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Spread Spectrum

Old 10-22-2005, 06:31 PM
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Default RE: Spread Spectrum


ORIGINAL: abel_pranger
There is no "driver circuitry in the receiver" in any of the Futaba, FMA, JR, Hitec, Berg or GWS receivers I use. They all provide logic level signals to the servos, which contain the driver circuitry. The Rx could care less if it is sending signals to a park flyer's pico servos or a 40%er's 'truck winch' servos.
True, as far as the servo signals are concerned.

Using a micro receiver to supply power to large servos is probably not a good idea, because it could overload the micro receiver power traces if the total servo load it excessive.

None of my giant model servos get their power from the Rx - I use a separate power bus to feed the servos.

I think that might have been what SoCalGliderGuider had in mind, but I've been wrong before.
Old 10-23-2005, 06:00 AM
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Default RE: Spread Spectrum

OK fellows train me.

I read in one case that the transmitter selects two discrete frequencies and operates on them.

The second case a fellow says that it spreads the energy across a large spectrum.

These two things conflict.

In the first case this radio is nothing more than an automatic channel selector and calling it spread spectrum doesn't meet what I thought was SS.

In the second case I don't know enough to comment.

I need help!
Old 10-23-2005, 07:31 AM
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Default RE: Spread Spectrum

Lets not rip this apart......... This a giant step in the right direction. It is great that, in the future, you will NEVER need a frequency pin again. as well as never having to scream, " I've been hit". Now when you crash you will know it's your own fault. LOL

mark


Old 10-23-2005, 08:40 AM
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Default RE: Spread Spectrum

OK, this is difficult to explain and difficult to understand. A bit of handwaving and a chalk board is usually required.

The ISM band of frequencies at 2.4 GHz has 80 frequency slots. Each of these slots is 1 MHz wide. This radio set selects two of these 1 MHz slots to operate on.

The global unique ID (GUID) has over 4.2 billion unique codes that are used to spread the energy in each of the 1 MHz frequency slots clustered around 2.4 GHz. The GUID code must be a digital word that is 32 bits long (2 to the 32nd power). It is this code that provides the pseudo-random function that spreads the bandwidth of a low bandwidth control information over a broadband channel. In essence, it takes the information and increases the spectrum bandwidth by a factor of 100. At the same time the signal energy drops by the same factor (in RF terms, this would drop the effective transmitter energy by 20 dB at any frequency in the band) So the signal would appear to just fall away and drop into the noise floor (the noise floor would appear to be slightly elevated, but totally random looking.)

This digital word is mixed in with the control signals and mixed with each of the carrier frequencies which results in the spreading of the signal over each of the 1 MHz frequency slots. This give a broadband signal (about 10 kHz of information spread over about 1 MHz of bandwidth.)

The receiver is uses the same 32 bit code. If it doesn't match up with what the transmitter is using, it rejects the signal as noise (which in essence it is). It does pull the correct signal out of the noise by auto-correlation when the codes match. Correlation has the effect of compressing the broadband signal to a narrow control signal.

I'm sure there are a few of the finer points that I have missed here. It's been about 20 years since I was working with this stuff.
Old 10-23-2005, 11:00 AM
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Default RE: Spread Spectrum

Thank you High Plains for your explanation.

I have some other questions but I will wait till later as I would not want to upset trioval00 with technical details which he apparently doesn't want to hear.
Old 10-23-2005, 11:23 AM
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Default RE: Spread Spectrum

Nice post, HighPlains. I just want to add a short note regarding the 2 frequencies mentioned in Dan's query. You mentioned them "This radio set selects two of these 1 MHz slots to operate on," but didn't mention why. The reason lies in the method chosen to deal with the issue of antenna directivity and blockage from the required direct path (at cm wavelengths) to the Tx by vehicle structure as it maneuvers. Two antennas are used and located such that at least one of them has a direct path view. Here is where the design choice comes in: two receivers are used, vs switching one receiver between the two antennas. It is easier to implement than switching between antennas, at the cost of requiring twice the bandwidth - the entire RF path is redundant. Some other systems do switch antennas to deal with the same blockage issue - the EHF SATCOM suite that is deployed on all USN ships larger than frigates, for example.

