you are confusing the simple Frequency Hopping feature of these HTs with the much more complicated: FHSS

this is no different than changing channels after each transmission, an example of which I gave above. 

[BoxCar]

15 minutes ago, MichaelLAX said:

I’ll read the link later, but I think you are confusing the simple Frequency Hopping feature of these HTs with the much more complicated: FHSS

this is no different than changing channels after each transmission, an example of which I gave above. 

Michael, it's you who is not understanding. The FCC has stated what emissions are allowed and if it isn't one of the listed emission types it is not allowed. You are reading the rule incorrectly by stating "It doesn't say it's not allowed" instead of seeing what is allowed.

[MichaelLAX]

that's possible, but I am not at my computer right now and it is much too difficult to read these rules on my iPhone.

But thank you for pointing that out, except I never said: "it doesn't say it's not allowed."

My point was in response to Lscott's comment:

Quote

Then the very name "spread spectrum" of course implies the signal is spread out over a range of frequencies. The permitted bandwidths on GMRS are only 20Khz and 12.5KHz. Having a signal hopping around many times per second would be effectively a wide band signal exceeding the permitted bandwidth. Again, IMHO, I think this would exclude the use of FHSS on GMRS. 

I have never said that the feature on my Pofung P15UV utilizes "spread spectrum," and perhaps he was confusing the two features.

But no point in debating before I read the rules he has linked.
 

[Lscott]

34 minutes ago, MichaelLAX said:

I’ll read the link later, but I think you are confusing the simple Frequency Hopping feature of these HTs with the much more complicated: FHSS

this is no different than changing channels after each transmission, an example of which I gave above. 

It's different because it happens under automatic control and is an intrinsic feature of the transmitter's frequency and modulation circuits, and not subject to operator control on a per frequency hop, nor even what the hop pattern that's implement, fixed or pseudorandom.

[MichaelLAX]

17 minutes ago, Lscott said:

It's different because it happens under automatic control and is an intrinsic feature of the transmitter's frequency and modulation circuits, and not subject to operator control on a per frequency hop, nor even what the hop pattern that's implement, fixed or pseudorandom.

Unless it is based upon a frequency hopping algorithm that is matched by both HTs, and just utilizes the FM modulation permitted by the FCC for GMRS, which I believe is the case with these HTs.

Further, I believe they are not using Spread Spectrum, as I indicated before. Spread Spectrum in a $30 HT?!?  I think not...

[MichaelLAX]

I have read the two links you provided:

I believe my HT utilizes F3E for sure, with the possibility that the Frequency Hopping feature might use some or none of the features of F1D and F2D; all of which are approved by the FCC for GMRS.

Again, I do not believe that Spread Spectrum is implemented in this $30 HT.

[Lscott]

3 minutes ago, MichaelLAX said:

Unless it is based upon a frequency hopping algorithm that is matched by both HTs, and just utilizes the FM modulation permitted by the FCC for GMRS, which I believe is the case with these HTs.

Further, I believe they are not using Spread Spectrum, as I indicated before.

Frequency hopping IS a form of spread spectrum transmission. Regardless of what algorithm is used to determine the next frequency hop some communication needs to take place between the two radios so they remain synchronized. Otherwise the RX radio would need to monitor several frequencies and try to "guess" where in a fixed sequence the TX radio is currently.

I did read about some radios used during Vietnam that used encryption, and I think some form of FHSS, and wasn't well received. The chief complaint was the delay in decoding the audio and for the radio's to sync up. The second(s) delay was critical under battle conditions where rapid communication was necessary.

https://en.wikipedia.org/wiki/AN/PRC-77_Portable_Transceiver

Types of spread spectrum modulation.

https://en.wikipedia.org/wiki/Spread_spectrum

[MichaelLAX]

According to a reviewer on Amazon (whom I have no idea if he really knows what he is talking about):

"Another interesting feature is the Hopping RX function of the GM-30. This does not actually hop frequencies, instead it secretly assigns a CTCSS/DCS tone but won’t display it in the settings...

"As a side note, it’s actually a good thing it doesn’t hop frequencies. Since GMRS radios have a very limited list of TX frequencies, you wouldn’t get too far, and it wouldn’t be that secure. Not to mention it would be generally poor radio etiquette since you would more than likely end up transmitting over other stations as you’re hopping around. So as far as creating the idea of it, it’s pretty neat. But to reiterate, I’m not too sure how practical it will be."

So getting back to my original reason for posting this thread; it was not to debate whether or not it is allowed given the FCC rules, it was: 

Quote

Does anyone have any experience with this feature on GMRS?

UPDATE: Lscott and I posted at the same time, but I think my comments in this post render our previous discussions moot. But thank you, as always you raise intellectually challenging information! 

[Lscott]

We don't have to agree, but the discussion was beneficial for those that choose to just sit back and read.  If it got people to think about other points then it served it's purpose. 8-)

[Lscott]

Oh, if anybody is interested in that book I mention in a prior post you can download a PDF copy from here:

https://dokumen.pub/qdownload/tapr-spread-spectrum-update-tales-from-the-rebel-alliance-9780964470750-0964470756.html