How many 2.4 GHZ system operating does it take to cause a lockout with the spekturm!

[DoctorAudio]

I have been reading and testing. What I would like to know is it possible for the other 2.4 GHZ systems to interfere with the Spektrum system. What would it take for one to be interfered with and cause a lock out?
The Spektrum uses two separate frequencies to transmit and rec. Others use many more. I wonder if that was a plan by the industrial leader in 2.4 GHZ technology (Futaba).
If a Futaba system would ping a freq. that is being used by the Specktrum would it try and use the other freqs. So what’s the chance of a Futaba to randomly switch to the two freqs that a spektrum is using and cause a lockout?

I guess that in the future we will all find out the hard way. I have had many years of testing out different theories and helping to come up with solutions. So here, I am posing a question what could happen. With all the manufactures coming out with products that run in the 2.4 GHZ range, it is bond to happen and mostly it will happen at some meet or fly-in. I know it is not likely to happen all the time. But for all that have spent the money to eliminate, glitches. They may have pushed themselves in to a corner, which would cause to loose interest in the 2.4 GHZ glitch –free zone.

I would love to take a tally that would show how many 2.4 GHZ systems it takes for one to interfere with the Spektrum system. But with, everyone one switching over it is bond to happen. Sooner or later!

[int2str]

How many people can use a Wireless access point at the same time?
How many people can use Bluetooth at the same time? (7 per device; but how many nets in range?)

There is a theoretical limit to all of this, but for all practical intends and purposes they do not apply.

Due to the digital encoding and GUID based binding, the chance of somebody sending to your receiver are about 1 in 4 Billion (IIRC). So let's rule that one out.

However, due to the limited nature of "channeling" sub-frequencies, you end up with a limited number of "channels" within 2.4 Ghz as well. How many? Who knows. But it would take quite a few, in very close proximity to flood the airwaves enough for the receivers not to be able to "lock on" to a given channel anymore.

We're at the beginning of all of this as well. I'm sure many more "DSM 2" (Spektrum) receivers can be next to each other than the earlier "DSM" (1?). As the technology matures, the receivers become more sensitive and more sub-channels become available.

And if you keep following the Wifi trend, we'll be looking at "Mimo" (multiple-in/mutiple-out or mixed-mode) system down the road.

Imagine the DX7-2012 model having 3 stubby antennas instead of one :D

[DavidH]

http://www.helifreak.com/showthread.php?t=65871
ClayK wrote:
Actually, that statement is misleading (borderline). Using modules in a system doesn't automatically add latency. A couple ms in latency isn't going to be noticeable to anyone. Until latency hits about 20 ms or larger, Of course, if you like getting staying with one radio technology without the possibility for future upgrade, by all means, get the radio that is a single band only. Personally, I like having the flexibility of choosing whatever band I want.

Second, there were some issues at BHam with a Spektrum system or two. Those issues may be model specific and not radio related, but nothing is perfect, so don't believe that 2.4 makes your radio system immune to any RF.


There were 175 registered pilots at the event. I would say there were 155-160 2.4ghz radios in use. At any given time there were probably 40-50 2.4ghz radios on at the same time.

from another forum
http://www.runryder.com/helicopter/t418414p4/
mavrick

In regards to Marcus’s flight I hate to say it but he locked out during that flight so I guest the dx7 is not bullet proof on 8.4v volts. I know this for fact as Colin bell was flying off to the right of Marcus, there where a few people standing between the two of them that I know that heard Marcus yell that he didn’t have it and then the heli hit the runway hard

David

[JKos]

One needs to understand more about spread spectrum to understand the whole situation. From a pure spectrum/frequency perspective, Futaba and Spektrum do not use the same "channels." Futaba uses 36 channels spaced further apart than Spektrum's 80 channels. The spreading codes, chip rate, and bandwidth used are different; the brands cannot control each other and simply look like noise to each other.

The systems do not transmit all the time. They actually have a relatively low duty cycle which means they are off more than they are on. So, two or more systems must be both transmitting and on the same small chunk of spectrum at the same time to have overlapping transmissions. But overlapping transmissions do not mean the data is not received. That is the beauty of spread spectrum. Multiple systems can easily operate on the same "frequency"/"channel" with zero issues. Reference cell phones as a great example.