Abel

Edited to add antenna directivity. Our usual omni antennas are omnidirectional only in one plane
Old 10-23-2005, 12:43 PM
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Default RE: Spread Spectrum

Highplains and Abel,

Let me see if I can nut shell what you two have said.

The system picks any two available channels out of 80 channels available. It picks two channels due to antenna orientation problems presented by an aircraft.

Within each 1 MHz channel the data is spread 1 MHz wide.

Questions.

If 4.2 billion codes are available, who assigns these codes?

Since the system picks two of 80, does that not mean that there are actually 40 times 4.2 billion or 160 billion possibilities?

Since two receivers are used to receive the two channels, why didn't they use only one frequency if it was just an antenna orientation problem?

I am not trying to bore anyone or be a smart A.., I am just trying to learn.
I was within a few days of buying the AeroSpectra scanner, but if SS is real close then I don't want to waste $900.
Old 10-23-2005, 02:17 PM
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Default RE: Spread Spectrum


ORIGINAL: iflyj3

Highplains and Abel,

Let me see if I can nut shell what you two have said.

The system picks any two available channels out of 80 channels available. It picks two channels due to antenna orientation problems presented by an aircraft.

Within each 1 MHz channel the data is spread 1 MHz wide.

Questions.

If 4.2 billion codes are available, who assigns these codes?

Since the system picks two of 80, does that not mean that there are actually 40 times 4.2 billion or 160 billion possibilities?

Since two receivers are used to receive the two channels, why didn't they use only one frequency if it was just an antenna orientation problem?

I am not trying to bore anyone or be a smart A.., I am just trying to learn.
I was within a few days of buying the AeroSpectra scanner, but if SS is real close then I don't want to waste $900.
Hi Dan-
I'll take a shot at the easy part and and leave the hard parts to HighPlains. I mentioned the antenna directivity and need for a LOS path between Tx and Rx in upper UHF/lower SHF part of the spectrum, and I'm sure your ham background contributes to understanding that. I'm making a guess that your question might be rephrased "why not simply connect various antenna elements together to provide for full spherical coverage and feed the composite signal to the Rx." One reason is that by so doing you are opening up the window of the antenna to noise, equivalent to saying the antenna gain is reduced. S/N is generally of at least as much concern to detection of the signal as is signal strength per se. Due to both practical and statutory limits on power transmitted, that alternative doesn't fit into limited S/N budget the designer has to work with. Another reason is that it would cause phase jitter in the resultant signal in much the same way that multipath does in the lower bands, with similar effect. The spacing between elements would be bounded on the lower end by the size/shape of the vehicle, and thus at least on the order of a wavelength and more at SHF. The respective lengths of the RF path to each antenna element would thus differ constantly as the vehicle changes attitude. The phase differences between pieces of the signal represent another element of noise that can be ill-afforded.
I suspect the design compromise (2 rx's each with their own antenna) made was primarily driven by economics. A viable product must be affordable, and that likely infers a highly eclectic approach to designing it. Modules developed for established appliance markets like cell phones are adapted wherever possible. If those other applications did not need more than one antenna input, a custom approach has to be pursued. Obviously a decision has to be made somewhere in the process as to where the better signal is coming from, and select that path. The design being discussed opted to provide for that selection at baseband, rather than upstream at RF. I would find that option easier myself, but though I have engineering background, it's been a long time since I did it regularly at the 'dirty hands' level.

Abel
Old 10-23-2005, 05:14 PM
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Default RE: Spread Spectrum


ORIGINAL: abel_pranger

I'm sure your ham background contributes to understanding that. Abel
Thanks Abel, nice explanation.

Yes, my electronics/ham does help me as well as fixing and programming digital computers since 64' adds to my digital understanding. However, my RF is a little on the weak side and I have the most experience at 50 MHz and below. I do have a video downlink on 900 MHz and three on 2.4 GHz. I have noticed the signal drop out with the 2.4 GHz systems when a turn is made. So while I have a lot of the basics, it the details I am missing. I like to learn new stuff and I thank you all for your patience and helping me to understand.
Old 10-23-2005, 05:38 PM
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Default RE: Spread Spectrum


ORIGINAL: iflyj3


ORIGINAL: abel_pranger

I'm sure your ham background contributes to understanding that. Abel
Thanks Abel, nice explanation.

Yes, my electronics/ham does help me as well as fixing and programming digital computers since 64' adds to my digital understanding. However, my RF is a little on the weak side and I have the most experience at 50 MHz and below. I do have a video downlink on 900 MHz and three on 2.4 GHz. I have noticed the signal drop out with the 2.4 GHz systems when a turn is made. So while I have a lot of the basics, it the details I am missing. I like to learn new stuff and I thank you all for your patience and helping me to understand.
Hi Dan-

Thank you for being so polite as to not mention that I answered the wrong question! I'm out of my element too, as my recent RF experience deals with military comms systems operating in the SHF-high and EHF bands. Lots of spread spectrum stuff, but it's all frequency hopping, unlike the system under discussion. I frankly don't know what is more prevalent in the commercial apps.

In my haste to reply before the SD/Philly game got underway, I read your question wrong. You asked why not one frequency, rather than why not one Rx. I shudda taken the "If 4.2 billion codes are available, who assigns these codes?" issue. The answer to that is nobody, it's just the number of possible bit permutations given the fixed length of the codeword.
Back to the issue of two frequencies vs. one, I don't have a clue as to the real motive of the designer(s) or the constraints they faced. I suppose in theory, 2X the bw allocated overall should ideally relate to a 3 dB improvement in robustness of the system. Can't begin to guess how much of that might be actually realized, though. Seems it would take some serious analysis with statistical variables like range-rate issues previously mentioned factored in. Maybe the answer is intuitive: something between 0-3 dB, good enough 'cause it doesn't cost much. It doesn't cost too much if 40 available channels is good enough.

Abel


Old 10-23-2005, 06:46 PM
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Default RE: Spread Spectrum

This might be a dumb question.. But has the AMA ok'ed the use of these systems at ama fields? I bring this up becuase I mentioned these were out now at a meeting today at the field and the "old gezzers" had a fit that the ama hadnt told them anything about them and wanted to out law them unless I could find something that says the AMA ok'ed them
Old 10-23-2005, 07:39 PM
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Default RE: Spread Spectrum

Great explaination Abel. It's been a long while since I've heard anyone discuss phase jitter, signal to noise ratio and noise fiqure. I wonder if the receiver(s) use I and Q quadrature dectection, the type of filters and how they deal with differental group delay or if it is all DSP now. At any rate, I am amazed they can offer so much technology for so little cost to the consumer. If you remove $160 (the cost of a computer radio) your left with $40. At wholesale this is maybe $15 for a lot of development.
Old 10-23-2005, 08:28 PM
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Default RE: Spread Spectrum

Hi,
It's still in the development phase for aircraft. As discussed in this forum, there's still a range problem with aircraft, especially at different orientations of the model relative to the TX. I also read that quick change of direction can also affect the system. IMO, eventually it'll all be worked out and probably be released as a high end system and used mostly IMAC and jet modelers. After a period of time it will filter down to the sport radio's.
Being a geezer myself, I'm not sure what their objection would be. Maybe someone with some knowledge on the subject should explain it to them, I just don't see a downside, except cost.
BRG,
Jon
Old 10-23-2005, 09:02 PM
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Default RE: Spread Spectrum


ORIGINAL: F106A

<snip>I'm not sure what their objection would be. Maybe someone with some knowledge on the subject should explain it to them, I just don't see a downside, except cost.
BRG,
Jon
Jon-

I agree there is no downside, in that the possibility of interference with pre-existing radio systems is zip-zero-squat. Not a technical issue but one of AMA policy and so, for drvcrash, Steve Kaluf is the man with the answers; he left his Email addy in a prior post to this thread.