While it is guaranteed that the hopping systems such as FASST will collide, it does not mean the data packets will be lost in every collision. While it is guaranteed the hopping systems will collide with Spektrum systems, it does not mean either system will lose a packet every time.

- John

[Pinecone]

Fly R/C magazine did a test with an X9303. They turned on and linked 44 Dx7s, then turned on the flew the X9303. It worked.

Realize, when you get to spread spectrum, you can deal with situations where the signal to noise ratio is upsidedown. IOW the signal is quieter than the noise. GPS works that way.

A WHOLE different way of thinking.

BTW WiFi "channels" actually overalap each other. :)

[DoctorAudio]

Great points made by all.
I do believe, that this is a problem that we will see come to the top.
What I am doing is posing the questions, so that we may find a solution that will not cost us more money in the long run!
If we are going to get thru this, we must take in account when lockouts happen. I know guys who have more money in the models than I do, and by far have more at stake.
But, I don't feel that these guys will suffer from the negative aspects of the spread spektrum technology. Here in the Northwest we have a far less amount of RC flyers then some other areas. We are much more spread apart; this is what I am saying.
The only time it will be of some serious notice, is when we all meet up for a Fun-Fly or a meet. I guess what I will do is keep my 72MHZ system ready to be dropped in when I see to many 2.4GHZ system in the air! I have a couple planes and helicopters that are still on 72MHZ. So I guess I have a back up and the option to choose.

Thank you guys for reading and your input in this thread!
Elvan Wilson II

[DavidH]

Here is what Spektrum was saying in early 2007
http://www.runryder.com/helicopter/t319060p2/

2/02/2007
This is off of Spektrum's site


What happens if the band is full (80 users for surface or 40 users for aircraft) and I turn on my transmitter?

In the unlikely event that all channels are occupied, the next transmitter will scan the band indefinitely until open channels are available. The transmitter will then acquire the channel(s) and begin transmitting. Only then will the system connect.


So yes, only 40 possible at a time.


Impound may not be a requirement by the AMA but that doesn't mean that a specific event can't impose an impound. As more and more Spektrum radios are used, and especially at very large events like IRCHA, there will eventually be a problem if some type of impound isn't used. There will need to be some way of governing how long a person has their radio on since it will affect the guy's who are flying as well as the guy's doing setup.


Now here is the same question with the answer a little different than the answer a year earlier
http://www.spektrumrc.com/FAQ/tm.aspx?m=70
What happens if the band is full (80 for surface or 40 for air) and I turn on my transmitter? - 12/28/2007 1:58:52 PM



In the unlikely event that all channels are occupied, the next transmitter will scan the band indefinitely until open channels are available. The transmitter will then acquire the channel(s) and begin transmitting. Only then will the system connect.

< Message Updated on 1/2/2008 9:52:09 AM >


Maybe something changed in a year that they could have more than 40 of the Spektrum systems on at one time. But a year ago Paul Beard was stating only 40 Spektrum would be able to connect and be used at the same time. That any other Spektums turned on, simply would not connect till it found open channels.
From the 9303X 2.4ghz manual page G-9
Specifications
• Model number—Airplane/Sailplane (JRP2910), Helicopter (JRP2920)
• Number of channels—9
• Modulation type—Direct Sequence Spread Spectrum DSM2/ DSM1 protocol
• Band—2.400 to 2.483GHz
• Spectral capacity—40 simultaneous systems
• Transmitter Current—180mA/DSM2 280mA/ DSM1
• Resolution—2048 w/R921 receiver

Maybe the magazine had a special radio since it was able to use more than 40 simultaneous systems together.
David

[Finless]

That was the way I understood it. The TX under the specifications for 2.4 must scan until it finds an open channel before it transmits. So for this reason unless there is a software problem in the radio another TX walking on one that is already transmitting shouldnt happen.

Bob

[JKos]

David,
Obviously the 41st through 45th radios simply found channels which they determined to be free. Without knowing more about the test execution, it's hard to speculate beyond that. If you spread out the radios across a flight line like at IRCHA, perhaps the radios at each end don't even notice the other end's radios.

- John

[JKos]

> The TX under the specifications for 2.4 must scan until it finds an open channel before it
> transmits.

Simply not true. You will not find that anywhere in the FCC documents under which our 2.4 GHz radios fall.