Abel
Old 10-23-2005, 09:11 PM
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Default RE: Spread Spectrum


ORIGINAL: drvcrash

This might be a dumb question.. But has the AMA ok'ed the use of these systems at ama fields? I bring this up becuase I mentioned these were out now at a meeting today at the field and the "old gezzers" had a fit that the ama hadnt told them anything about them and wanted to out law them unless I could find something that says the AMA ok'ed them
Tell them to read the AMA Safety Code. What's good for the FCC is good for the AMA. The AMA doesn't "Okay" the frequencies we use now, the FCC does.

I've got a "geezer" in my club trying to stir up a fuss about these systems too. WHY WHY WHY WHY WHY WHY WHY WHY WHY???? What possible motivation could these people possibly have to object to the biggest advancement in R/C radio systems in nearly thirty years? Heck, I just about had to backhand one of the YOUNGER members for drinking the geezer's Kool-Aid today! [:@] [:@] [:@] [:@] [:@]

2.4GHz is nowhere near the old 72MHz band, and these radios have built-in frequency control. How can this be a BAD thing? What about no more worries about park flyers on the other side of the hedge row shooting you down? What about no more conflicts over who has the pin? What about no more accidental shoot-downs?

I guess I just didn't drink the Kool-Aid.
Old 10-23-2005, 09:13 PM
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Default RE: Spread Spectrum


ORIGINAL: drvcrash

This might be a dumb question.. But has the AMA ok'ed the use of these systems at ama fields? I bring this up becuase I mentioned these were out now at a meeting today at the field and the "old gezzers" had a fit that the ama hadnt told them anything about them and wanted to out law them unless I could find something that says the AMA ok'ed them
I believe the AMA says if it is a legal frequency to use for R/C, it's OK with them.
Old 10-23-2005, 09:18 PM
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Default RE: Spread Spectrum


ORIGINAL: abel_pranger

I shudda taken the "If 4.2 billion codes are available, who assigns these codes?" issue. The answer to that is nobody, it's just the number of possible bit permutations given the fixed length of the codeword.

Abel
Ok, that is the one that will get me. Out of 4.2 billion codes, I'm the one that will end up with the same code as someone else at the field. Now, where is that frequency board with 4.2 billion slots?
Old 10-23-2005, 10:51 PM
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Default RE: Spread Spectrum


ORIGINAL: iflyj3


ORIGINAL: abel_pranger

I shudda taken the "If 4.2 billion codes are available, who assigns these codes?" issue. The answer to that is nobody, it's just the number of possible bit permutations given the fixed length of the codeword.

Abel
Ok, that is the one that will get me. Out of 4.2 billion codes, I'm the one that will end up with the same code as someone else at the field. Now, where is that frequency board with 4.2 billion slots?
AMA is working on it. Targeted cost is $420,000,000/unit, quite a bargain when considered on a per slot basis. Get your order in with your 2006 renewal and they'll let your club lock in the price.

Abel
Old 10-23-2005, 11:36 PM
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Default RE: Spread Spectrum

10 cents a slot!

Wow, every 2nd person on the planet can fly now without interference.
Old 10-24-2005, 11:17 AM
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Default RE: Spread Spectrum

One thing to be aware of, which may help explain the limited range....

The path loss of a radio signal between a transmitting antenna and receiving antenna is proportional to frequency squared. At 2.4 GHz the path loss is 1100 times greater than the path loss at 72 MHz (.072 GHz). This is a whole lotl! Path loss is a result of physics and cannot be changed.

Some, but not all, of this extra loss is probably made up in the SS receiver through processing gain, and some can be made up by more efficient antennas. There is still a large deficit to overcome.

It is not by accident that our RC systems operate at 72 MHz! 72 MHz is a good compromise between antenna efficiency and path loss. Now if we could only do a spread spectrum system at 72 MHz.....!


Old 10-24-2005, 02:45 PM
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Default RE: Spread Spectrum


ORIGINAL: JPMacG

One thing to be aware of, which may help explain the limited range....