- John

[DavidH]

David,
Obviously the 41st through 45th radios simply found channels which they determined to be free. Without knowing more about the test execution, it's hard to speculate beyond that. If you spread out the radios across a flight line like at IRCHA, perhaps the radios at each end don't even notice the other end's radios.

- John

John,
All I can do is read what the people that designed and built the radio system say. It is said in more than several places that if 40 Spektrum systems are operating. That the 41st when turned on will have to wait till there is a clear channel for it operate. I am not speculating, just reading what Paul Beard has said several times and reading what is in the instruction manuals for the specs of the system. If I can't believe all of that, I really don't know what is correct then.
If the 41st thur 45th radios found clear channels. I just don't understand how they did it under what has been told about how the system operates.

David

[JKos]

David,
When the radio scans the channels, they either think they see an unacceptable signal level or they declare the channel "clear." If a channel is declared "clear," it can choose that channel. There is some threshold at which the scanning process determines which side of the fence a channel lies on. Think of it like the squelch control on a CB radio. If the channel is above the squelch level, that channel is deemed already in use.

If Spektrum tx B does not "see" tx A on channel X (i.e., the detected level is below the threshold), it will feel free to use that channel. What we don't quite know is just how far tx A has to be from tx B before tx B doesn't "see" tx A. Nor do we know the test conditions under which the successfully used 45 Spektrum tx's at once.

> If the 41st thur 45th radios found clear channels.

That's not an "if", they had to have found two channels they deemed clear (enough).

- John

[helisforfun]

Has any one seen this May 2008 Fly RC Testing with 40 DX 7s turned on?

Link to a scan of the article
http://gallery.mac.com/bobsweb#100112

[int2str]

Neat article.

2.4Ghz is easily one of the best things that has happened to the RC hobby!

- No unsightly antennas anymore
- No frequency pins
- No glitching (only blackouts :D)
- No huge TX antennas to bend :D

I've been in the radio hobby for many years now, and it's made it so much nicer!

[Pinecone]

The issue is, spread spectrum is not anything like you are used to with narrow band. The signal is spread out over 1 MHz or more of bandwide and hashed with the GUID. So to another Spektrum radio, others radios are just noise. If there is too much noise the Tx won't use that "channel" but if the noise floor is low enough, it will use that channel.

With spread spectrum the signal can actually be below the noise floor and still work. GPS works like this. It is like talking to someone at a loud party. If you measured the dB, the party would be louder than the person talking, but you can still hear and understand them.

Another case in point, the WiFi channels, actually overlap one another.

Coming from a background with electronics and ham radio, spread spectrum makes my head hurt. :)

Also the duty cycle of Spektrum is only about 10%. So 90% of the time they aren't transmitting. So there is the possibility of systems operating on the same channel due to not actually interfering by not transmitting at the same time.

So at some point there will be a need to impound 2.4 systems, or at least develop some form of control of the number of units turned on. But the ones turned on later, will be the ones where the overall noise floor has been raised to the point that they will not find a "free" channel and will not transmit. The ones already linked should not be effected.

But enough systems on, and fades and lost packets might occur at some interval.

[Danal Estes]

So at some point there will be a need to impound 2.4 systems, or at least develop some form of control of the number of units turned on.

Not really. Consider the most recent IRCHA. Largest RC even ever of any kind. 712 registered pilots. Multiple flightlines. Vast majority of people flying 2.4G (only 150 72Mhz radios impounded). No issues.

Physical demonstration that impound is unimportant for 552 2.4G radios at one facility. Radios were mostly Spektrum with a few Futaba and XPS mixed in. Everything just worked. Especially impressive when you realize heli people do more setup than most (thus increasing the odds of radios being on in the pits as well as at each pilot station).


Again, this was the largest RC event EVER, and OF ANY KIND. If they didn't impound 2.4G, why should anybody else?

[Danal Estes]

I would love to take a tally that would show how many 2.4 GHZ systems it takes for one to interfere with the Spektrum system. But with, everyone one switching over it is bond to happen. Sooner or later!

No, it isn't "Bound" to happen. These things tend to self limit. How many pilot stations at the field? How many people in the pits? As you can see above, 550+ people in the pits with multiple flight lines isn't enough.

You just can't get enough "density" in the real word to have this problem.