The path loss of a radio signal between a transmitting antenna and receiving antenna is proportional to frequency squared. At 2.4 GHz the path loss is 1100 times greater than the path loss at 72 MHz (.072 GHz). This is a whole lotl! Path loss is a result of physics and cannot be changed.

Some, but not all, of this extra loss is probably made up in the SS receiver through processing gain, and some can be made up by more efficient antennas. There is still a large deficit to overcome.

It is not by accident that our RC systems operate at 72 MHz! 72 MHz is a good compromise between antenna efficiency and path loss. Now if we could only do a spread spectrum system at 72 MHz.....!
Good points, JP-

The rule you cited works well across the VHF and UHF bands, so is valid in comparing the 72 MHz path loss to that at 2.4 GHz. Don't try it for wavelengths under 20 mm though!
Your post should remind folks already daydreaming about using the Spektrum radio being marketed for park flyer application to larger models - better be thinking about effective range as well as bigger servos. The 3D guys might have a stronger case for flyin' up close and personal!

Abel
Old 10-25-2005, 10:41 AM
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Default RE: Spread Spectrum

Actual gound level test has resulted in 0.7 miles before serious glitching. See:

[link=http://www.flyingcirkus.com/forum/Flew_and_Biff-_tested_JR_Spread_Spectrum/m_96209/tm.htm]Crikus Report[/link]

This is at 1/10th watt. FCC allows these 2.4 GHz units to transmit one watt. Note that a full flying field was the back ground for the test.

About the sequency: You can have the same "code" as another flyer as long as your timing is different than his. Even if you accidentally hit the same minor frequency (sequence) the redundancy of the data stream (CRC checks and such) will just throw that part out. Since that "part" is one thousandths of a second (or less) you will see no impact on your eye-hand closed loop control. the high end miltary stuff uses adaptive logic. If there is a big broad band frequency spike in the middle of the spectrum being used, the adaptive logic shifts to the outer bands. Also if there are too many hits during the sequency the adaptive logic changes the sequency or even goes to random shifting.

As to wither your flying field will allow these is really the meat of the argument. No reason not to as per the manufacturers suggested use. Actually would be safer for all flyers.
Old 10-25-2005, 11:41 AM
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Default RE: Spread Spectrum


ORIGINAL: SoCal GliderGuider

Actual gound level test has resulted in 0.7 miles before serious glitching. See:

[link=http://www.flyingcirkus.com/forum/Flew_and_Biff-_tested_JR_Spread_Spectrum/m_96209/tm.htm]Crikus Report[/link]

<snip>
That shows range should be ample for a larger sport model...........just so you leave the model sitting on the ground.

Abel
Old 10-26-2005, 10:38 PM
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Default RE: Spread Spectrum

ORIGINAL: drvcrash

This might be a dumb question.. But has the AMA ok'ed the use of these systems at ama fields? I bring this up becuase I mentioned these were out now at a meeting today at the field and the "old gezzers" had a fit that the ama hadnt told them anything about them and wanted to out law them unless I could find something that says the AMA ok'ed them
Excerpt from article by Dan Williams (D II frequency coordinator) referenced by Steve Kaluf in a piece on the AMA site:
*******
Q: Will the AMA Safety Code change, and
when will I be able to fly my models at my
AMA chartered club with SS?
A: AMA’s goal with the system testing is to
make changes to the Safety Code as
necessary to accommodate the new systems.
The Academy wants to encourage the
technology—not hinder it.
The code reads, “I will operate my model
aircraft using only radio-control frequencies
currently allowed by the Federal
Communications Commission … ” These
frequencies are currently authorized, so there
should be no problem as long as the club
doesn’t have additional safety rules regarding
the use only of our current frequencies.
*******
Kaluf's piece is here: http://www.modelaircraft.org/spreadspec.asp
Williams' piece is here (includes more from Kaluf): http://www.modelaircraft.org/comp/Sp...m/Spectrum.pdf
Old 10-27-2005, 12:41 AM
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Default RE: Spread Spectrum

Not sure why Horizon does'nt have them yet, I saw one at my LHS 2 days ago...so they are definately out there...maybe I'll have to go back for a closer look...

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