Spektrum systems will never interfere with one another; the Xth system, when scanning for clear channels, will just not find any and therefore not transmit. Futaba won't interfere with Spektrum (and vice versa) because Futaba hops every 4 milliseconds (or so, they are a bit unclear in their technical doc) and therefore they can't 'stay' on the Spektrum's 'channels' (a really misleading term, by the way). And on and on... they are all mutually non-interfering in any realistic numbers. You'd need hundreds or thousands of people standing shoulder-to-shoulder to create a problem... and then there wouldn't be enough room for their aircraft!!

Spread Spectrum is, as pinecone said, very weird stuff. In some ways, the more background in RF you have, the worse off you are to understand the strengths and limitations of SS. Start by reading about GPS, and then about SS theory in general. Here's one resource: http://www.maxim-ic.com/appnotes.cfm/appnote_number/1890

[DavidH]

Not really. Consider the most recent IRCHA. Largest RC even ever of any kind. 712 registered pilots. Multiple flightlines. Vast majority of people flying 2.4G (only 150 72Mhz radios impounded). No issues.

Physical demonstration that impound is unimportant for 552 2.4G radios at one facility. Radios were mostly Spektrum with a few Futaba and XPS mixed in. Everything just worked. Especially impressive when you realize heli people do more setup than most (thus increasing the odds of radios being on in the pits as well as at each pilot station).


Again, this was the largest RC event EVER, and OF ANY KIND. If they didn't impound 2.4G, why should anybody else?


Yes there was 712 registered pilots. But that does not mean every one there was a pilot. Vendors were included in the count along with wives, girlfriends. There was probably well over 200 that were on the registration list that didn't even have a radio there. I know I was registered. I did not even have a heli or radio at the event. Most of the vendors and booth workers had nothing to fly.

I just had an event with 175 registered pilots, All but about 15 of them were actually flying at the event. There were 15 72 mhz radios in the impound. There were some reported issues with 2.4 ghz at times. At any given time there was probably 30-40 2,4 ghz on at the same time.


David

[Pinecone]

If you have an event with several hundred 2.4 radios there, you will get to the point where you will have to control the number turned on.

As you say, they won't interfere, but the new ones won't find clear channels. So you have an event where no one can fly because so many radios on in the pits, that the flight stations can't get free channels. :)

[Danal Estes]

Points well taken about absolute counts and who was flying, etc. Still, this was the largest RC event of any kind ever in history, and non-impound (for 2.4) was a non-problem.

Also, I've been to several events with registration numbers in the 70 to 110 range, only a very very few 72Mhz, LOTS of pilots, lots of planes/helis, and LOTS of flying, no impound. No issues.

The day somebody is standing at a flight station and can't link up, we need to discuss impound. Until then, I'll take these events as physical proof that impound is not needed for 2.4g radios.

Impound is also a PITA for the event organizers/workers. ;)

[DavidH]

The day somebody is standing at a flight station and can't link up, we need to discuss impound. Until then, I'll take these events as physical proof that impound is not needed for 2.4g radios.


That was one of the issues I was referring to as happening. It is a reality. It will and does happen.

I agree impound is a PITA for event organizers. But now with 2.4ghz they are presented with another problem. People just walking up at events and going out to the fligth line and flying. The event organizer does not know if they have AMA, plus they have not signed the proper paperwork to fly at the event. To me as a contest director that is a serious problem. All it will take is one of these people that do this is have an accident at the event and then not have AMA.
I plan to address the issue next year, by issuing wrist bands that can be seen to all the registered pilots. That way a visible scan can be made at the flight line and determine if the pilot has AMA and is registered.
David

[Danal Estes]

David, The wristbands are a great idea! Probably more visible than nametags, which is what some events have used.

[Pinecone]

Exactly, when it starts happening, then there will hve to be some form of impound. And it WILL happen eventually at the larger events.

Wrist bands are a very good idea, easily visible and easy to use. Many events I deal with use wristbands to make sure people are where they are supposed to be. I race SCCA road racing and they use different colors for spectators, crew and drivers.

[Ed1955]

I guess that's the beauty of belonging to a small club. I will never have to worry about that at my home base club because it's limited to 30 members. Don't get me wrong, I would love to see four hundred Trex 600's on the flight line but it's just not going to happen at the home club.
Ed